On Fri, 21 May 2004 18:36:36 +0100, Jim Ingram wrote:


"Seri" wrote in message
.. .
I have a new boiler (finally) and am considering doing some of the
gas pipe
re-routing myself (a possible topic for another time), before I
even
consider this I thought it was a great excuse to purchase an 'okay'
little
flammable gas detector and add another toy to the toolbox.

I'm a paranoid type of person when it comes to things that can kill
me, so
as soon as the thing arrived today I carried it over to the
existing cooker
a opened a hob the minimum amount to measure the amount (over
1000ppm but
less than 10000ppm).
Next I recalibrated it and tested the existing gas piping put in
some years
ago by the previous owner, one of the joins in the existing
pipework manages
to just get the detector to tick over to the first level (above
100ppm but
less an 1000ppm), this made me realise, I don't actually know if
this is an
allowed amount, I had (possibly stupidly) assumed that no gas would
be
allowed.

To cut this simpler, what is the allowed maximum PPM of flammable
gas from a
join?

I am not a gasfiitter however I can tell you that the Lower
explosive limit of methane in air is ~5%(v/v) (the minimum
concentration of gas required to sustain flame propagation) which
equates to 50,000 ppm so not much chance of an explosion in your
case. I suspect that the regs quote an allowable amount of leakage in
terms of volume of gas. I think what gasmen actually do is to turn
off the gas supply and connect a manometer (pressure gauge) to the
gas pipe work for a period of time and calculate the amount leaked
from the pipework volume and the pressure drop.

Have a look in the Gas Fitting FAQ below.
Whilst a Flammable Gas Detector is a useful (but not absolutely essential)
gadget for tracing leaks it is NOT the tool used to certify that gas pipe
work is gas tight.
The limit for Domestic Natural gas pipework is NO drop of pressure in
2minutes when measured with a U-gauge and less than 0.25mBar when measured
electronically.

The method is fully explained in the FAQ.

Even tiny leaks can have gas concentrations by the pipe that
are approaching the 'LEL'. This means that were you to
be so foolish as to trace them with a naked flame they would be big enough
to ignite and support a flame.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html