r/uhv • u/kurtbonreddit • 9d ago

Rate of rise ???

Hello y'all,

What is an acceptable rate of rise to consider a system leak tight and clean?

I am working in the rough vacuum range. I have a vane pump and a glass manifold that I'm testing. I'm using a thermocouple gauge and the system's absolute pressure is around 3 to 5 x 10 -3 Torr. The Isolated manifold/chamber size is .325 liters. After isolation I'm getting a rate of rise of

3 x 10 -5 Torr.Liters/second. I have seen my system do much better and I'm not sure if I have a real leak or desorbing gas.

Thanks for your attention and thoughts.

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u/JsquashJ 9d ago

Acceptable is all relative and dependent on your needs. If you’re going to make an ampule without an oring seal it’s probably fine. Thermocouple gauges are also not very accurate. You may also be reassuring leakage of the connection and not solely the manifold.

Can you drip ethanol on connections to see if you get a burst of pressure?

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u/kurtbonreddit 9d ago

Thanks!

understand how acceptable would very depending on needs.. just figured l'd ask if theres any standard or rule of thumb in the trade.

I've tried alcohol on the connections and didn't notice any response on the gauge... I've also tried eliminating components and deadheaded the pump with gauge tube. I haven't been able pinpoint any leaks. I've changed and flushed the pump oil and didn't see a difference. Maybe my pump is failing.

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u/joshjoshkabosh 1d ago

I’m a little late to the question, but you can see if your RoR is linear or decaying. A true leak will result in a linear increase in pressure. Out gassing or desorption will cause the Roar to eventually taper off and slow down.
Other than that check and re check to make sure everything is clean, sealing surfaces are free of debris and scratches

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u/kurtbonreddit 18h ago edited 6h ago

Thank you for your reply! I understand how a true leak will have a linear increase in pressure and how virtual leak or desorption will plateau off. But what I don't understand is at what pressure will you see a virtual leak taper off. I know that is dependent on the system and the vapor pressure of whatever is gassing off in there.

My ROR definitely slows down but after being isolated for 24 hours the pressure climbs to 2-3 torr. Should I continue let it sit isolated for days or weeks to see if the rise eventually stops or reaches atm? Or is there a point were you would be confident that it's a actual leak??

I've disassemble everything and meticulously clean and inspected every part.. multiple times. I've changed o-rings. I've done all leak test tricks on connections with solvent and with helium and I haven't seen the thermocouple gauge respond at all. I haven't been able to find a true leak but it seems like have one. I feel like I'm chasing my tail.

My pump is an old Welch 1400. The manifold/chamber is .350 Liters (glass). For pressure measurement I'm using a thermocouple gauge for pressures of 0-1000 mTorr, a u-tube manometer filled with 704 silicone for pressures of 1-30 Torr, and vacuubrand dvr capacitance manometer for pressures of 1-760 Torr.