OPERATING INSTRUCTIONS

for the

G-01 GAS-MEMBRANE CONTROL BOX.

Safety Aspects of the G-01 Control Box

The G-01 Gas Membrane Control Box is made with Swagelok stainless steel
fittings. They are rated by the manufacturer as follows.

+-----------------------------------------------+----------------------+
| **Part**                                      | **Pressure           |
|                                               | rating/psig**        |
+-----------------------------------------------+----------------------+
| ¼" tubing                                     | 7,500                |
+-----------------------------------------------+----------------------+
| 1/16" tubing                                  | 6,800                |
+-----------------------------------------------+----------------------+
| Black hose                                    | 5,000                |
+-----------------------------------------------+----------------------+
| Quick-Connect couplings (when connected)      | 4,000                |
|                                               |                      |
| \[NB. These must be connected/disconnected    |                      |
| only at atmospheric pressure.\]               |                      |
+-----------------------------------------------+----------------------+
| Valves in the G-01 box                        | 6,000 & 5,000        |
+-----------------------------------------------+----------------------+
| Plug valve (on the line to the dac)           | 3,000                |
+-----------------------------------------------+----------------------+

These are the maximum working pressures recommended by Swagelok. They
are not the bursting pressures: there is a large safety factor built
into them according to US standards.

The maximum pressure that seems to be available from commercial gas
cylinders is 3,000 psig, which is reduced to about 2,500 psig by a high
pressure regulator. Thus, the rated presures for all components are well
above the pressure at which the system will operate.

The gas membrane welds will also support this pressure so long as the
membrane is properly located onto its matching pressure plate. In
addition, the membrane is fully enclosed in the thick steel casing of
the diamond anvil cell when the cell is assembled. Pressure should only
ever be applied to the membrane when within a properly assembled cell.

The volume of the pressureised gas behind the membrane is only about 0.5
ml at 200 bar. Thus, the maximum gas volume at STP is under 100 ml. The
rate at which gas can be delivered to the membrane is deliberately
restricted by means of the attached capillary tubing to a maximum of 12
litres per second (at STP) under a pressure difference of 200 bar. Thus,
the low volume of gas at the membrane, together with its total enclosure
by the cell, ensures that there is no danger to an operator even should
a leak develop in the membrane.

> **Making Swagelok connections.**
>
> The DXR-GM cell and G-01 control box use Swagelok connections
> throughout. In the course of experimental work you will sometimes need
> to break or re-make some of these couplings. Swagelok issue the
> following rules for these procedures.
>
> **1. New installations - making a connection for the first time.**
>
> Assemble the components of the joint and make the joint finger-tight.
> Mark each part with a pen or pencil. Hold BOTH parts of the joint with
> spanners. Then for ¼\" diameter tube, tighten by 1¼ turns. For 1/8\"
> or 1/16\" tube follow the same procedure but tighten by only ¾ turn.
>
> **2. Re-making joints.**
>
> Assemble as above and make finger-tight. Then with spanners make a
> further ¼ turn (all tube sizes).

**A: Connecting to the pressure control box (PCB).**

1\. Any inert gas (nitrogen, argon, helium) may be used to pressurise
the system, provided a high-pressure regulator is used with a 2000 or
3000 psi cylinder. The cylinder and regulator are not supplied with the
cell. A length of black flexible hose (rated to 5000 psi) is supplied
for connecting cylinder and PCB. One end of the hose terminates in a ¼\"
diameter stainless steel tube which is bent at right-angles. This end
should be connected to the back of the PCB. Follow the Swagelok
procedure indicated above. Thus: the nut on the fitting should be
finger-tight. Mark the nut and the fitting with matching marks. Holding
BOTH the nut and the fitting with suitable spanners, tighten the nut 1¼
turns. Do not allow the fitting to turn.

The other end of the flexible tube terminates in a ¼\" or 6 mm diameter
stainless steel tube (depending upon customer requirements). This should
be connected to the gas cylinder reduction head.

2\. **An extension tube** 1 meter in length (1/16\" diameter capillary
tubing) is supplied with all gas-membrane cells. This is intended to go
between the cell and the PCB and may be bent into any convenient form.
One end should be plugged into the side of the PCB, the other end
connects to the valve on the cell. Longer extension tubes can be
supplied on request, for example, for remote operation in synchrotron
experimental areas.

3\. The cell plugs into the extension tube, or the side of the PCB, by
means of a quick-connect coupling. Push back the sleeve of the socket on
the PCB, while pushing the connecting stem fully into the socket.
Release the sleeve, and pull gently on the stem to make sure that it is
locked in place. To release the quick-connect, hold the stem firmly and
push the sleeve back. Pull the stem out of the socket. A schematic of
the control box is shown in Figure 1.

# These operations must be carried out ONLY

> ***while the system is at atmospheric pressure.***

![](media/image1.png){width="5.260416666666667in"
height="3.4944444444444445in"}

**Figure 1:** Schematic layout of control box.

**B: Gas-membranes for cryogenic use.**

Any inert permanent gas may be used with membranes at ambient
temperature. However, membranes intended for **cryogenic** use must be
pressurised with helium gas. Cells for cryogenic use are supplied with
quick-connect fittings which contain non-return valves to conserve
helium. Details of the membrane connections within the cryostat shroud
vary with the individual cryostat.

**1. Fitting to the cryostat.**

The single-tube membrane assembly is usually supplied with 1 meter of
1/16\" diameter stainless steel capillary tubing in the form of a coil.
When setting up the DXR-GM cell in your cryostat, you will probably want
to (1) alter the tube length, and (2) bend it.

(1)The tube is easily cut with a standard metal saw. (2) When bending
the tube, it is essential to avoid collapsing it. *Always bend it over
some round-section item, such as a piece of pipe, or a screwdriver.* The
tube may be bent close to the membrane provided care is taken not to put
strain on the entry of the tube to the membrane itself. One convenient
way of using the coiled tube in a cryostat is to pull the coil so that
it spirals up the length of the cold stations. Alternately, the tubing
may be straightened out.

The membrane tube leads to the PCB. At some point it will pass through
the cryostat shroud or top plate, the details depending upon the
specific cryostat used. A Swagelok union is supplied. Outside the
cryostat, the exiting tube is joined to a Swagelok T-piece. This T-piece
carries (1) a valved quick-connect coupling; (2) a safety relief valve,
see Figure 2.

![](media/image2.png){width="3.951388888888889in"
height="5.1930555555555555in"}

**Figure 2.** Schematic of cryostat connections for one-tube
gas-membranes.

The safety valve is to protect the membrane in the event that the
cryostat warms up accidentally. Since He gas at 200 bar pressure (the
maximum this system is designed to use) undergoes a gas-solid phase
change, great caution should be used if operating below 10 K. From 12 to
6 K the volume of gas in the membrane doubles. The safety valve is
adjustable for the range 3000 to 4000 psi (200 to 280 bar). It has been
pre-set by unscrewing the cap on the valve to the point at which gas
will almost certainly leak out once the system is pressurised. Tighten
it gradually until the desired range of pressure operation is obtained.

> Great caution should be used if operating below 10 K

**2. Evacuating the membrane.**

If the membrane is to be used at low temperatures, it must first be
prepared by evacuation and filling with helium gas, to prevent ice and
other contaminants from blocking it. The following procedure is
suggested.

1\. Connect the PCB to your He cylinder using the black flexible tube
supplied. Open the bleed valve and the shut-off valve and flush He
through the PCB. Then turn off the gas supply and close the shut-off
valve. The system is now filled with He as far as the shut-off valve.

2\. Now assemble the components of the membrane supply system: the PCB,
the meter-long extension tube, and the gas-membrane itself. The
green-handled shut-off valve may be assembled either next to the
cryostat or on the extension tube, using the above Swagelok fitting
instructions.

3\. On the front left-hand side of the PCB is a ¼\" capillary tube for
which the gas flow is controlled by the bleed valve. Fix this tube to
your vacuum system. Open the bleed valve but keep the shut-off valve
closed. Evacuate the entire assembly.

It is recommended that the membrane assembly and its capillary tubing be
heated during evacuation (e.g. with a heating tape, or a flame) to
desorb water vapour and other contaminants.

4\. Close the bleed valve (i.e. isolate the PCB from the vacuum system).
Open the shut-off valve and fill the membrane with He to 1 atmosphere
pressure, using further He from the cylinder if necessary.

5\. You may wish to repeat steps 2 to 4 one or more times to ensure
freedom from contamination.

6\. Disconnect the vacuum system from the tube at the front of the PCB.

**Note:** Each time the quick-connect couplings are used, a very small
amount of air enters the system. Therefore, for best performance at very
low tempercatures, re-evacuation of the system may prove necessary from
time to time.

**C: Use of gas-driven membranes.**

1\. Gas-pressurised membranes must be operated with great care. The
maximum deformation of a membrane along its axis of rotation depends
upon several factors, which include its width, overall diameter and
thickness, and its weld and elastic characteristics. For the membranes
supplied with DXR-GMW cells ***a maximum deformation of 0.40 mm is
provisionally considered to be safe***. Operating procedures should take
account of this.

2\. ***In an ideal experimental situation*** in which the anvils do not
indent their supports, and the gasket itself has been correctly
pre-indented, as the membrane drives the piston all the load is
transferred to the gasket/sample, resulting in an increase of pressure
on the sample. In such a situation, the membrane should perform safely
and reliably to very high loads, because the elastic deformations of the
cell components during the run remain well within the maximum safe
deformation limit of the membrane.

3\. ***In X-ray cells*** in which the anvils are mounted upon beryllium
supports, indentation of the beryllium takes place progressively with
increased loads. Cells which have been above 50 GPa generally suffer
indentation of 150 to 200 µm per plate. Thus, a total indentation of
0.40 mm, the safe limit of the membrane, can arise from this cause
alone.

4\. Possibly the greatest risk to the membrane is presented by ***the
possibility of an anvil failure***. This can result in a sudden
downwards movement of the piston, with the consequent possibility of
inversion of the membrane.

5\. It follows from these considerations that high pressures should be
reached in stages in membrane-driven cells, and that ***at all times
great care should be taken to limit the possible deformation of the
membrane***. The following recommendations are designed as a guide to
safe membrane use.

6\. As gas pressure is applied to the membrane it should be plotted
against the pressure at the sample, as estimated by the ruby R-line or
other method. Any significant departure from non-linearity should be
investigated immediately.

**Recommendations.**

1\. At the start of each run ensure that the retaining ring is ***very
tightly*** screwed down: use of a spanner is recommended.

2\. Then insert the four M3 left-hand/right-hand extraction screws into
the body until they are in good contact with the bottom of the piston.
The pitch of these screws is 0.50 mm. Release them by half a turn, that
is by 0.25 mm. The downwards movement of the piston is now limited by
these screws to 0.25 mm.

3\. Additionally, one of the four screws may be replaced by a micrometer
probe or transducer in contact with the bottom of the piston, with
digital readout (direct or via a computer), so that a continuous record
of piston movement is available. ***It is strongly recommended that this
be done.***

4\. With increase of gas pressure the membrane will eventually move the
piston until it is in contact with the four M3 extraction screws. At
this point, clamp the piston in place using the four M3 (chamfered cap
head) screws supplied with the cell. Now reduce the pressure in the
membrane to ambient. Re-tighten the clamping ring. Then unscrew the
extraction screws by half a turn to give a further 0.25 mm freedom. In
this way, high pressures may be reached in stages.

5\. As gas pressure is applied to the membrane it should be plotted
against the pressure at the sample, as estimated by the ruby R-line or
other method. Any significant departure from non-linearity should be
investigated immediately.

6\. ***Indenting gasket blanks.***

If the initial thickness of the gasket blank is greater than 0.20 mm,
indentation should be done in stages using the above procedure.

**D: The cell assembly.**

1\. Handling and alignment of the cell are outlined in the accompanying
instructions.

2\. Load the sample and insert the piston into the cell body.

3\. Place the membrane assembly on top of the cell, taking care that the
small pegs on the side of the membrane assembly engage in one set of the
grooves at the top of the cell. This prevents the membrane assembly from
turning as it is being screwed down.

4\. The large screw ring should already be threaded onto the capillary
tube of the membrane assembly: it should now be screwed down
***tightly*** to hold the cell components together.

5\. Proceed with the protocol of ***Recommendations*** as above.

> **The cell must be disassembled ONLY whilst**
>
> **at atmospheric pressure.**
>
> **Gas pressure must be applied to the membrane assembly ONLY**
>
> **when it is correctly assembled as part of the cell.**

**E: Pressurising the cell.**

\[NB. The G-01 Pressure Control Box requires ½ hour warm-up time.\]

1\. The operation starts with the shut-off valve of the PCB open, and
the bleed valve partly open. The shut-off valve (green handle) next to
the cell must be open.

2\. Slowly adjust the regulator on the cylinder to bring the pressure on
the digital display to the desired value. For fine adjustment, the bleed
valve may be closed slightly to increase the pressure, or opened
slightly to reduce it.

3\. Allow at least 5 minutes for pressure in the cell to stabilise: it
takes time for gas to flow through the long capillary tube.

4\. The shut-off valve next to the cell may now be closed to maintain
the cell at the chosen pressure. ***Note this pressure as it will be
necessary to know it when re-pressurising the device.***

5\. With the cell valve closed, the rest of the system may be returned
to atmospheric pressure. This may be done by turning down the regulator
on the gas cylinder and using the bleed valve to exhaust the system
until atmospheric pressure is indicated on both regulator and digital
display.

6\. The cell, still under pressure, may be disconnected from the rest of
the system (now at atmospheric pressure) by carefully pressing on the
quick-connect sleeve while holding the stem.

> **No attempt should be made to disconnect**
>
> **(or even to turn) the quick-connect fitting while the**
>
> **indicated pressure is above atmospheric.**

7\. The cell, with pressure maintained by the cell valve, may now be
used while separated from the PCB.

**F: Re pressurising the cell.**

1\. To change the cell pressure, the cell must be re-connected to the
PCB by the quick-connect coupling. The pressure in the whole system must
then be brought as close as possible to the pressure at which the cell
valve was closed \[see section 16\]. The cell valve may now be opened as
both cell and PCB should be at the same pressure.

2\. The pressure in the whole system may now be changed.

> **No attempt must be made to adjust the cell valve while there is a
> significant pressure differential between the cell and the**
>
> **rest of the system. This valve may be opened or closed**
>
> **ONLY while the cell and the digital display**
>
> **are at essentially equal pressure values.**
>
> **G: Alternative operation mode 1.**

1\. If the cell is connected to the PCB by a long capillary, the cell
may be used with the cell valve open and pressure maintained
continuously through the PCB. The long capillary has sufficient
flexibility to allow the cell to be mounted on other equipment. This
method is recommended if accurately-monitored pressure is required.

**H: Alternative operation mode 2.**

1\. Four M3 x 20 screws are supplied so that, once pressure has been
applied by the membrane, they may be used to clamp the piston in place.
Experience will show by how much these screws must be tightened to
maintain the pneumatically-applied pressure. These screws may be used:
(a) as backup to pneumatic pressure maintained by the valve (sections
16-18); or (b) instead of pneumatic pressure maintenance.

**Supplementary information.**

1\. Digital display unit: see manufacturers instructions.