Gas Flow Operating Procedures

The chamber gas is mixed 63-37 (by weight) Ar ethane. The major maintenance required bu experimenters consists of monitoring the gas supply system, and changing out empty bottles. The gas system manual should be checked for details. Bottles containing this gas may be found stored in the racks next to the Hall A access ramp, and should be returned and clearly marked when empty.

The four FPP straw chambers are connected in parallel to the gas system. (see Figure ). (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 .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 /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 /hr when combined with the leak rate of 3 /hr results in a complete exchange of gas in the chambers roughly every 1 - 2 days. At this level of consumption, a full gas bottles connected to the FPP system last approximately 10 days. When a bottle is nearing empty ( 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.

Check both the amount and the pressure of the gas in the cylinder to be used. Replace the cylinder if necessary and you are authorized to do so.

2.

Visually ensure all the gas lines have been connected to the FPP detector packages correctly.

3.

Gas pressure at th egas panel on the spectrometer should be in the range 10 - 15 psi. If not, it is likely that either there is a problem with the gas input or that the chambers are set to draw too high a gas flow.

4.

Enable the gas flow by opening the rotameters. Set the flow rate to about 7 /hr (this corresponds to about 20 units on the scale) to ensure one complete exchange of chamber gas every 20 - 40 hours.

5.

If the chambers have been continuously maintained under gas, no purging upon startup should be necessary. In less than 30 minutes, the mineral oil output bubbler in the gas panel should indicate smooth, continuous flow. Otherwise, gas flow must be increased to purge rate for one day, before decreasing to normal rates, before high voltage can be turned on.

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. 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.