Negative Gas Pressure

[rogerthebodger]

A manometer measures the difference in pressure between 2 points.

In your case it measures the pressure between the pressure at the test point and the atmospheric pressure so is the atmospheric pressure changes the reading will also change.

Add to this any temperature changes will affect the reading.

I am very aware of changes in atmospheric pressure as living at 2000 metres above sea level when we travel to sea level any packets we take to our boat get squshed.

 

[davebrent]

Hello again, we have just tested with water manometer, a monument 30mb, and have observed a 13.5 mb fall in 30 minutes where our weather station reflects an outside temp fall @ 19:00 today from 20.4 C to 19.3 outside and 24.5 C to 23.5 inside, please see attached photos of our test setup and water manometer fall. We are leaving the electronic manometer overnight again. We have just seen and note the water manometer has Rothenberger natural gas hose , not LPG rated, whereas the digital manometer has the corrent Rothenberger Neoprene hose which is LPG rated, not sure how big a difference that will make but will replace the water manometer hose and re-test tomorrow, kness are aching a bit now from climbing in and out of boat. Thanks again to all for sticking with this puzzle as we are now nervous of out boat safet inspection as it seems we are needing to arrange a visit as temperatures are rising

 

[greeny]

Digital manometer. Its a bloody digital pressure gauge. No wonder there are inconsistent variations over time.
Water manometer. SG 1.0. Can't be wrong. Why do we use these modern bright shiny things.

 

[Birdseye]

There is no such thing as negative pressure. What your gadget is indicating is a pressure below ambient. However simply because you gas is turned off and the lpg system is therefore sealed whilst ambient pressure varies throughout the day and night, there will always be a pressure difference. And then you can add in the issues mentioned in posts above. To my mind the worry would be if the gadget didnt show a pressure difference overnight since that would show a leak either because lpg leaked out or air leaked in..

 

[davebrent]

Digital manometer. Its a bloody digital pressure gauge. No wonder there are inconsistent variations over time.
Water manometer. SG 1.0. Can't be wrong. Why do we use these modern bright shiny things.

Thank you for your reply, sorry we do not understand the difference, we have switched to digital manometer over night as we are concerned the negative pressure may result in water being sucked into the installation and would rather avoid additional issues, thanks again

 

[davebrent]

There is no such thing as negative pressure. What your gadget is indicating is a pressure below ambient. However simply because you gas is turned off, and ambient pressure varies throughout the day and night, there will always be a pressure difference. And then you can add in the issues mentioned in posts above. To my mind the worry would be if the gadget didnt show a pressure difference overnight since that would show a leak either because lpg leaked out or air leaked in..

Thank you for your reply, sorry we struggle to understand our observation, perhaps we should simply refer to an unexpected fall, however the scale of fall is a little alarming, will try an find another boater or campervan owner in a postion to perform a similar test, it was not our intention to leave the device overnight, we have just been suprised with next day readings, tahnks agin

 

[JumbleDuck]

the manometer is intialised to zero by pressing the hold buttton for 5-10 seconds whilst not attached

to confirm, the manometer is left connected to the test point after having run the gas hob with the manometer reading 30mb with flame,

we then close the bottle tap, the flame goes out and the manometer reads zero, we then leave the manometer attached to the test point

Thanks. I think your next test should definitely be do to do all the same things with the manometer but not connect it to the test point.

 

[JumbleDuck]

There is no such thing as negative pressure.

There is when it's gauge pressure, which is a perfectly valid sort of pressure.

There used to be a garage in Milton Keynes with a big sign outside saying "Have You Tried Our High Pressure Vacuum?"

 

[davebrent]

Thanks. I think your next test should definitely be do to do all the same things with the manometer but not connect it to the test point.

Thanks JumbleDuck, hope I have understood test recommendation and have performed a few additional tests, boat is a trailer boat on driveway so easy to perform tests if my knees can take climbing in and out

the gauge was reading negative 41 at 22:00 since 19:00, sorry did not take photo

please see atached photos reflecting before and after weather station readings

manometer reading after 10 minute test with pipe not connected, no movement

manometer reading after10 minute test having reconnected manometer to test point, test point was left open for first test, fall of 3 mb

manometer reading after 10 minute test having represurised then burn off to zero mb, fall of 6mb

the maonmeter was reset before each test

manometer has been left connected and will upload reading tomorrow morning

Thanks again for all your help with this

 

[davebrent]

Another thought

Acceptaing the installation volume has no bearing on pressure, is it correct a smaller installation will have a greater surface area to volume ratio and therefore may cool or warm faster, our installation being a smaller installation may suffer the fall ( or rise ) effect at a greater or faster timeframe, just a thought

 

[davebrent]

Good Morning, we hope all is well, have taken reading this morning and we are reading a fall of negative 91, we note the outside temp has risen from last night, however the inside temp has fallen, thank you in advance for your thoughts

 

[JumbleDuck]

Thanks. It looks as if you have a real effect.

It's very odd. I wondered if starting with the stove running meant that the pipe would be cold with recently vaporised gas so that its subsequent expansion reduced the pressure, but a back of an envelope calculation puts this orders of magnitude from what you are seeing.

So, given that the volume of the pipework is constant, the only possibilities are that ...

1. the gas is getting colder or
2. the quantity of gas is reducing or
3. the atmospheric pressure is rising

Between you an VicS I think (1) and (3) are ruled out. Does hose absorb propane, I wonder?

 

[rogerthebodger]

Does hose absorb propane, I wonder?

I did read that the reason flexible LPG pipe must be replaced is the plasticizer being leached out of the hose due to the LPG.

If that would cause this effect I don't know.

 

[davebrent]

Hello, thank you for your comments, for info the lpg hose to gas bottle is new with manufacture date March 2019, this was replaced in preparation for our boat safety examination, have been busy applying bottom paint today, will climb on board a little later and take another reading as manometer was left connected and will post result, we are starting to think we must be experiencing common behaviour of butane, just rarely observed, we are happy the installation is holding the fall in pressure, we have just been so suprised at the scale of the fall with concern for any adverse affect on the instalation or appliances, but all seems well, there is always lots of other stuff to worry about, thanks again.

 

[davebrent]

Hello again, have just taken reading, seems pressure has recovered a little with increase in temp, please see attached, will probably monitor again as we head into autumn and winter, thanks again for all