Pressure drop on test guage, nothing at meter

[nip_it_up]

Hi all,
I am wondering if someone can make sense of this. I have been trying to determine if there is a leak on an existing water supply pipe. Attached a dry pressure guage to garden tap and turned tap on. Pressure went to about 3.5 bar then I shut off supply at the water meter. Over the next hour pressure dropped to around 2.5 bar. I assumed there was a leak but could not find anything. Next day I took a meter reading and 5 hours later there was no change at the meter, indicating no leak! I have tested the pressure guage independently with a stop end and pump and seems to hold.
Very confused, what am I doing wrong and is this the correct way to test a supply pipe?

 

[Deleted member 120897]

Could it be that you didn’t tighten the gauge up on the tap properly? Try it again, but obtain some gas leak detection fluid, and spray the joint connecting to the tap, if it bubbles, you’ve found your ”leak”.

 

[nip_it_up]

I tried the test again after the meter check. This time I used the washing machine valve under the sink, and isolated the the water going upstairs and to the boiler, still same result. I did not use detection fluid as this was a wet test. The connection to the guage remained dry at all times.

 

[Chuck]

If you have any 'plastic' pipe in the setup you may be seeing the effects of this 'relaxing' after a the change in pressure.

A tricky point about testing in the way you are doing is how to rule out the possibility that the isolating valve at the meter end is not drop-tight.

The pressure in a sealed rigid container filled with water is very sensitive to leaks, changes in temperature and deformations of the container. I wouldn't personally worry too much about transient changes in pressure of the size and timescale you have described.

I'd suggest using the water meter observed over several hours to establish wheter you have a leak and only switch to pressure drop-rate measurements to pin down where the leak is located.

 

[nip_it_up]

Ah, so we should only use meter reading to be sure of leaks. Generally are there any more ways to check for leaks without resorting to a gas tracer test.

 

[Chuck]

Generally are there any more ways to check for leaks without resorting to a gas tracer test.

Leaks generate sound (both audible and/or ultrasonic) which can be detected with specialist equipment. Not gear any normal domestic plumber would have but there are specialist leak-finding firms who do this sort of thing.

 

[John.g]

If the stopcock is passing even very slightly then the pressure test will only reflect the mains static pressure which may go up as well as down.