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 ). All gas connections are made using POLYFLO^TM tubing and TJNAF-specified connectors. Gas enters the chamber assembly through a bubbler containing (edible) mineral oil. Each chamber has a volume of approximately 30 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 /hr. Gas is exhausted from the VDC package through a second bubbler also containing mineral oil. A typical chamber leakage rate measured against this load is 3 /hr. 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 10 hours. At this rate of consumption, a full gas bottle connected to a single VDC detector package lasts approximately 10 days. 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.

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.