Hi I wonder something, I have an valve working with helium gas but ı can not test with helium right now. When ı change the fluid by nitrogen. How can I verify actual the pressure drop if I do my tests by helium gas.
Explain to us WHAT you are proof-pressure testing with Helium... and what kind of pressure-loss sensor you are using...
Your question is a bit 'off'... perhaps You mean: "if you are supposed to pressure test with helium... and all of the pressure test 'plumbing/valves' are designed for Helium... then is it OK to pressure test with nitrogen... or NOT"?
Are you testing for pressure drop over time or pressure drop from flow? Pressure drop over time indicates leakage which helium is commonly used to test for. Is this your case? If so, nitrogen is not a substitute.
Please clarify what the working gas is supposed to be, nitrogen or helium?
If helium, then nitrogen as a test gas is completely inappropriate, since N2 is a substantially bigger molecule than helium. Moreover, helium is an industry standard leak testing gas, and there are instruments designed specifically for detecting helium leaks.
Sounded to me like the OP wants to measure a flowing pressure drop (total pressure loss) across an open or partially open valve, i.e. a Cv curve. But, as Wil said, you need to be more specific about what testing you are contemplating.
It is still tricky to measure with nitrogen and then translate the flow data to helium, given the big difference in JT coefficient. But for low expansion, low delta-P you might have some success, just comparing volumetric flow rates.
Pressure drop testing as a leakage test is more problematic, as others have noted.
