Confusing Gas Pressure - any ideas?

Dec 28, 2017
100
59
North Wales
Funster No
51,749
MH
A Class Adria Vision
Exp
Since 2016
Hi all, I use a manometer to check for gas leaks in my MH and, up till now, when turning the gas on (at 30mbar) it pushes the fluid up 4 inches, ready to test.
I've just changed all my gas bottles for Gas-It bottles and filled them with Autogas, I've also changed the regulator to a (brand new) Cavagna Auto Changeover unit, marked as a 30mbar unit.
All my appliances in the van work fine, but when I test for leaks using the manometer, it's pushing the fluid up 7 inches.
My question is - Does this indicate that the regulator is allowing a higher pressure through than 30mbar?
I'm guessing that a trip to my local dealer is on the cards but thought I'd check here first. All advice greatly appreciated.

Stonie
 

dave newell

Free Member
Oct 31, 2008
3,271
4,360
Telford, Shropshire
Funster No
4,733
MH
Home converted PVC
Exp
26yrs
What you are seeing is the regulator "lockup" pressure. If you run an appliance, the cooker for example, and monitor the pressure with your manometer you should see it settle to whatever height indicates 30mbar. My manometer is calibrated to read mbar.n

D.

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Jan 19, 2014
4,580
14,057
south shields
Funster No
29,767
MH
Chausson 620 welcoome
Exp
since 1990
What you are seeing is the regulator "lockup" pressure. If you run an appliance, the cooker for example, and monitor the pressure with your manometer you should see it settle to whatever height indicates 30mbar. My manometer is calibrated to read mbar.n

D.
what would you expect a typical lock up pressure to be... would it be somewhere in the region of the regulator working pressure or higher..?
TIA..
Andy
 
OP
The Rolling Stones
Dec 28, 2017
100
59
North Wales
Funster No
51,749
MH
A Class Adria Vision
Exp
Since 2016
Thanks both, I followed your link, The Wino, and you're right, it seems low on that scale.
Dave, I tried your idea and it dropped an inch, I'm still not sure why 30mbar has always been 4" but now it's higher.
Confused still but happier!
 
Aug 6, 2013
10,972
13,953
Kendal, Cumbria
Funster No
27,352
MH
A Class
Exp
since 1999
Thanks both, I followed your link, The Wino, and you're right, it seems low on that scale.
Dave, I tried your idea and it dropped an inch, I'm still not sure why 30mbar has always been 4" but now it's higher.
Confused still but happier!
The regulators used on MHs & caravans aren't precision devices. They rely on a spring that holds a valve open. This is connected to a fabric diaphragm which is moved by the gas leaving the regulator. The other side of the diaphragm is connected to outside air. When output pressure rises to the required pressure (38mB in this case) the diaphragm moves to close the valve. As it approaches closure the output pressure will drop so the valve opens. In reality it never closes and is never fully open so regulation is achieved by the valve varying the amount of gas allowed through it. This is the normal operating mode of the valve and is why pressure is measured whilst gas is being used by an appliance. When the last appliance is turned off no gas can flow through the regulator so output pressure is determined by how quickly the regulator valve closes and the pressure taken to cause it to fully seal. This is not the normal operating mode and the pressure measured in this state will vary and is not relevant to normal operation. The manometer measurement used for a leak test doesn't really matter: all you need to see is a difference in liquid levels that stays the same for a reasonable period of time after the cylinder or supply tank is turned off.

38mB = 15.25” water
= 0.551 psi
= 1.122” mercury

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Last edited:

dave newell

Free Member
Oct 31, 2008
3,271
4,360
Telford, Shropshire
Funster No
4,733
MH
Home converted PVC
Exp
26yrs
what would you expect a typical lock up pressure to be... would it be somewhere in the region of the regulator working pressure or higher..?
TIA..
Andy
In a 30mbar reg lockup pressure will typically be around 40mbar. Lockup pressure will always be higher than normal operating pressure.

D.
 

Freespirit1

Free Member
Sep 3, 2013
738
470
West Wales
Funster No
27,912
MH
A Class
Exp
Short while after a gap of many years.
If you want to measure pressure in inches of water pressure or mm for that matter - just use a clear piece of soft plastic tube - make a “U” shape in it - fill U to half full with water - connect top of U on one side to source of pressure leaving other side of U open to atmosphere.

Open pressure source to U measure difference in water level in U to give you inches water pressure.

Use conversion factor given in post No. 2 by @The Wino to give mbar - basically, divide the inches by 0.402 :)
 
Last edited:
OP
The Rolling Stones
Dec 28, 2017
100
59
North Wales
Funster No
51,749
MH
A Class Adria Vision
Exp
Since 2016
Thanks for your detailed explanations everyone. I really appreciate your expertise and I'm happy that my set up is fine. Really looking forward to getting away soon and testing the refillable gas bottles. Think I'll wait till the snow's cleared first :)

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