Device for Use in a Shared Commercial NMR Instrument Version II "
by A. G. Redfield, Brandeis University
II, Upstream from the Stop Tube
Most of what follows is likely
to be modified by your circumstances. In our case the small size and
low ceiling of the 500's room dictated that our system be very portable,
whereas some labs could have permanent setups at or near the instrument.
This is especially likely for shuttlers to be used with physically larger
magnets (600 and higher) as will eventually be desirable.
Furthermore, I recently concluded
that it is not needed to have the pressure/vacuum introduced at the
top of the stop tube, requiring, or nearly so, that a set of heavy valves
by lifted up and down each time the stop tube is used. Instead, the
air I/O can enter at the side into annular space just above the top
of the main glass shuttle tube, and transmitted to the inside of the
stop tube by many small transverse holes near its bottom. The upper
end of the stop tube might be a solid rod of around 1/4 inch diameter,
and would not be completely straightforward to design. This is worth
trying because it is annoying and fatiguing to deal with the air I/O
as it is now done.
Engineers will hate our electronics
system based on monostable (one-shot) timing and be tempted to use a
modern system based on an embedded computer/timing system. But the present
system is completely adequate. A problem might seem to be that the timing
settings cannot be stored and reproduced as are parameters on commercial
instruments. Perhaps, but air pressure parameters could not be documented
so easily, and because of subtle differences between instruments they
could not be easily ported.
For these reasons we will
not present such details as we did above.
Section TW TOWER and components
on it. The tower is useful probably in any case. We bought a heavy
aluminum 84x19" full-size rack made by Bud (they used to make aluminum
passenger railroad cars and "Budliners"), strongly recommended, with
many 12-24 tapped holes on both sides of its vertical aluminum beams
(the electronic supplier that sells it does not sell these screws which
are unusual in the US; you get them from McMaster Carr or equivalent.)
It has two very heavy (2 1/2 x 6 x 1/4") tranverse aluminum angle base
pieces which I mounted upside down and on both sides of the rack's
vertical channel pieces (The front of the tower is defined as
the side of the rack that will be closest to the magnet). Then to hold
the front wheels I cut two pieces of 1 1/2 by 3/16 angle aluminum 25
inches long and fastened them with at least 4 1/4-20 screws to the top
(what would have been the bottom) of the base piece, extending out 11
1/4 inches from the front of the rack's vertical channels It thus extends
11 3/4 inches out from the back surface of these channels. The 1 1/2
" vertical surfaces of these angle pieces touch the inner edges of the
vertical channels. Two 11" x 1 1/2 x 3/16" aluminum angles 11 long are
screwed to the top of the front base piece angle, next to the long 1
1/2 " angles, so that they extend out exactly as far as the longer ones
beyond the front. The bottoms of each pair of angles form a 3" wide
surface to which I attached, at the most frontward points, stainless
steel front casters. To mount the back casters I screwed a pair of 11"
long 1 1/2 " aluminum angles to the base piece, next to the longest
angle, toward the outside of the rack and ending in back, exactly even
with the end of the longest piece. These are also screwwd sideways to
the back ends of the long angle pieces To the bottoms, at the back end,
of each set of these angles I attached a rear caster. The latter were
swivel casters, but the front casters were not but were oriented toward
the front; this is desirable because the front of the tower is less
stable because the casters have to be close together to fit under the
magnet The result is a rack which fits through all doors in the building,
rolls freely, does not tip over very easily, and to which all kinds
of things can be fastened.
An old piece of heavy plywood
was hand-cut to fit in the space between the horizontal angle pieces,
to form a wood base, and heavy cider-blocks were placed on it, leaving
room for the reservoirs (below), to increase stability. The reservoirs
are made inexpensively from polyvinyl chloride (PVC) 3" diameter tubes
normally used for sewer piping in houses, and standard fittings, cut
with a wood saw and cemented with standard cement for this purpose.
Each is about 6 ft high. Heave plywood boards about 4" wide are screwed
onto the outsides of the rack, extending backward, and connected at
the back with a cross board. This U-shaped assembly is vertically a
few inches below the top of the sewar pipes, and its back board is even
horizontally with the back of the base of the rack assembly (with the
bottom casters). The three reservoirs are attached to this assembly
with elastic "bungie" cords available at hardware stores. Small strips
of 3/4" plywood nailed into the base wood constrain the bases of the
reservoirs, forming a 3" wall at the back and extending a few inches
along the side, to constrain the bottoms of the reservoir tubes.
The bottom of each reservoir
is closed with a standard cap. At the top, each tube has standard fittings
(that can be found only in big stores or catalogs like that of McMaster
Carr (with great care)) to reduce the size, and couple to a brass T
of nominal 1/2" size (inside diameter ~1/2 "). The long side of each
T is vertical, and the upper vertical end is coupled to a large quick-disconnect
fitting, to go via a large flexible tube to the valve manifold (below).
The side connection of each T goes to a smaller diameter quick disconnect
and flexible tube that crosses via an overhead trough to the rolling
table. This plumbing will be discussed more later.
A wooden trough that is instantly
removable connects the tower to another smaller (49x19") Bud aluminum
rack that sits on a relatively inexpensive white plastic and aluminum
rolling lab table. The trough is made by nailing (with stainless steel
nails) two "one by fours" with their four inch surfaces vertical, to
a same-size bottom with the four-inch surface horizontal (and no top
cover). The length of the trough is determined by the size of the laboratory
and the need to keep powers supplies away from the magnet; ours is about
six feet long. It is convenient (for moving in and out) to have the
sides longer than the bottom by about 1 foot so that the ends form forks
that can snag the wires and tubes that the trough must carry, during
installation. The ends of the tops of the side wall pieces of the trough
are tastefully cut off with a 45 degree angle cut. The trough is supported
at each end by horizontal ~8" light aluminum angle pieces (1x1x1/8)
bolted at convenient places on the sides of the two racks, with short
(3") pieces of the same angle stock, at their outer ends, to keep the
trough in place but allow it to be in stalled and removed easily. The
bottom of the trough is thereby about 76" off the floor when it is installed.
No wires or tubes run across the floor to the magnet or tower. The trough
is vastly superior to commercial metal or plastic wire troughs.
The heavy (1 1/2 by 3/16)
pieces of aluminum angle stock that are supplied with the tower's rack,
for its top, are not used there. Instead these and lighter 1/8 thick
pieces of aluminum angle stock about 20" long are bolted horizontally
across the front or back of the tower at various heights to support
anything you want. Most important, to support the valve manifold, two
of these are mounted across the front of the rack about 3 feet apart
vertically, with 7/8' diameter holes in their centers. A long (7-8 foot)
3/4" diameter rod passes vertically through these two holes to support
the valve manifold and let us raise and lower it . I made Delrin bearings
for the rod out of 1 1/4 x 2 pieces of Delrin 1/2" thick with 3/4 holes
that were screwed in above the 7/8" holes in the aluminum with small
screws. The 3/4 rod had the valve manifold attached at its upper end,
and the entire rod was raised by a rope attached to a short foot at
its bottom. The rope went through each wheel of a pair of double pulleys,
one attached to the foot of the rod and the other attached high on the
frame. In this way we have a 4-times mechanical advantage in lifting
the heavy manifold. A nautical cleat fastened to the tower at a convenient
level was used to fasten the end of the rope. Thus the valve manifold
and rod, total weight around 15 lb, could be easily raised and lowered
next to the magnet, and lowered to get through the door.
The valve manifold will be
described shortly. It is attached rather loosely to a platform formed
from the top of two short horizontal (~6" long) pieces of angle stock
running roughly forward to back. The vertical sides of these pieces
point downward and are attached to an assembly of aluminum pieces that
I will not describe in detail. They form a rectangular box about 5"
wide on both dimensions and 6" high. Near the lower end there is a horizontal
1/8 thick aluminum plate with a 3/4 " dia. hole, for the long 3/4 rod,
described just above, to pass through loosely. Another similar plate
and hole is located inside the box about 5" above the first, for the
3/4" rod to pass though loosely also. The two holes are about 4 1/2
and 10 inches from the top surface of the box, on which the manifold
will sit. To get through doors, the 3/4" rod passes through this box,
extending about 3" above the top of the box, even with the tops of the
When moving the tower, a
commercial collar (whenever possible use these, stainless steel ones,
not the cheap set screw type but the ones which squeeze the rod with
a single cap screw, it is always cheaper to use them than using some
machining) fixes the position of the valve manifold vertically on the
rod, and also the rod is limited in how far it can go down by the height
of the highest fixed hole (3/4" bearing) fixed on the tower's front,
through which the rod passes. The heights of the collar and of the crosspiece
that holds the bearing are set so that the 3/4" support rod is about
even with the tops of the solenoid valves, as mentioned, and both are
just below the top of the lowest door through which the tower has to
pass when the rope is released and the manifold is as low as possible.
Once the tower is in the
500's NMR room, we can insert a 6" wood spacer between the collar and
the box to eventually get the valves very high. The spacer is made from
two 6" long pieces of 3/4" plywood attached to a third narrower one
so that they form a groove 6" long, and can surround the 3/4" rod snugly
but not tightly. The (vertical) groove between the outside pieces is
about 3" deep horizontally, and there are hook-like 1/2" knobs cut on
the outer edges of these pieces, at both the upper and lower ends, that
tend to keep the spacer in place. When it is, the valve manifold is
pushed upward 6" relative to the rod, compared to when the spacer is
not there, so that the end of the rod is 3" below the bottom of the
valve manifold, just barely going through the top one of the two 3/4"
holes in the support box for the manifold.
Section PL, plumbing.
This covers all the plumbing from the top of the stop tube back all
the way to the sources of pressure and vacuum. Very tedious. The purpose
is, first, to provide regulated pressures (low and high) and suction,
and then to connect these via electric solenoid valves, in the correct
order and time, to the common output connected at the top of the stop
tube. At this point you must READ PARTS of the MRC article which I will
not repeat here. It even gives the valve manufacturer's part numbers
but there was a misprint: the valves are Asco numbers 8262G212 for pressure
and 8262G264 (thanks to Boris Itin for finding this). Later I will give
an equally tedious description of the electrical system that mainly
turns these valves off and on.
General remarks: it is good
not to have leaks in the system before the solenoid valves (though very
small ones can be tolerated). Downstream (toward the magnet) of these
valves small leaks are not at all a problem because they are not likely
to be large compared to the flow through the shuttle tube (however,
note that this latter flow is nearly shut off as soon as the shuttle
is at either end of its travel), and the valves are open usually only
for a small fraction of the cycle.
The valve manufacturer recommends
that the valves be mounted with their magnetic axes vertical, to aid
in clearing particle dirt, but this is not a big problem because the
air is well filtered, as is the vacuum (joke). I have had problems after
modifications or glass breakage. In the latter case the shuttle itself
often sticks in the glass tube and it has to be taken out and washed
with water and dried with paper towels pushed through the shuttle tube,
after any NMR tube brakage. We recently had one valve stick for no obvious
reason (it was the last one to open, to let the pressure equilibrate
to ambient pressure) but this fixed itself as soon as I had time to
identify the valve, and to determine that it was not an electric problem.
I followed this recommendation, and also arranged the valves so their
bottoms were also on the same plane so they could sit on the platform
formed from two pieces of angle stock, mentioned above. It is also desirable
to make the manifold as compact as easily possible, and clustered around
the 3/4" aluminum tube that supports it (not in a straight line, for
example) and with space for this rod to pass through between the valves,
vertically, close to their center of gravity. It is also a good idea
to be able to add more valves later, easily. It was hard work to design
the manifold. In general it was a poor idea to use plastic plumbing
which breaks, so brass was used everywhere except as noted.
The entire manifold can be
disconnected mechanically and electrically in a few minutes, and lifted
off for inspection and maintenance. Unfortunately a strong person has
to get on a ladder to do so. Getting the manifold back on is especially
strenuous. The valves seldom are expected to need replacement (although
I have one spare of each type), but their internal parts have to be
inspected for wear probably ever half million cycles and this is easy
if they are vertical. The Asco Company seems to think you will know
how to take them apart. You have to slide off the top nameplate horizontally
with the help of a screwdriver, and you can eventually figure this out.
Then the solenoid coil slides up off the top of the valve. A spring
then has to be removed to get access to the large hex nut that has to
be unscrewed. I invested in a large wrench of the "box" type that encloses
the nut like a socket wrench, but it is a single assembly. I took apart
each valve, and put it together again, before assembling them, so that
they would be easy to take apart later. Then you can take out the guts.
BE SURE NOT TO GET CONFUSED ABOUT HOW PARTS GO TOGETHER, ESPECIALLY
FOR THE NORMALLY OPE VALVES, AND INCLUDING THE NAME PLATES. ONLY DISASSEMBLE
ONE VALVE AT A TIME.
Asco does not tell you what
to look for when inspecting their valves, so I can't either. I have
not had to replace one yet. I have a repair kit for both of the types
of valves I used.
We have an electronic counter
(Veeder-Root) that has a battery that will last 7 years and counts the
number of cycles, to help us decide when to opent the valves up for
Ordering the parts for the
manifold (from McMaster-Carr) was a major job. Read what follows carefully
when you do are ready to build it, and not before, you will forget what
I do not use Teflon tape
for plumbing even though it looks good and does not soil your hands;
I use standard plumber's compound to save time and frustration. Some
of the details may be important, and changing them could complicate
easy construction of the manifold.
Tour through the plumbing:
I now describe the gas plumbing in tedious detail, going upstream, starting
from the top of the stop tube. The stop tube itself ends at the top
simply cut off on a lathe and then beveled slightly on the outside ("chamfered")
to facilitate connecting the to the manifold. Upstream of the stop tube,
everything remains connected together forever. The first thing upstream
is a "vacuum quick disconnect" that adapts any 5/8 tube via a compressed
O-ring to a brass solderable tube. They can be bought cheaply from vacuum
supply houses. It is adapted to nominal (US size) 1/2 plumbing by drilling
through a commercial brass plug of this size and soldering it in. This
is screwed into one side arm (oriented vertically) of an aluminum 1/2"
US nominal Tee (aluminum to save weight, this is a big Tee). The opposite
vertical side arm has a similar plug screwed into it to which is adapted
a silicon pressure gauge (Honeywell XCA505DN, marketed by Newark Electronics,
5 volt supply, +- 5 pounds per square inch). This is adapted to the
brass 1/2 nominal plug with a plate that pushes it onto a questionable
O-ring seal with the gauge's surface. These gauges contain parts that
break if you drop the gauge. After you receive and study the gauge you
will figure out how to seal it. It is a permanent part of the manifold
assembly so it will not be likely to be dropped.
All plumbing of what follows
is 3/8" US nominal plumbing and 3/8" (~11 mm) inside dia. rubber, copper,
or plastic tubing except the solenoid valves which are only available
as 1/4" nominal, and the 3 flexible tubes that are about 60 cm long
and connect the movable manifold to the fixed tops of the reservoirs.
The long (~8ft) plastic or rubber (for vacuum) tubes that go from the
reservoirs to the table, via the trough, are also 3/8" inside diameter
reinforced Tygon or thick rubber, not because pumping speed is critical
for them but to avoid kinks that could pinch off the air flow.
Returning to our tour, the
side arm of the aluminum Tee is adapted connected with a male (M) to
female (F) aluminum elbow, (known as a "street elbow"). This elbow is
important: it does not affect the speed of gas flow much but it allows
a higher degree of flexibility of the connection between the tower and
the stop tube. (Note also that the entire manifold can swivel about
the 3/4" aluminum rod, on around a vertical axis, which we arrange for
the same reason.)
The other connection of the
1/2" aluminum elbow is adapted with standard brass fittings to a 6"
length of rubber tubing whose other end is adapted to the central Tee
of the manifold (which is female gender). This is the side-arm hole
of a FFF Tee of the type which looks as if it were milled from a square
block. (Tees have three connected holes and are sold in three forms,
FFF (all female) and FFM (or is it MFF?) and FMF which have one out
of the three connections being male and with FFM having this male being
one of the two end connections, and FMF in the middle.)
The overall shape of the
manifold is more or less like a two-pronged fork whose handle is the
rubber hose already described, oriented towards the magnet, then a perpendicular
cross piece made of three block-type Tee's, and two arms pointing away
from the magnet. (see fig. PA). The six
valves are connected, three on each side, to the outer sides or ends
of the arms. The cross piece consists of a central FFF Tee, already
mentioned, screwed directly into FFM Tees via their M connections, and
all in the same orientation. The two arms each consist essentially of
a single standard (non-block) FFF Tee with its long part pointed back
away from the magnet coupled to the middle F connection of one of the
two cross-piece Tee's. All five Tee's are oriented so that the plane
defined by the 3 connections is horizontal. The side-arm Tees and the
two cross-piece Tees are not directly coupled, they are connected by
a short (but not he shortest available, you need some space here) M-M
connector (a "stud"). Now if you put it together, there are 6 female
connections always connected to the rubber tube via the Tees: two at
each end of the arms, and four pointing out though the relay. The current
is off on any relay when its control line from the timing card is up
(+5) so the strobe for sideways away from the manifold. The solenoid
valves will be screwed into these holes via short MM 1/4 nominal to
3/8 nominal adapters. But wait, don't put it together now, first you
have to screw the valves onto the T's, at least the side ones. And before
you do that you have to read the next three paragraphs.
First you have to carefully
read what I wrote in MRC to determine which wire driving the valves
go to + voltage and which to -voltage!!! These wires are both red and
you can't tell which is which, each valve has to be tested!!!. (A green
wire on each valve is supposed to go to ground for safety reasons but
is not so important for these DC voltage valves). IMPORTANT!! LABEL
the wires after determining the one that is +.
Second you have to read the
obscure paper sent with the valve to understand which way the gas is
supposed to flow in the valve, indicated only by letter on the two ports.
This may not be so important if you use the valves I suggest that are
rated for considerably higher pressure than the pressure you are likely
to use; but you never know. Obviously, for the three normally closed
valves used for pressure the flow direction will be toward the rubber
hose; but for the other three, that is two normally open for vacuum
at the start of a cycle, and one normally closed for atmosphere relief
at the end, the flow is away from the rubber tube. Mark these valves
before assembling the manifold.
OK, after several hour you
got it together. Plan now how to attach it loosely to the aluminum platform
mentioned above, with a transverse scraps of aluminum angle stock and
two long screws. Perhaps don't build the platform until now, after further
thought. Before, this plan and install the plumbing to/from the three
reservoirs. The valve that releases to atmosphere at the end has nothing
connected to its female connection hole, not even a plug. The low pressure
valve is connected to medium wall 1/2" inside diameter tygon tubing
with standard fittings and this and the remaining two large flexible
tubes are clamped semi-permanently with screw-type hose clamps. The
remaining two lines from the reservoirs, vacuum and higher pressure,
each go to a pair of electrically and functionally parallel solenoid
valves. I used copper tubing to connect the upstream ends to the hoses,
to get a pleasing compact connection rather than a multitude of bulky
Tee's, at the cost of a slightly complex connection procedure (and a
similar reverse disconnection, if the valve bodies ever have to be disconnected
which is unlikely). First I assemble copper adapter elbows, 1/2" nominal
pipe thread to 1/2" nominal copper "sweat" (solder) connections by soldering
4 of them onto 4 pieces carefully cut to be equal in length, about 3"
long. These were then screwed into the four valves (with pipe compound)
on the side that was to be further from the rubber tube to the magnet.
Then, with the four tubes pointing downward, the four solenoid valves
were coupled to the Tee's as shown in fig.
PA. Finally the Tees were all connected together as already described.
Then I soldered the lower ends of the two pairs of the nearby
copper tubes together and each pair to a short horizontal output tube,
using two short pairs of copper tubing and two each of an elbow and
a Tee for the entire solder part of the job. The long flexible tubes
that go down to high-pressure and vacuum reservoirs are clamped directly
onto the two open ends of the short copper tubes with screw-on tubing
These three large diameter
Tygon or rubber tubes that go to the reservoir have quick-disconnect
pressure connectors (McMaster Carr) (not to be confused with the vacuum
quick disconnect used above at the top of the stop tube). They could
be eliminated in favor of screw-type hose clamps, but use of quick-disconnects
encourages removal of the manifold for inspection. (Here and below,
"quick-disconnect pressure connectors" refers to a type of connector
that usually has a one-way valve inside one side which is normally the
high-pressure female side; but in our case we used so-called fast-flow
types in which this valve is not included. For the particular variety
we ordered, the female's were brass but the males were brass-plated
magnetic steel, and we had to re-order stainless steel ones.) The quick
disconnects used here were 1/2" nominal size. For these connecters in
general we used an orientation for gas flow from male to female; as
a result only one of the two pressure pairs of connectors has to be
color-coded to assure proper connection because a vacuum line cannot
be connected to a pressure line. Each of these three connections are
standard, using hose clamps and barbed tubing connections where needed.
The corresponding smaller lines to the relay rack on the table, via
the trough, are nominal 3/8" size and entirely similar otherwise.
The vacuum reservoir also
has a small diameter side-connection to a 4 foot small-diameter hose,
going to the sample sucker. (An annoying feature of our present system
is that the vacuum lines must all be connected and the electronics plugged
in to use the sucker. This could to a considerable extent be alleviated
with a small reservoir in this line having a one-way valve at its downstream
The three long supply hoses
are semi-permanently connected at the table, using hose-clamps to the
three vacuum/pressure gauges (see MRC for the types). Pressure of about
20 PSI comes from a regulator on the lab wall, fed from the spectrometer's
carefully filtered 80 PSI air, with a gate valve in front. A small tube
connected reversibly to a taper fitting at the wall regulator goes to
the higher 0-10 PSI regulator for the higher pressure, and feeds the
lower pressure 0-5 PSI regulator, as well as the line to the tower.
Manual pressure-relief valves at the output of both regulators are useful
for setting the pressures. All these plumbing and the vacuum connections
are straight-forward. Unfortunately all vacuum gauges sold in the US
seem to cover only 0-30 PSI vacuum and are marked in inches of mercury.
These valves and gauges are mounted on recycled pieces of rack-panel,
on the upper part of the relay rack that sits on the rolling table.
The vacuum originates in
an oil-free pump (type given in MRC). This is a vane pump and can be
used either for low vacuum or pressure, apparently. It has been trouble
free but I have a repair kit anyway. It is in a sound-reducing box in
the lab down the hall where I do off-line testing. The box is made of
3/4 roofing plywood to whose inside is cemented the fake anechoic chamber
lining made of cheap foam (left over from the treatment of our 500 MHz
NMR room). This is not a simple box but its internal top extends not
quite all the way across, and an outer top covers all but a small section
on the other end, and is about 4" above the inner top, so they form
together a duct. This duct is lined with the anechoic foam. The box
has a cooling fan mounted on a hole in the lower part of one wall to
draw air into the top duct, very important, and should have a temperature
turn-off switch to avoid a fire in case the fan stops and the pump motor
We had a vacuum line installed
for this pump down the hall to the NMR room, with gate valves at each
end. The vacuum is sensed by a standard electromechanical regulator
at the pump, which turns the pump on and off to keep the vacuum around
1/2 atm. This switch tends to stick in the on-position so we installed
a solid-state relay to run the pump, controlled by the regulator switch.
We also have a pressure line installed, to ship the dried air pressure
back from the NMR room to the development lab. It is not connected yet
so we use standard nitrogen gas tanks to supply the brief tests we usually
do in this room.
SECTION EL, electronics.
Tower. The only electronics in it is a +5 volt regulator chip
that converts the 12 volt relay supply voltage to 5 volts for the silicon
pressure sensor . All lines go to two ten-position terminal strips which
also hold the 5 v. regulator. These terminal strips are bolted to the
vertical arn of a piece of 1x1 aluminum angle stock, that also holds
down the valves on their platform by means of two screws through holes
in the platform. The 12 wires from the relays are screwed to these strips
and then all the relay + wires are connected together to the input 12
volt supply. (see Fig. EA), lower section.
The two pairs of minus wires from the valves that operate in parallel,
for the vacuum and the high pressure, are screwed into the same terminals
of the blocks. Back-biased power diodes are installed here between the
four - control wires and the +12 volts, with the cathode side to +12,
to protect the solid-state relays. All wires but the output of the pressure
gauge go through a 6 conductor cable about 70 cm long, to an archaic
8-pin connector (Cinch Jones) which will be replaced by a type that
is easier to connect and disconnect. (We will standardize on a low cost
24 pin round plastic connector made by AMP, CPC series.) The signal
from the silicon pressure gauge also goes through the same connector
but is otherwise carried by a coax cable taped to the 6-wire cable to
reduce noise. The upper end of the coax and 6-wire cables are firmly
taped to tubes of the manifold, to avoid breakage.The 8-pin connector
is disconnected when the tower is removed from the NMR room. The pressure
signal is again routed through a small coax line, taped onto a six-wire
cable after this connector. These lines then all go via the trough to
the table, where they are semi-permanently connected to a circuit board.
Table. Solenoid valve
control. Most of the electronics in general is on circuit boards
that live in a Vector type CCA13C/90 cage. The circuit boards are mostly
standard Vector type 3677 proto-boards with 0.1" hole spacing and convenient
layout of connectors and pads. The cage has its own small +5, +-15 V
supply module. Interconnections are via soldered wires between board
connectors or 26-wire ribbon via standard IDC connectors or barrier
strips, to the cage, to a few BNC cable connectors, or to the wires
The four minus end wires
from the valves, and the ground and +15 go to a connector-less circuit
board via a barrier strip, that slides into this cage and normally lives
there (fig. EA, top section) This card
has four nearly identical solid-state relays on it controlled via a
ribbon cable from the timing card in a conventional way. The +inputs
from the solid-state relays all go to a +5 supply voltage, and the -inputs
go to the ribbon.
The timing control card (fig.
EB) runs these four control lines and another that turns the Helmholtz
coil on & off that is used for very low field operation (below). These
are controlled by 7407 open collector drivers. The inputs of this card
are only 3 wires: most important, a positive gate from the Varian console
that coincides the "raisetime" of the sequence. It comes from spare
(coax) output 1 on the back of the Varian console to the end of a long
coax cable that dangles off the magnet in back. To this is connected
another long coax line that travels via the trough to a coax connector
on the back of the cage. The other input lines to this card select either
this strobe, or an internal test strobe generated on the card. There
are also three potentiometers on the front panel connected to the timing
card via the ribbon connector, one of which determines the length of
this test strobe, and another which determines the time between strobes.
A third one controlls the time of the high pressure . These are connected
via the same (and only) ribbon cable as the solenoid control lines.
There are also two wires that transmit the position of the "Varian-stop-test
switch on the front panel.
I will not describe how the
555 chip that generates the strobe works (bottom left of
Fig. EB), you will have to get help from a local electronics technician,
nor how the "one shots" (officially called monstable multivibrators)
type 74221 work. Basically the latter convert a TTL negative or positive
edge into a pulse whose length is determined by a soildered-in capacitor
and a resistor. They are about the simplest thing you find in digital
elactronics. The circuit diagram EB uses conventional symbols for some
logic gates, and box symbols for the timer and one-shot units. The one-shots
are in dual packages and each section is represented by a single box.
Generally the pin numbers are written near the wire-lines and often
the function is written just inside the box near the wire; if not, the
positions of the wires are similar for similar units. Letters like (HY)
near each box or gate indicates to me where the unit is located, as
row H, column Y on the circuit board, to help me trouble-shoot. Generally
I leave out of the diagram a 0.01 microfarad capacitor which I ALWAYS
solder across the 5 volt power line to ground., right at every chip.
These cards are intended for standard logic with three long lands that
are for ground, and four others for +5 V. Lines going to square boxes
with letters in them or numbers up to 22 denote edge connections, I
use A for +5, and B and Z for ground. It is good practice to connect
the distant ends of ground and + 5 together with cross-wires, and connect
them with .01 microfarad capacitors, and ground the grounds B and Z
to the cage, and I always do. Lines going to circles with numbers inside
them denote connections to the ribbon connector.
The 555 chip generates pulses
at a rate determined by a variable resistor on the front panel and the
falling edge of this triggers the one-shot (EX), whose length is determined
by another variable resistor. The output of this is the test stroe.
It, and the Varian strobe input via a BNC connector go to a 74253 data
selector (lower right part of figure) that selects which strobe to use
as selected by the panel switch.
The selected +strobe (Varian
strobe or on-board generated test strobe) is convert via logic to a
strobe which drives the vacuum control line and turns on the vacuum
(upper left, to ribbon line (8). Note however that the vacuum relays
are normally open so this strobe. In fact only these vacuum, normally
open, solenoid relays, of the four, have current running through them
most of the time, to keep them closed (because they are "normally open";
normal means, in this case, with no current running through them strobe
must turn off the current these relays is up during the selected).
MRC explains why I do this.
A minor point is that this
current heats up these relays, which could shorten the life of their
coils or internal parts. For this reason I inserted a 3 ohm resistor
between the "plus" line of the solid state relay R1
and the + 12V supply, to decrease the steady current through these solenoid
valves; the valves only need about 1/2 of their stated current to stay
closed once they are closed. But to assure that the valves actually
close, it is desirable to apply the full voltage to them just at the
beginning of closed period, and doing so results in faster closing at
this time which is the most crucial of the sequence. I do this by use
of the 0.0048 farad capacitor connected from the plus terminal of the
solid-state relay R1 to ground, so that the
capacitor charges during the time (~0,1 sec or more) that the valve
is open (and the solid state relay is off, and the vacuum is raising
the sample or keeping it up), and then when the solid-state relay closes
full voltage (12 volts) is applied to the solenoid, and then decays
to about 8 volts in about xx milliseconds. The resistor and capacitor
are mounted on a spare location in back of one of the pressure regulators,
because the capacitor is too large to fit in the Vector cage.
Now going forward in time,
the negative edge of the strobe goes to the "A" input of the next one-shot
and turns it on for a time controlled by a front panel potentiometer
mentioned above. This applies a negative strobe to the high-pressure
control line and turns on the high pressure for only a short time (~15
msec) depending on the fron t panel potentiometer setting.
The end of this strobe turns
on another one shot that controls the low pressure time, set by a screwdriver-adjusted
potentiometer typically to about 0.3 sec. It has to be longer than the
"doptime" (see MRC).
Finally the end of this strobe
turns on the last solenoid relay that opens the manifold to atmosphere
for about 0.3 sec. Three very similar steps.
This board also generates
two other important signals:
1.The "rear scope trigger"
goes to an edge terminal (11) and then to a BNC coax connector on the
rear, to trigger the storage oscilloscope sweep. It is identical to
whichever strobe is being used, the internal test strobe ore the Varian-generated
strobe. A TTL amplifier isolates this output. See below under "pressure
monitor". This signal also goes to the Veeder Root counter that tells
the number of cycles that the system has done.
2. The Helmholz coil used
for very low field measurements, and located on top of the magnet, can
get hot if it is run all the time. So we turn it off during most of
the time that the selected strobe is off (down) and the sample is at
the center of the magnet. This can be done by using the selected strobe
directly (passing through the TTL inverter at (FY) and edge pin (22)
to a coax connector on the rear of the Vector cage because the Helmholz
circuit is turned on by a negative strobe. However, there is a delay
between the time the vacuum is turned off, and the same time that the
high pressure is actually applied to the sample because of delays in
turning on the solenoids, then a delay in actually opening the solenoid
valves, and finally a delay in filling the shuttle tube with high pressure.
We do not want the field to start turning off before the sample starts
to leave the upper position at the stop tube. So the time at which the
Helmholz coil is turned off is delayed by "stretching" the strobe that
controls the coil, by about 0.1 sec. One-shot (HY) does this; it is
triggered by the falling edge of the selected strobe and turns on for
aboput 0.1 sec. Its positive output is "Nor'd" by use of the second
inverter connected to the output to lengthen the output at the rear
connector. It is necessary to check with an oscilloscope to see if the
delay in turn-off is as long as needed, and also that the current through
the Helmholz coil is on by the time that the sample is at the stop tube.
fig. EC shows the entire electronics of the coil. Basically, a solid
state relay turns it on under control of the output just described.
It is in series with the connection of the Helmholz coil to a large
power supply which is set to regulate current. The size of the current
is determined by the Varian computer under control of the NMR operator.
Staring at the right side
of fig. EC, 8 lines of a binary representation
of the user-entered parameter "cur" are presented by use of simple commands
(see Varian user programming manual, which does not tell you that the
most significant bit of the number that appears is on pin 1 and not
pin 8 as I initially assume!). This is a number from 1 to 255 (if cur
is zero the program is written to turn off the shuttle strobe entirely,
and a high field R1 run results). The current in the Helmholz coil,
if it is on, is supposed to be proportional to this number, and the
system is adjusted so that if cur is nearly 255, the Helmholz coil exactly
cancels the main magnet's fringe field. We determine exactly what this
number is by using a Hall magnetic field probe and seeing at what number
of "cur" the field in the center of the coil is zero. The number cur
is fed directly into the 8 most significant digit inputs of a digital-to-analog
converter, while the four least significant inputs are set to zero.
The output of the converter is then proportional to Cur and equals about
1 volt when cur = 255. This voltage is fed via a cable to the input
of the power supply that controls its output current when it is in the
voltage-to-current operating mode. The power supply is an excellent
0-10A 0-50V Kepco supply. We mounted a toggle switch on the top back
of the supply to switch from the external-voltage-control, constant
current mode, to the normal mode using the front panel controls. THE
FRONT PANEL CONTROLS HAVE TO BE TURNEDTO THEIR MAXIMUM VALUES when the
supply is externally controlled (at least for this power supply). More
precisely, the current control should be all the way on (clockwise)
but the voltage control should be set very slightly below 50 v or the
supply mat turns itself off and will have to be turned on ). You will
have to consult the manual of the particular power supply that you have,
to learn how to wire this constant current mode; and be sure that you
can get or buy a power supply that regulates current; high speed is
In theory the output circuit
(left side of Fig. EC) does not have
to be grounded anywhere but it probably works best if the + output of
the supply is grounded, as it is. A back-biased diode is connected across
the Helmholz coil which conducts no current and normally does nothing,
but when the magnet is switched off suddenly by the solid state relay,
a larger reverse voltage could appear on it and destroy the relay. (This
diode may not be needed, but it is hard to get practical information
on these relays. Aside from this, the main thing is to connect the input
and output voltages with the right polarity This must be a DC-to DC
relay, be sure not to get one for AC output. Also, one with plenty of
voltage and current capability, and mount it on a heat sink or panel).
A series "shunt" resistor connected to a digital voltmeter ("DVM") monitors
the current through the coil and is not used much in running; it was
mainly used initially, to verify that the current output was proportional
to the parameter "cur".
The control voltage (off,
or zero volts, to turn on the current) comes from a coax connector on
the back of the Vector cage, as described already at the end of the
last section. The pull-up voltage is supplied by a little modular supply
of the type used to supply portable electronic consumer devices. I had
some trouble with this and had to put a resistor to the + voltage in
order to turn off the current completely; perhaps the 7407 driver does
not turn off completely.
Slightly exotic circuitry
and mechanical design is used to have the negative output of the A/D
converter floated at the ground of the Kepco supply, to reduce "ground
loop" problems. This is suggested by the "earth" symbols, a horizontal
line with cross hatches /// below it. Consult an engineer who understands
about ground loops.
Yet a third smaller Bud aluminum
rack sits on a dolly made of heavy plywood and casters, and holds the
Kepco supply and a medium-size rack panel on which is mounted the other
things shown in fig. EC. This rack is
not wheeled into the lab except if very low field measurements are planned.
The time-course of the pressure during the field cycle is monitored
using an excellent and inexpensive 60 MHz Tektronix digital storage
oscilloscope. Its horizontal sweep is triggered from the selected strobe
("rear scope trigger", usually its trailing edge so we can see the key
part of the sequence at the end of the sample's visit to low field.
We have described how the output of the silicon pressure sensor is shipped
via the trough to the table's relay rack. This signal could be fed directly
into the oscilloscope's vertical input, but this is inconvenient when
looking at small pressure changes because zero voltage directly from
the silicon pressure gauge is indicated by +3.5 volts. This is avoided
by connecting three forward-biased small diodes to this point, whose
other (base) end goes via a 22K resistor to -15 volts supplied by the
Vector cage's supply. The a BNC connector on a panel that goes to the
oscilloscope vertical input is connected between the diodes and the
resistor to give about zero volts with zero pressure.
Power supply. The
+-15v, +5 V supply is a small unit inside the Vector cage. A 12 V 5A
open supply sits of the bottom shelf of the lab cart on the end furthest
from the 500 magnet. This shelf carries the Helmholz coil in transit
also. The middle shelf carries the shuttle tube and all other long pieces
and miscellaneous junk. The trough and a professional-grade 6 foot ladder
have to be hand carried. The voltage of 12 volts was selected for invalid
reasons; 24 volts is probably better; 24 volt valves are more standard
and easier to get.
See MRC. Fig. ED shows the operational
amplifier circuit used. It is powered by two 9 volt batteries in the
same box. Probably the coil wire and number of turns are not critical.
It was wound onto a small brass spool formed by soldering 3 annular
brass cylinders, all 1" diameter with a hole through which passed a
3/8" dia.brass tube about 1.3 meters long. Two of the discs were soldered
at the bottom about 1 cm apart to form a spool for the wire; the third
was about halfway up the rod as a guide. The outside of the coil of
wire is about 20 mm dia and it is probably #30 wire.
After tests using stir bars,
we (Dan Miner and I) inserted the coil in the magnet (with the Varian
upper stack removed and no probe) and the reset button was pushed to
zero the output voltage, and subsequently the integrators drift control
was adjusted continuously. Then the coil was moved to different heights
at different speeds and a good speed was found, fast enough to allow
us to estimate the voltage change, but not too fast to give non-reproducible
results. The latter may occur if the induced voltage is so high that
the op-amp output is overloaded. Then we recorded a series of voltages
with displacements from the center of the magnet going up, and also
from the top of the magnet (at the upper edge of the uppermost flange
that is permanently part of the magnet) and especially between these
points. The latter voltage should correspond very accurately to 11.75
T, to allow us to convert the voltages to gauss intervals. Dan Miner
then took the deflections and fitted them with a spline-fitting program
to generate the graphs (Fig. EF and
Fig. EG) which we use to set the height of the upper stop (see part
This calibration has been
used for our three papers published so far, and I am not worried about
the calibration, because the relaxation rates do not vary strongly except
at fields below about 2 Tesla, where errors in the calibration may be
small. However, as this is being written I am trying to check this calibration
by field-cycling NMR at about 4 Tesla, and at lower fields by use of
Hall magnetometers calibrated using a teaching NMR setup (Teachspin).
Future sections. I
did not write a sections on the modifications we make to standard Varian
library sequences. There is some information in an earlier report. But
hey, no one else has this machine so there's no hurry, and it is easy.
I will also write more on measurement of the fringe field when I have
done it better.
I have been trying variations
of the upper plug, making it nearly half as long, and eliminating the
groove at the top. It is now made with only a cap at the bottom (where
the sample is). It is still hollow but we no longer make it with such
a thin wall (drilling the inside with a smaller drill), and this makes
it easier to make. We no longer rely on the fact that the plug will
float; the wire at the top makes this unnecessary, although it is good
to have the plug as light as possible. The end sections are made to
fit the NMR tubes as closely as possible and are slightly longer. (Frank
can machine the clearance with better than 1/2 mil clearance, that's
nearly 100 mkicrons.) The clearance in the middle is also a little less.
The flats at the bottom are still used. Probably we will go back to
a longer plug, or some compromise length. All this is to reduce sloshing
and bubbles still more. All the above is being tried with 5 mm tubes.
I am also experimenting with 5-minute epoxy for cementing in the plug,
and also for cementing the tube in the adapter. This may allow a lead
of only 4 hours between sealing and running, rather than 18 hours. This
epoxy turns out to be less viscous as well as setting fast. The theory
is that if the epoxy and sample meet only in the close clearance at
the top, they will not mix much before the epoxy sets in 5 minutes.
When I have more experience I will revise the relevant sections above.