Gases used in connection with certain laboratory testing procedures must be essentially free from impurities and pollutants which would otherwise undermine the usefulness or accuracy of the test.  For these applications, previously manufactured gases may have to be filtered and cleansed by the on-site user, before introduction into a test instrument. 
For example, in the field of gas chromatography, manufactured carrier gas must be subjected to further filtration at the laboratory site, before it can be utilized in a test instrument.  For that purpose, filters have been developed, for in-series installation between the gas storage tanks and the chromatograph. 
Most laboratories locate their compressed gas tanks or cylinders in a centralized storage area.  Gas is distributed from the cylinders to a plurality of instruments in various locations, by means of copper or stainless steel tubing.  Traditionally, gas purifiers or filters were installed at the storage area, adjacent the gas cylinders.  However, this approach requires that the operator leave his instrument unattended, each time the filters are periodically changed or checked for condition. 
In response to this need, point of operation cartridge panels were developed.  These panels are designed for convenient installation, adjacent the chromatography instrument.  Point of operation cartridge panels have also been used in connection with other gas utilization devices, which require gas having special purity requirements.  These panels enable the operator of the instrument or appliance to monitor gauges and check the condition of indicating filters, changing the filters as necessary. 
Each panel typically has a plurality of cartridge type filters, interconnected in series fashion between the incoming line and the instrument.  Each filter is especially designed to remove a particular contaminant, such as oxygen, moisture, or hydrocarbons. 
The cartridges are secured within cartridge mounting heads, having a coupling mechanism for detachably locking the cartridge in place.  Each mounting head also includes internal passageways, which route incoming gas through the filter and then pass the cleansed gas on to another filter or to the gas utilization apparatus. 
To replace a cartridge in a prior art panel, a cartridge retaining nut is first unscrewed from the head.  Then, the spent cartridge is removed, exposing the downstream side of an inlet check valve and the upstream side of an outlet check valve to atmospheric pressure.  This relative drop in pressure actuates each valve, respectively, into a closed position, isolating the cartridge mounting head from upstream and downstream components.  Gas flow through the head and the entire panel is therefore interrupted while the cartridge is being replaced.  This necessarily causes down time in the operation of the instrument, and may corrupt gas in the line if the check valves develop leaks.