Why Closing Manometer Port Causes Pressure Increase?

Maybe you just found a tny leak.

No leaks. Try it yourself - close the "+" port tightly and you will see: 1.1, 1.2, 1.3 ..10...20 mB and so on steady increase.

Sorry but unless this manifold was helium leak checked to UHV conditions I don't believe you that no leaks exist. At the same time the actual leak could easily be orders of magnitude less than this mysterious effect.

I think that I will take you up on the offer to try it myself. Who do I charge for my transportation and lodging expenses to get to your facility? Being a CR4 member I'll give you a big discount for my billable hours.

Maybe a drifty sensor? Are you plugging one port or both since it is a differential. If blocking only one port you may be seeing barometric pressure changing.

Closing a port on a differential manometer makes it an absolute manometer, so you're seeing the pressure change on the open port. With both ports blocked you're probably seeing "sensor drift", but it could be that one port is a little less airtight than the other.

I'm pretty much a total amateur in this area but given what you've said my guess is that the results you are getting definitely sound like a leak. - You must remember that once closed the instrument is a very small system so the effects of any leak will be massively amplified. Connect it back to the main system and the leak effectively disappears. Such a small leak should not interfere significantly with performance of the overall system unless the system is very small.

From the figures you've given I'm guessing that your system is intended for medium vacuum.

To answer in more detail my first question would be- exactly how does the particular type of manometer actually work? My guess is that being differential and with what you've said that the instrument either works by measuring the flow rate through a calibrated channel or uses a common bellows between two sealed chambers. (However it could be using some other method - like two isolated calibrated sensors with an electronic comparator.)

You seem to imply that the instrument is heated, is that to give a calibrated fixed temperature or is it part of the mechanism that measures the pressure? like a Pirani gauge. (just looked that up )- https://en.wikipedia.org/wiki/Pressure_measurement#Thermal_conductivity

Further note if it is a Pirani type gauge then I would guess you should be getting a pressure rise on a closed chamber without a leak. A gas when heated expands and in a closed space that means a rise in pressure. However it should also produce an opposing effect producing an apparent pressure drop due to heating reducing the temperature differential between the gas and the heating element. Afraid you probably need a real expert to answer that one...

Try hooking the port to an inclined manometer(Such as those made by Dwyer or others),which can read in very low ranges of pressure, to the opened port under question.If the pressure is really increasing, you will know by observing the liquid manometer.If not, you have a problem internal to your manometer.Make sure your port is open.The liquid will act as a plug,same as valving it closed.The advantage is you get to see what is Really happening inside your instrument in question.

Probably leak and/or drift, indeed, thank you !

I was hoping for an effect of an absolute pressure manometer, but have proven to myself the idea was hopeless.

What is the weather doing in the vicinity of this blocked manometer? And what does <...gradual changing...> mean? +/-50mBar is the differnce between a pleasant sunny day and the most atrocious storm.

No no, in 2-3 minutes pressure rises to 20-30 mB. The more or less the same at two manometers : Testo 510 and a more cheaper Pyle.

you have about a 10mB a minute leak in that volume you are measuring. IF we knew the volume we could tell you the size of the leak.

I don't think you need to worry too much Yuri B. Like I said before the volume inside the instruments will be tiny and so the leak(s) must (should) be very small. It actually occurs to me that the most likely place for the leaks is in the valves that close off the chamber themselves.- In fact its quite probable that most such valves leak a little bit and that they are probably still within specifications. A look in the instruments manuals might be of help.