Negative Gas Pressure

D

davebrent

New Member
Joined
6 Jun 2012
Messages
18
Visit site
Hello fellow boaters, 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 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
 
The only thing I could suggest is that the gas left in the line at atmospheric pressure has condensed into liquid causing the lower pressure. This would be harmless.

Are you using Butane or Propane, as this would likely not happen with the higher pressure Propane ? Not an effect I have ever observed, but these gases are both used as refrigerants, so will easily condense subject to the external temperature and indeed are stored in your cylinder as a liquid at its saturation point.


I have thought about this some more, and think this is a reasonable explanation for Butane. At 0 Bar Butane (R600) has a saturation point of -0.49C. The residual gas in the line was superheated, i.e. above its saturation point, however, it will want to settle to its saturation pressure so a very small qty will condense into droplets of liquid, hence the lower pressure oberved.

For propane (R290) which is a far higher pressure gas, its saturation temperature at 0 Bar is -42.11oC, and this is simply too cold to achieve any residual condensation.

However, I am going to ask the expert at Chemours if this is a correct explanation.
 
Last edited:
Thank you for your reply, our concern was more with the size of the negative pressure where we saw negative 75mb and any adverse affect on appliance gas seals and similar.

We are currently experiencing warmish temperature and are thinking of colder times. We guess there must be a negative imit once the gas has condensed, we are just not sure if current temperatures reach that limit.

Thanks agin
 
haha will configure a camera to catch the thief...

We are thinking perhaps temperature and time, we are confused by the difference between negative 30mb in the morning and negative 75mb mid afternoon where temperature was rising

Thanks
 
I know THE UK technical expert at Chemours (formerly Du Pont) for my day job. I have asked him the question.

Incidentally this relatively high saturation temperature for Butane is why it is useless on cold winter days. I always convert to Propane, it is higher pressure, burns cleaner, and I understand you get more thermal energy per unit volume of liquid, which is what you are essentially buying.

I will come back here when Mark at Chemours answers my question.
 
A further thought, from memory a boat safety examination has a gas tightness test over 10 minutes, we think 5 mins stablise, 5 mins testing, guess we would be unlucky if the butane gas were to start to condense mid test, this may reflect as a leak. We think the test is performed at circa 20mb to avoid regulator lockout, will the gas condense at this level, we did not observe this in our testing, but will re-connect manometer and repeat our test.

thanks to superheat6k above, we appreciate and find interesting your analysis and attention to this
 
A further thought, we have always turned the gas off at bottle and burnt off the remaining gas at hob when leaving the boat, will leaving the lines under pressure make any difference...
 
We sea boaters are fortunate not to have to suffer the indignity of some idiot with a clip board telling us our boats are safe to use. If one ever sets foot on my boat he will be soon after working out the temperature of the River Hamble !
 
  • Like
Reactions: pvb
Thank you Rogershaw,

so using the formula with a starting pressure and temperature to a later cooler temperature

from
30 mb / 30 C = 1.0

to
?? mb / 15 C = 1.0

?? must equal 15 mb to balance i.e. a drop in pressure

just not sure how to apply formula when starting pressure is zero
 
davebrent said:
Thank you Rogershaw,

so using the formula with a starting pressure and temperature to a later cooler temperature

from
30 mb / 30 C = 1.0

to
?? mb / 15 C = 1.0

?? must equal 15 mb to balance i.e. a drop in pressure

just not sure how to apply formula when starting pressure is zero
Click to expand...

We are talking about absolute pressure of it starts at 1 atmosphere 14.7 psi if I remember correctly.

The temperature is also absolute in the degrees kelvin.
Formula
0 Kelvin = -273,1°C


{\displaystyle {\frac {P}{T}}=k,}

where:

P is the pressure of the gas,T is the temperature of the gas (measured in kelvins),k is a constant.
 
The starting pressure is not zero.
The 'pressures' on the manonmeter are relative to atmospheric.
So 1000mB +/- 30 mB or whatever.
The temperature is absolute, in K, where 0degC is 273K
30mB is 3% pressure so 3% temperature would be something like 10degC change.

It suggests to me that a run of gas pipe is getting warm with the stove in use?
Or just in the sun?
 
Rogershaw said:
We are talking about absolute pressure of it starts at 1 atmosphere 14.7 psi if I remember correctly.
The temperature is also absolute in the degrees kelvin.
Formula
0 Kelvin = -273,1°C
{\displaystyle {\frac {P}{T}}=k,}

where P is the pressure of the gas,T is the temperature of the gas (measured in kelvins),k is a constant.
Click to expand...


So then P1 is the current atmospheric pressure .... say 1013mb
T1 is the current temprature in °K ................ say 298K ( 25C)

If the manometer falls to -30mb P2 is 1013 - 30 = 983mb

Therefore T2 = 298 x 983 / 1013
=289K​
= 16C​
​
If the temperature fell by about 9C that would explain a fall in pressure of 30mb​
 
Last edited:
Thank you to all for your time and consideration, all seems to account for the behaviour we observe, with just a few puzzling elements left...

1. not sure why in the morning the manometer reads negative 30mb then negative 75mb mid afternoon at a warmer time
2. have just recorded a negative 101mb this morning from midday yesterday, hope we are close to a limit as to how far this will fall, just thinking of are we getting close to appliance limits on pressure

the size or volume of the installation is probably a factor, our boat is small at 21 foot, we have a gas hob, gas water heater and gas warm air heater, we have roughly measured 6 meters of 3/8" copper pipe, longest run is circa 2 meters from gas bottle to gas hob with single inline valve mid run, for fittings we have 2 off quarter turn valves, 2 off 3 way tees, 2 off reducing coupling to appliances circa 50cm total 5/16 pipe to applicances, 1 off straight coupling, 4 off bulk head fittings, 1 off inline test point, 1 off bubble tester, with 24" 3/8 hose to regulator mounted directly to 4.5kg butane bottle

there are greater minds than ours working this puzzle, thanks again to all where we really do appreciate all comments
 
davebrent said:
Thank you to all for your time and consideration, all seems to account for the behaviour we observe, with just a few puzzling elements left...

1. not sure why in the morning the manometer reads negative 30mb then negative 75mb mid afternoon at a warmer time
2. have just recorded a negative 101mb this morning from midday yesterday, hope we are close to a limit as to how far this will fall, just thinking of are we getting close to appliance limits on pressure

the size or volume of the installation is probably a factor, our boat is small at 21 foot, we have a gas hob, gas water heater and gas warm air heater, we have roughly measured 6 meters of 3/8" copper pipe, longest run is circa 2 meters from gas bottle to gas hob with single inline valve mid run, for fittings we have 2 off quarter turn valves, 2 off 3 way tees, 2 off reducing coupling to appliances circa 50cm total 5/16 pipe to applicances, 1 off straight coupling, 4 off bulk head fittings, 1 off inline test point, 1 off bubble tester, with 24" 3/8 hose to regulator mounted directly to 4.5kg butane bottle

there are greater minds than ours working this puzzle, thanks again to all where we really do appreciate all comments
Click to expand...
Cannot explain the 75mb fall when the temperature actually rises

101 mb is a heck of a fall. Its going to need a temperature change from about 30C to 0C to explain that by Gay-Lussac’s Law !

The volume of the system is not relevant provided it remains constant

Id like to cross check your readings with a simple U tube manometer
 
It sounds to me as if you are setting your "manometer" to zero at standard supply pressure and there is a small leak, but it's not clear without more information. So, if you don't mind ...

* A manometer measures pressure difference. What is the other pressure (atmospheric?) and how is it connected (port?) Or is yours actually a pressure sensor - they are often inaccurately sold as manometers, particularly when designed for low pressures.

* How do you zero it?

* Have you tried leaving it running unconnected to the gas system? Drift in the electronics could be an issue.
 
The manometer was from amazon,

Digital Manometer,Dual Port Air Pressure Meter Pressure Gauge HVAC Gas Tester,Large LCD Display with Backlight(9V Battery Included): Amazon.co.uk: Business, Industry & Science

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

for info we have a gasboat 4208 dual fuel regulator set for 30mb

think it would be interesting if anyone else was in a position to perform the same as a comparison

thanks for your time
 
we do have a small water based manometer but got frustrated with the setup, we were forever having water shoot out unless the gas bottle was turned on very slow, seems the regulator lockout varys in sensitivity, perhaps not an issue with a larger water based manometer.

We will go back to the water based manometer and see if we can setup for the negative test and will report back
 
You must log in or register to reply here.

Other threads that may be of interest

B
Help to test new Anchor App on Android
Replies
1
Views
921
Brendan01
B

Share this page

Top