The equipment relies on valves to channel flow and, for accuracy and repeatability, this internal hardware must itself be leakproof.
One major instrument manufacturer found that standard pneumatic valves are themselves too leaky, producing erratic test results. They turned to Humphrey Products, Kalamazoo, Mich., for a custom valve manifold with virtually zero leakage.
Company engineers started with a standard, poppet-type cartridge valve with 1/8-in. orifice, flow rating of 25 scfm ar 100 psig, and maximum pressure rating of 215 psig. They modified it to hold O-ring poppet securely under high pressure. A new stem effectively traps the Buna O-ring, keeping it from blowing out or distorting under high test pressure.
A Humphrey Mizer valve serves as the pilot operator. It consumes only 0.5 W of power, resulting in little heat dissipation, and that ensures the system will not affect instrumentation accuracy.
Valve manifolds were tested for 200,000 cycles and periodically checked for leaks. In each case, pressure drop was less then 0.002 psi over 30 sec at 100 psi - equivalent to 0.0025-scc/min flow - 1/20th of the leak rate permitted by the customer's specifications. The unit is now in full production.
O-ring seals in the valve poppet give extremely low leakage.