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Old 15th Apr 2018, 5:20 pm   #1
mole42uk
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Default GPO Indicator Gas No5

I recently bought a GPO Indicator Gas No5 in an auction. The instrument is clean and cased but no further information is known. The meter scale reads "Percent Explosive" at the top. I was told it is for sampling the atmosphere in underground spaces before venturing in with tools.

Has anyone any information about these devices?
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Old 15th Apr 2018, 7:37 pm   #2
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Default Re: GPO Indicator Gas No5

If you're a member of the Telecommunications Hertitage Group, there are lots of documents about this on their resources web site.
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Old 15th Apr 2018, 9:03 pm   #3
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Default Re: GPO Indicator Gas No5

Ah, so you've got an IG5 then! We used them a lot when I was at BT, (http://www.jmwlimited.co.uk/IG5_Indi..._Detector.html)
They are pretty robust,they had to be, the quickest way to kill one was to suck water into it, they have a filter that goes in the sampling line to prevent debris and water being sucked into the sensor, they are a handy tool for checking for flammable gas in chambers and cabinets, although iirc they are designed for natural gas and don't like petrol fumes.
Somewhere I have my original BT training manual and I'm sure it has the operating manual in it, I'll dig it out and copy it for you.
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Old 16th Apr 2018, 6:36 am   #4
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Default Re: GPO Indicator Gas No5

Mine seems to be an early one, it has a canvas case but looks otherwise just like the one detailed in the link above.

I bought it because no-one else wanted it at a tools auction, and I liked it because it has "PERCENT EXPLOSIVE" on the meter dial. That attracted the schoolboy in me!
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Old 16th Apr 2018, 8:18 am   #5
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Default Re: GPO Indicator Gas No5

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I bought it because no-one else wanted it at a tools auction...
In that case I assume you paid somewhat less than the 198 quid in the linked page!
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Old 16th Apr 2018, 11:14 am   #6
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Default Re: GPO Indicator Gas No5



It was 196 cheaper.....
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Old 16th Apr 2018, 11:22 am   #7
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Default Re: GPO Indicator Gas No5

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Ah, so you've got an IG5 then!

Somewhere I have my original BT training manual and I'm sure it has the operating manual in it, I'll dig it out and copy it for you.
I'd love to see that if you have time!
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Old 16th Apr 2018, 12:35 pm   #8
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Default Re: GPO Indicator Gas No5

ISTR that these had a piece of metal hanging down, so that it would cause ripples when it hit water. It was then time to stop lowering the probe before it sucked in water.
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Old 16th Apr 2018, 12:41 pm   #9
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Default Re: GPO Indicator Gas No5

Mine has that piece of metal. I wondered what it was for.
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Old 16th Apr 2018, 1:44 pm   #10
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Default Re: GPO Indicator Gas No5

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It was 196 cheaper.....
Yes, that's the sort of price when I start feeling sorry for a vaguely interesting lot on the basis that I've not lost much even if it proves useless.
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Old 16th Apr 2018, 5:36 pm   #11
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Default Re: GPO Indicator Gas No5

the sensor is a heated wire, in a bridge cct. the resistance of the wire changes when the gas is passed over it thus altering the bridge and changing the display on the meter. they take a load of U2 1.5v batts. the switch and meter will have a batt test position. the sensor is damaged by the petrol vapour that leaves a coating on the wire. there was a filter unit to be inserted in the sensor line if petrol contamination was expected. and yes when a petrol tank in a garage leaks it finds its way into the cable ducts.

you should be able to test it using the gac cooker, just turn it on for a second and then test the air around it.
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