Negative Gas Pressure

D

DaveBrent

Free Member
Joined
Aug 23, 2020
Posts
17
Likes collected
9
Funster No
74,852
MH
Boat Owner
Exp
Since 2011
Hello fellow travellers, we hope all is well.

We have made an unexpected observation whilst preparing our small motor boat for her boat safety inspection.

Background, we have purchased an electronic manometer in order to test gas tightness and are pleased all appears well as reflected by our bubble tester.
We decided on a manometer tester as we have seen youtube videos where small leaks may pass bubble testing and we currently have no gauge but are considering adding as a backup.

The butane gas was turned off and the gas was purged at the hob following testing, the manometer was left connected to the test point overnight reflecting zero mb pressure.

To our suprise the following morning the manometer was reading negative 30mb and negative 75mb by mid afternoon.

The negative was confirmed at the hob by holding a flame against the burner where the sound of gas/air movement was heard but the flame was not blown, seems air was being drawn into the burner.

This was firther confirmed the following day with a manometer reading of negative 60mb. This time we turned on the gas and watch the manometer move toward zero then to positive 30mb, we then lit the hob burner, turned the gas off at bottle and watched the manometer reduce to zero.

Our amazon purchased manometer has two terminals and is connected to the positive terminal and seems to be trusted by many gas engineers and has always returned to zero after testing.

We struggle to account for these observations and although pleased the negative pressure is being held, we are a little concerned with the size of the negative pressure and any adverse affect on our installation.

We ask if anyone is able to account for this behaviour or is in a position to apply a similar test or advise of anything we should be considering.

Thank you in advance
 
We would not normally leave a gauge connected, so I have never found this, however, the volume being tested is very small so a small difference in temperature can have an affect on the pressure. I know butain is water soluble and can dissolve into a water gauge, but I'm assuming you are testing with an electronic gauge
 
Thank you for your reply, yes, we are primarily using an electronic gauge, but have tested the effect with our small water manometer, please see attached.

The highest fall we have recorded was 101mb from 10pm to 10am next morning.

We are happy the installation is holding the negative pressure, but are concerned the pressure may hurt our appliances where we have a small gas stove, gas air heater, gas water heater.

We posted on a boationg forum and it has been suggested the butane gas is condensing, we are just suprised at the scale of the fall and worried about appliances.

We hope to find another in a position to perform a simialr test to see if they experience a similar result.

Thanks again
 

Attachments

  • neg 01.jpg
    neg 01.jpg
    169 KB · Views: 56
  • neg 02.jpg
    neg 02.jpg
    151 KB · Views: 53
  • t1.jpg
    t1.jpg
    63 KB · Views: 50
  • t4.jpg
    t4.jpg
    165 KB · Views: 51
  • t5.jpg
    t5.jpg
    55.2 KB · Views: 55
I think you are over thinking things , if it tests ok on a 5 minute test Which is standard for LPG (no more that 10 mins) then it will be fine
By leaving it on for hours You are testing (as gasman says) expansion & contraction due to temperature changes & such a small volume IT will change Bearing in mind things can cool down at night , possibly atmospheric pressure which changes all the time as well

Subscribers  do not see these advertisements

 
why are you testing for so long? sure its a 2 minute test? plus temprtature stabilization?
 
In the past it was a 5 minute test, then it was changed to a 2 minute test, inline with NG.
 
Thank you, we guess this has always been happening and has not caused issue yet.

It was suggested this effect would be less with Propane with a lower boiling point.

Our only concern was with the boat safety test itself as we have observed a fall over 6mb over a 10 minute test. For boat safety we understand the test is over 10 minutes with 5 minutes stabilise 5 minutes test, fingers crossed any fall is not seen as a leak.

However, it has also been suggested that the boat safety inspector may only use the bubble tester where the makers Alde quote a 10 second test and boat saftey scheme quote a 60 second test, we seem to pass these with no bubbles.

Thanks again for your time and consideration.
 
matt34 said:
why are you testing for so long? sure its a 2 minute test? plus temprtature stabilization?
Click to expand...
We did not leave the device connected to perform an over night test, was just the end of a long day and knees were aching from climbing in and out of our trailer boat on the drive. We are just reacting to having seen an unexpected fall, or rather the size of fall. Thanks
 
I have not worked on boats since I worked on the round the world boats before they moved to Portsmouth, so I am not up to date on the regulations, but if it has passed the required test, why are you worried?

Subscribers  do not see these advertisements

 
Scotch mist said:
I think you are over thinking things , if it tests ok on a 5 minute test Which is standard for LPG (no more that 10 mins) then it will be fine
By leaving it on for hours You are testing (as gasman says) expansion & contraction due to temperature changes & such a small volume IT will change Bearing in mind things can cool down at night , possibly atmospheric pressure which changes all the time as well
Click to expand...
You are probably right, our hope was to find another in a position to try the same for the comfort of a similar result, we do agree an unusual test, but this test was never our intention, the scale of fall has just been suprising. We read an air test after new or amended install is up to circa 120mb, the negative fall we are seeing is not far off that, guess our instllations are being tested all the time as we sleep, thanks agin for all
 
DaveBrent said:
We did not leave the device connected to perform an over night test, was just the end of a long day and knees were aching from climbing in and out of our trailer boat on the drive. We are just reacting to having seen an unexpected fall, or rather the size of fall. Thanks
Click to expand...
dont worry about it if it passed a 2 min test then fine (y)
 
When I used to Do a Pressure test On sewage pipes in domestic houses
with a water gauge connected , all it would take is for the sun to pop out from behind the clouds to blow the water out the gauge

Subscribers  do not see these advertisements

 
DaveBrent said:
You are probably right, our hope was to find another in a position to try the same for the comfort of a similar result, we do agree an unusual test, but this test was never our intention, the scale of fall has just been suprising. We read an air test after new or amended install is up to circa 120mb, the negative fall we are seeing is not far off that, guess our instllations are being tested all the time as we sleep, thanks agin for all
Click to expand...
Even 120mb is not a high pressure, it is just higher than working pressure, it won't harm the installation.
 
Thanks to all for your help and comments, we have just looked up 120mb as just over 1.7psi, probably less than a f@rt in the wind, probably time to focus on other things, was interesting for a while, thanks again
 
It looks like it's measuring the changes in pressure rather than the absolute pressure.
I wonder if it's the change compared to the original pressure or like a water gague the change compared to atmospheric pressure.
As others have said temperature will result in fairly big changes in pressure which may account for the reduction but if it is behaving like a water gague and comparing to atmospheric pressure any changes there will also have an effect.
Its an odd one where a longer measurement is less accurate or to be pendantic is accurate but not telling you what you want to know ( if theres a leak).
Presumably you test with the system pressurised otherwise there would never be any leaks I wonder how the regulator responds to any atmospheric changes?
 
mitzimad said:
im struggling to see how you get a negative reading ?
Click to expand...
We understand the reading as being a relative reading, not an absoulte reading, so the fall is simply relative to the start, thanks
 
The Wino said:
It looks like it's measuring the changes in pressure rather than the absolute pressure.
I wonder if it's the change compared to the original pressure or like a water gague the change compared to atmospheric pressure.
As others have said temperature will result in fairly big changes in pressure which may account for the reduction but if it is behaving like a water gague and comparing to atmospheric pressure any changes there will also have an effect.
Its an odd one where a longer measurement is less accurate or to be pendantic is accurate but not telling you what you want to know ( if theres a leak).
Presumably you test with the system pressurised otherwise there would never be any leaks I wonder how the regulator responds to any atmospheric changes?
Click to expand...
The electronic gauge seems to be having as the water gauge, as a comparison against atmospheric

The only worry we are left with is if this effect were to happen during our boat safety exam. We have seen late evening a fall of 9mb over a 10minute period, however this is a fall from a zero start and the fall is being held.

We think the boat safety test starts with a pressurise to regulator lockout circa 30mb, then a fall to 20mb to release regulator, then a 5 min stabalise, then a 5 min test, we may see a fall in the 5 min test if either the gas were to start to condense and or the temperature were to fall, fingers crossed the boat safet test guys are familiar with these behaviours and we get an all clear. thanks again for your help
 
The negative reading would be probably be because overnight the gas/air or whatever it was in the pipes cooled. It might also have been influenced by changes in atmospheric pressure. If the air pressure was low when it was connected and subsequently increased which might be the case if a storm passed away then this would give a negative reading. The manometer is measuring the differential pressure between the pressure in the pipes and the atmosphere.

BTW what manometer did you get as I'm after one? Typical prices seem around £35.
 
DaveBrent said:
We understand the reading as being a relative reading, not an absoulte reading, so the fall is simply relative to the start, thanks
Click to expand...
We understand another way to think of this is to think of a temperature gauge that starts at 20C if the temperature were to fall to 18C the gauge would measure negative 2C, not really a negative temperature, rather a change of negative 2C from a start of 20C to an end of 18C, thanks
 
DBK said:
The negative reading would be probably be because overnight the gas/air or whatever it was in the pipes cooled. It might also have been influenced by changes in atmospheric pressure. If the air pressure was low when it was connected and subsequently increased which might be the case if a storm passed away then this would give a negative reading. The manometer is measuring the differential pressure between the pressure in the pipes and the atmosphere.

BTW what manometer did you get as I'm after one? Typical prices seem around £35.
Click to expand...
We bought ours from amazon
Amazon product ASIN B07CLCTGQWseems to be rated by a number of gas engineers and we have been happy having compared to water gauge.
The water gauge is fine, only ours is small and we kept blowing the water our during test setup

It has been suggested we measure atmosperic change, by disconnecting the gauge and plugging the test pipe, this may tell a story with what may be happening atmospericaly, have just remembered this recommendation but have not got around to it yet, thanks

Subscribers  do not see these advertisements

 
  • Like
Reactions: DBK
The Wino said:
It looks like it's measuring the changes in pressure rather than the absolute pressure.
I wonder if it's the change compared to the original pressure or like a water gague the change compared to atmospheric pressure.
As others have said temperature will result in fairly big changes in pressure which may account for the reduction but if it is behaving like a water gague and comparing to atmospheric pressure any changes there will also have an effect.
Its an odd one where a longer measurement is less accurate or to be pendantic is accurate but not telling you what you want to know ( if theres a leak).
Presumably you test with the system pressurised otherwise there would never be any leaks I wonder how the regulator responds to any atmospheric changes?
Click to expand...

Many year ago I did a course which included testing large long gas pipes in an industry installation. The tests could take days to complete and corrections had to be made for both temperature and atmospheric pressure. I too think the negative pressure was due to changes in atmospheric pressure, try it again noting the barometer reading at both times.
 
  • Like
Reactions: DBK
I notice that it can measure gauge and/or differential pressure. Which were you using?
 
SomeoneElse said:
Many year ago I did a course which included testing large long gas pipes in an industry installation. The tests could take days to complete and corrections had to be made for both temperature and atmospheric pressure. I too think the negative pressure was due to changes in atmospheric pressure, try it again noting the barometer reading at both times.
Click to expand...
The photos above show our weatherstation reading over a half hour period, we think the 10,11 reading top-ish left reflect pressure, both a 10 minute water gauge test and 10 minute electronic gauge test recorded a fall. We think we are being affetced by many things, atmospheric, temperature, but mostly the condesing of the butane gas within the installation, we were just suprised at the scale of fall and have been posting in the hope someone else was in a position to test the same. Given the installation seems to be holding the fall, we may simply be left with the volume difference of butane gas vs butane condensing toward liquid, although we understand boiling point being circa minus 0.5C, thanks
 
tonyidle said:
I notice that it can measure gauge and/or differential pressure. Which were you using?
Click to expand...
We left on gauge, the lcd reflects diff when in that mode as used by us when tightness testing, thanks

Subscribers  do not see these advertisements

 

Join us or log in to post a reply.

To join in you must be a member of MotorhomeFun

Join MotorhomeFun

Join us, it quick and easy!

Join us

Log in

Already a member? Log in here.

Latest journal entries

Back
Top