Gas Flow Operating Procedures

Chamber gas is delivered to a given VDC detector package via HAWGS, the Hall A Wire-chamber Gas System. Complete details of this system are presented elsewhere in this manual.

Each VDC detector package consists of two VDCs connected in parallel (see Figure 5.3). All gas connections are made using Polyflo$\rm ^{TM}$ tubing and Jefferson Lab specified connectors. Gas enters the chamber assembly after bypassing an overpressure bubbler containing 15 mm of (edible) mineral oil. Gas is exhausted from the VDC package through a second bubbler containing 5 mm of mineral oil. Each chamber has a volume of approximately 30 $\ell$ and is operated slightly above atmospheric pressure. Standard flow rate set points are clearly labeled next to the control panel flow meters. The gas flow through the chambers may be independently varied and is typically set to 7 $\ell$/hr. A typical chamber leakage rate measured against the 5-mm mineral oil load is $\le$ 3 $\ell$/hr. The flow rate of 7 $\ell$/hr when combined with the leak rate of $\le$ 3 $\ell$/hr ensures a complete exchange of gas in the chambers roughly every 8 hours. When a bottle is nearly empty (say 90%), it should be changed since the quality of the gas at the bottom tends to be low. Gas bottles may only be changed by authorized personnel.

Figure 5.3: Gas flow schematic

1.

The status of the gas handling system should be monitored carefully as well as logged every shift. Any substantial deviation from the median parameters can result in a change in the operational parameters of the VDCs 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. Further, it is critical that gas flow has been maintained for 24 hours prior to any power up.