Gas Flow Operating Procedures

The chamber gas is mixed 63$\%$-37$\%$ (by weight) Ar ethane. The gas is mixed in the Hall A gas shed which is located next to the entrance to the Hall A truck ramp. One needs key #8, which is located in a key box in the Hall A counting house, to get inside the shed where the gas mixing is done. The argon and ethane bottles which feed the gas mixing system are located outside the shed and can be exchanged when they are empty. The mixed gas is sent down into Hall A and to each of the detector huts. There are two each of argon and ethane bottles connected to the gas system and a Matheson 8590 controller switches between the two bottles when the gas pressure in the bottle drops below a certain level. At this point the one bottle can be replaced while the other is being used. The procedure for changing gas bottles is outlined below:

1. Warning: High pressure gas bottles contain significant stored energy and are potentially hazardous. Handling of gas bottles should be done only by qualified, trained personnel.

2. For smoothest operation, used gas bottles should be replaced before their internal pressure drops below the desired regulator output pressure.

3. Two possible cases exist in which a gas bottle needs to be replaced: only one empty gas bottle on a system or both bottles empty on a gas system.

4. For case 1 the sequence of steps is as follows:
   1. Check in the Hall A Gas Shed. If all bottles have sufficient pressure each of the Matheson 8590 controllers will have one green "RUN" LED lit and one yellow "READY" LED lit. A red "EMPTY" LED lit indicates a bottle with low pressure, the corresponding bottle needs to be replaced. If a red "EMPTY" LED is lit the central "ALARM" LED should also show red. Nothing further needs to be done here; go outside to the Gas Bottle Pad.

   2. Visually verify that the corresponding pressure gauge on the flex line is showing a low pressure. A low pressure is not necessarily zero. Close the bottle valve for the empty bottle.

   3. Disconnect the empty bottle from the high-pressure flex-line. The in-line check-valves will prevent gas escaping from the manifold. Replace the bottle's cap, and move the empty bottle to the EMPTIES storage rack. Note that ethane bottle fittings, type CGA-350, have left-handed threads.

   4. Place a full bottle of gas in the on-line rack, remove the bottle cap, and connect the bottle to the flex-line.

   5. Open the new bottle's valve, check for leaks at the bottle fitting. The corresponding pressure gauge should now read full bottle pressure.

   6. The ALARM state of the Matheson 8590 controller should have automatically reset. Check inside the Hall A Gas Shed. Each controller should show a green "RUN" and yellow "READY" LED lit. If not, re-check the installation of the gas bottle.

5. For case 2 the sequence of steps is as follows:
   1. Check in the Hall A Gas Shed. If all bottles have sufficient pressure each of the Matheson 8590 controllers will have one green "RUN" LED lit and one yellow "READY" LED lit. If a Matheson 8590 controller shows two red "EMPTY" LEDs lit and the central red "ALARM" LED lit, both bottles of the corresponding manifold need to be replaced. Nothing further needs to be done here, go outside to the Gas Bottle Pad.

   2. Follow steps 2. through 5., as detailed immediately above, for both bottles.

   3. The ALARM state of the Matheson 8590 controller should have automatically reset. Check inside the Hall A Gas Shed. Each controller should show two yellow "READY" LEDs lit. If not, re-check the installation of the gas bottle. Press either of the two buttons labeled "LEFT BANK" and "RIGHT BANK". The lit LED above the button you pressed will change from yellow "READY" to green "RUN". You will most likely need to reset the Low Supply Pressure shutdown at this point.

The four FPP straw chambers are connected in parallel to the gas system. (see Figure 5.22).

Figure 5.22: Drawing of the gas panel on the hadron detector stack.

1.

(The FPP chambers are also in parallel with the VDC chambers.) All gas connections are made using POLYFLO$^{TM}$ tubing and TJNAF-specified connectors. The chamber volumes range from approximately 120 to 220 $\ell$. Gas pressure in the chambers is typically a few Torr above atmospheric pressure. The gas flow through the chambers may be independently varied and is typically set to 7 $\ell$/hr, leading to a replacement of the chamber volumes about every 15 - 30 hours. Gas is exhausted from the FPP chambers through a bubbler containing $<$ 1 mm of mineral oil. A typical chamber leakage rate at this flow rate is 25 - 50 $\%$. The flow rate of 7 $\ell$/hr when combined with the leak rate of $\le$ 3 $\ell$/hr results in a complete exchange of gas in the chambers roughly every 1 - 2 days. At this level of consumption, a full gas bottle connected to the FPP system lasts approximately 10 days. When a bottle is nearing empty ($\approx$ 90$\%$), it should be changed since there may be heavy contaminants in the gas. Gas bottles may only be changed by authorized personnel.

Gas-handling Procedures

1. Typically gas is continually flowing though the chambers. If at all possible, gas flow should be continuously maintained, even in no-beam time periods. This avoids time loss to reconditioning and maintains the desirable steady-state operating condition. If the chambers are not being used in an experiment, the flowmeters for the front chambers are set to 20 and the flowmeters for the rear chambers are set to 60. When the chambers are used in an experiment the standard setting for the front chambers is 40 and for the rear chambers it is 105.
2. Gas pressure at the gas panel on the detector stack should be in the range 13 - 15 psi. With the large leakage rate of the FPP chambers, we typically run at near the limit of the capacity of the gas mixer to supply the gas flow demanded by the FPP and VDC chambers. Therefore it is possible to demand too much flow rate from the mixer. If the gas pressure drops below 13 psi drop the flow to the FPP chambers and contact Jack Segal or Howard Fenker to determine the cause and remedy for the situation.

The status of the gas handling system should be monitored carefully as well as logged at least once per 8-hour shift. Any substantial deviation from the median parameters indicates a change in the operational parameters of the FPP and should be immediately investigated.