# How does a manometer work?



## Zebedee (Oct 3, 2007)

I don't mean that exactly of course, but I do have a poser for you! :?

Having installed a BBQ gas point yesterday, I set up a water manometer to check for leaks _(cold tea actually - it shows up better!)_ and switched on the gas.

After a few seconds to let it stabilise, I switched off the gas, marked the water (_cold tea :roll: _) level and waited.

As always seems to happen, the water level dropped by at least an inch in the first 5 minutes, so I touched nothing else but made another mark to indicate the new water level.

Then I came on here and wasted more than half an hour . . . during which time the water level had dropped a further 5mm at the most. Such an insignificant drop that it indicates a more than adequate gas tight system.

*Why does the level drop so much during the first few minutes? *

It seems to do this even when there is no leak, then it falls (almost) no further even if left for a much longer time??

What am I missing??

Dave


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## 101578 (Oct 28, 2006)

I could answer this but it's tooo sexist :lol:


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## Zebedee (Oct 3, 2007)

Leaky said:


> I could answer this but it's tooo sexist :lol:


With a username like yours I wouldn't take any notice if you did!!

Dave 8O :lol: :lol: :lol: :lol:


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## 101578 (Oct 28, 2006)

Zebedee said:


> Leaky said:
> 
> 
> > I could answer this but it's tooo sexist :lol:
> ...


  :lol: :wink: i thought the thread was about hunky chunky men :roll:

Leaky's wench


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## time-traveller (Apr 23, 2007)

Zebedee said:


> I don't mean that exactly of course, but I do have a poser for you! :?
> 
> Having installed a BBQ gas point yesterday, I set up a water manometer to check for leaks _(cold tea actually - it shows up better!)_ and switched on the gas.
> 
> ...


I don't know what YOU'RE missing Dave, but obviously I must be!

I would have thought that they whole idea of a test rig would be to indicate a leak by means of a falling (tea) level. But you say that it is what you would expect. If you accept a falling level as normal, what would you be looking for that would indicate a leak? :?


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## johnc (May 1, 2005)

*Why does the water level drop*

I can think of two reasons.

(1) Some of the gas is being a absorbed by the water.

(2) Change in temperature casuing the gas pressure to change.

General gas law states

P1*V1/T1 = P2*V2/T2 where the temerpature is in degrees Kelvin

Isn't Physics wonerful !!!


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## twinky (Aug 11, 2008)

I'm a bit confused why you switched on the gas after connecting the manometer. Wouldnt that just blow the tea straight out the end?

I can only relate to when the gas board came to connect up the mains gas onto my house. 
The fitters manometer had a flexi rubber tube before the bend and he just blew into the end and pinched the tube to hold the liquid up.
Then he fitted it on to the test point next to the inlet and let go of the hose.
The liquid settled imediately and he watched to see if there was any drop.

The point is although the pipework was sealed it contained no gas - just air, during the test. And he only watched for a few seconds.

As it happens he refused to connect me up because he said the manometer had shown a leak in the house. I suggested that it maybe the cracked rubber tube on his manometer and he changed it. No leaks found and he connected me up.


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## Zebedee (Oct 3, 2007)

time-traveller said:


> I don't know what YOU'RE missing Dave, but obviously I must be!
> 
> I would have thought that they whole idea of a test rig would be to indicate a leak by means of a falling (tea) level. But you say that it is what you would expect. If you accept a falling level as normal, what would you be looking for that would indicate a leak? :?


This is the crucial bit Traveller.

_"It seems to do this even when there is no leak, then it falls (almost) no further even if left for a much longer time?? "_

The bit that baffles me is why the tea level always seems to fall about an inch in the first few minutes . . . . but then it stops and falls no further however long it is left (within reason of course).

What is so special about the first few minutes??

Dave


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## Zebedee (Oct 3, 2007)

twinky said:


> I'm a bit confused why you switched on the gas after connecting the manometer. Wouldnt that just blow the tea straight out the end?


No - it's only 30 millibars, which is a very low pressure.

It was easier than puffing - that's all.  

Dave


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## Zebedee (Oct 3, 2007)

*Re: Why does the water level drop*



johnc said:


> (1) Some of the gas is being a absorbed by the water.


Possibly John.
I thought of that, but so quickly over such a tiny surface area???



johnc said:


> (2) Change in temperature causing the gas pressure to change.


Not yesterday. It went suddenly cooler just before I did the test, so the effect would have been the reverse.



johnc said:


> Isn't Physics wonderful !!!


Yes, when it answers your questions . . . but I can't work this one out.

Thanks.

Dave


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## spykal (May 9, 2005)

Hi Zeb


It is usual to wait 5 minutes before taking a manometer reading to allow for temperature stabilisation.

Where did you attach the manometer, which side of the regulator? 

Before taking a reading and after turning off the gas at the bottle did you equalise the pressures either side of the regulator by burning off excess gas pressure at the stove ?

Mike


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## Zebedee (Oct 3, 2007)

Hi Mike

I just used my newly installed BBQ point and connected the manometer to that.

Shouldn't have needed any equalisation should it? :? 

I was checking for leaks in the whole system in fact, as I had not turned off any of the gas cocks - so the whole system was open right back to the cylinder.

Dave


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## spykal (May 9, 2005)

Zebedee said:


> Shouldn't have needed any equalisation should it? :?
> 
> Dave


Hi Dave

I think on LPG systems it is usual to equalise the pressure either side of the regulator before doing a manometer leak test. If you have a high pressure tail between the cylinder and the regulator the high pressure there needs to be equalised with pressure in the piping on the low pressure side.
This can be done by closing the cylinder valve and then releasing the pressure from the system by burning off gas at the stove until the manometer reading drops a little, ( from memory say from 12" water to 8")

This is when I get my friendly neighbourhood gas engineer to call and give me a reminder course :lol:

Mike

Mike


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## Zebedee (Oct 3, 2007)

Thanks Mike.

Hadn't thought of that  .

I have a stop valve right next to the regulator on the low pressure side.

Won't need to equalise the pressure if I use that - will try it again later and see if I get the same result as before.

Thanks again   

Dave


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## rumik (Aug 19, 2007)

The gas in the system will be very (i.e bone) dry. When you connect the manometer some water (tea) will evaporate into the dry gas. How much water is represented by 1" drop in level? If the manometer pipe is very small bore then this might explain the initial drop. 

The high pressure tail will hide a small leak in the system by continuing to feed gas into the pipework vis the regulator until the pressure has equalised each side of the regulator. Whilst this would interfere with the test it doesn't explain the initial drop in water (tea) level.


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## Fatalhud (Mar 3, 2006)

It could be the fact that you forgot to add milk and sugar :roll: :roll: 
Alan H


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## Zebedee (Oct 3, 2007)

Fatalhud said:


> It could be the fact that you forgot to add milk and sugar :roll: :roll:
> Alan H


Plonker!! :lol: :lol:

Your sense of humour is worse than mine - and that's saying something! :lol:

Dave


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## Fatalhud (Mar 3, 2006)

I am trying :wink: 

Alan H


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## Pudsey_Bear (Sep 25, 2008)

VERY :lol: :lol: :lol: :lol: :lol: 


Kev.


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## oilslick (Oct 3, 2007)

*Adiabatic compression!*

I reckon it is the time i takes for the temperature to recover after you compressed the existing gas (air) in the u tube.

Grant


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## Pudsey_Bear (Sep 25, 2008)

Hmm compress gas it warms up, thus expansion, cools down contracts, level changes, clever.



Kev.


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## Zebedee (Oct 3, 2007)

*Re: Adiabatic compression!*



oilslick said:


> I reckon it is the time i takes for the temperature to recover after you compressed the existing gas (air) in the u tube.
> Grant


Hmmmmm????

Does a paltry 30Mb count as compression though Grant?

And the gas entering the system from the cylinder would be cooled anyway, due to latent heat of vapourisation, so that would expand as it warmed up and counteract the effects of your adiabatic compression.

Dunno. :? :? :roll:

Where's Boff when you need him!! 

Dave


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## Pudsey_Bear (Sep 25, 2008)

Somebodies just being clever now, I'm off.

Kev.


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## oilslick (Oct 3, 2007)

*30mb is a compression!*

any compression is a compression!!

The previous quoted gas law PV/T = constant shows that the relationships between the pressure volume and temperature are proportional to each other.

As 30mb is basically a change in atmospheric pressure of about 3%, then we will initially see a rise in temperature of 3% (from absolute zero, -273) = about 10°C. As the gas cools back down to ambient the volume will then shrink back 3%.

If your pipe had about 1 metre of air in it then thats about 3cm...

Grant


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## Pudsey_Bear (Sep 25, 2008)

I need a lie down.

Kev.


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## Zebedee (Oct 3, 2007)

Thanks Grant.  

I'm convinced, I think, but might have to go and have a lie down with Kev.  


Aaaarrrrrrrggggggggggghhhhhhhhh . . . . what am I saying! 8O 8O   

Dave


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## oilslick (Oct 3, 2007)

*ok!*

OK you guys have a lie down. (Together?)

By the way I didnt mention the possible effects of the incoming gas... I expect it has already lost its temperature effects from evapourating from the gas bottle in the vans pipe work a while ago (afterall you have hardly drawn any gas off.

Grant


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## Zebedee (Oct 3, 2007)

*Re: ok!*



oilslick said:


> OK you guys have a lie down. (Together?)


Hope I don't offend Kev _(I'm sure he's a lovely lad  )_ but I think I'll decline thanks! 8O 8O



oilslick said:


> By the way I didnt mention the possible effects of the incoming gas... I expect it has already lost its temperature effects from evapourating from the gas bottle in the vans pipe work a while ago (afterall you have hardly drawn any gas off.
> Grant


OK Grant, now I really am being awkward!

I hadn't used the van for a couple of weeks, so when I turned the gas tap on to pressurise the tea-ometer there was quite a hiss of gas entering the system - as there always is under these circumstances.

That's what made me think the cooling effect might be significant.

Dave

Oh . . . me head hurts.

I will try it again later, under much more strictly controlled circumstances, and try to eliminate as many of the variables as I can.


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## spykal (May 9, 2005)

Hi Zeb

Following your description above I am wondering if before testing you made sure that you had purged all of the air out of the entire gas system?
To do this you would have needed to burn off gas or release the air/gas mixture at all the appliances before testing.

Anyway I think you should have used a Womanometer they are so much less hassle and are always right :lol: 


Mike


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## Zebedee (Oct 3, 2007)

spykal said:


> Anyway I think you should have used a Womanometer they are so much less hassle and are always right :lol:
> Mike


I've got one of those Mike, and as you say, even when she is *not *right (_it has happened twice in 40 years of marriage 8O :roll: _) she still insists that I am wrong!! 8O

Not sure about the reduced hassle though! :? :? :wink:

Dave


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## Pudsey_Bear (Sep 25, 2008)

Err, excuse me, there will be no lying down with Kev, unless, you have certain anatomical parts about your person, Which although I'm not certain, I don't think you have I like hills and valleys, not lumps n bumps, or as it is said in fifty first dates (brill film) I prefer taco to sausage.

Kev.


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## Zebedee (Oct 3, 2007)

Where's the emoticon for a big sigh of relief! 8O :lol: :lol: :lol: 

Dave


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## Pudsey_Bear (Sep 25, 2008)

You can go off people you know, I'm hurt now.

Kev.


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## teal (Feb 23, 2009)

I thought the general idea is to fit the rubber pipe to a connector/ bleed nipple and turn the gas on until it rise to 8 inches or metric and turn off gas and waite 10 minutes in which it should only drop a small amount then stop, if the amount drops below a certain level then you have a problem . The caravan people use a pressurised pump to 50 lb and this also should only drop a little, as for tea instead of water that takes the biscuit.


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## teal (Feb 23, 2009)

About hundred years ago i was being tested by the Corgi rep and after many questions before pratical he said said how long is your manumeter and i replied " about 15 inches" he nearly died laughing he meant what display does it go up to. I can just see him now if i said wait a mo i am going to fill it up with tea


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## Zebedee (Oct 3, 2007)

teal said:


> I thought the general idea is to fit the rubber pipe to a connector/ bleed nipple and turn the gas on until it rise to 8 inches or metric and turn off gas and waite 10 minutes in which it should only drop a small amount then stop, if the amount drops below a certain level then you have a problem . The caravan people use a pressurised pump to 50 lb and this also should only drop a little, as for tea instead of water that takes the biscuit.


Thanks for that. 

Can you elaborate a bit as I don't know what you mean by dropping a small amount, or dropping below a certain level? :?

Wonder if it was biscuit crumbs in the tea-ometer that caused the anomaly? Hadn't thought of that! :? :lol: :lol:

Dave


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## teal (Feb 23, 2009)

Hi , regarding how much drop when testing for leaks,if it goes down after about 10 minuets there is a problem, leave for another 10 m and if still dropping even by a small amount turn off gas and investigate. I would try the fluid test around the fitttings also is the flexible gas pipe old?.


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## sugdenr (Mar 13, 2008)

*Gas Test*

There is no way that the water is absorbing significant amounts of the gas. If you have a slow leak then it will leak down completely over time, but a drop then stop etc is not a simple leak.
Firstly did you close all the gas taps to every other appliance to ensure that you are testing only what you have just fitted? Try isolating different parts of the system and testing them, does the same thing happen?


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## Zebedee (Oct 3, 2007)

teal said:


> I can just see him now if i said wait a mo i am going to fill it up with tea


It wasn't just tea . . . if you don't mind!!! 8O

It was Earl Grey! :roll:

Any technical reason for not using tea? #-o

It simply makes it easier to see. :albino:

Dave


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## spykal (May 9, 2005)

Zebedee said:


> Any technical reason for not using tea? #-o


Yep........ everything stops for tea :lol:

Mike


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## Zebedee (Oct 3, 2007)

spykal said:


> Zebedee said:
> 
> 
> > Any technical reason for not using tea? #-o
> ...


Including gas leaks!!! 8O

See, I'm not as daft as some people think!! 8) 8) :lol:

Dave


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