Reddit reviews yaufey Single-Stage Rotary Vane Vacuum Pump 3CFM,1/4HP,220ml,5pa for HVAC/Auto AC Refrigerant Recharging,Wine Degassing(R134,R12,R22,R502),Medical with Manifold Gauge Set
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High Efficiency and Low Noise， Heavy-duty Aluminum Alloy CasingOil Drain Plug Positioned at Bottom of Oil ReservoirLightweight, Durable Construction for Easy Handling & Long Life UseFast Design Removes Moisture & Non-Condensable Gases that can Destroy any A/C Refrigeration SystemExhaust Port with Exhaust Cap, Direct Drive Motor Allowing Easy Starting Which is Maintenance Free
So, I get that something can be water tight but not air tight. In this case, I've detached the hose from the pump, and immersed the manifold (from this: https://www.amazon.com/gp/product/B01H72W8Z8), which contains the valve, along with the vacuum chamber. It's totally detached and not connected to the pump at all.
I will try pumping down and keeping the valve closed. IIRC the last time I did that, the pump doesn't have any effect on the chamber at that point.
I am using Buna-N (1/8") to seal the endcap. It's liberally smeared with vacuum grease. The end cap stays on the chamber purely by vacuum adherence- I don't have any screws keeping the endcap on tightly. Do you really think that makes a big difference?
The link to Terra Universal is interesting, because they show a similar vacuum leak that settles to level. They solve the problem by re-pumping once, then it stays at the final vacuum level achieved with the pump.
I take your point about the battery or arduino being an outgas source. I am trying to eliminate that but my previous attempts have failed. I tried various MEMS/chip based solutions but their ranges have not been as good as my BMP183.
I tried using a bulkhead union as a vacuum feedthrough but had trouble sealing it.
For analog gauges, do you mean like this: