Smith's Instruments "Air" gauge

Oct 04, 2003 4 Replies
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Hi ppl,



I recently picked up a box of old British car bits, and in it are two gauges labelled "Air". One is a Smith's, a dual gauge. The other is a generic thingie that looks like a more modern Smith's.



The scales are dual units of Lb/cu in, and kPa. Both have an air inlet (of course) and the guts connect this to a spiral copper flat tube. In the Smith's it rotates a teensy gear to move the needle. (There are two sets of guts in the Smith's).



Pardon my incredible ignorance, but, what is this measuring? I thought vacuum, and indeed one needle in the Smith's bounced around a very little bit, once connected to a manifold port on my little Spit, but the other was as stuck as I am on this.



Anyone know the principal of operation with this copper coil thingie?..and of course...what they are measuring?



TIA, Mark



Mark



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N

It's measuring the primary and secondary air pressure on a HGV air braking system. The guages you bought are for a lorry or coach to monitor the air brake pressure.

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K, Thanks.

Mark

Mark

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R

I think you probably mean Lb/sq.in.

Have you tried blowing? What is the full scale deflection? If it's more than 100 kPa/15 psi it can't be a vacuum gauge! As the others have suggested it's probably an air brake pressure gauge.

It works like one of those party whistles that unroll when you blow them, except that the spiral doesn't fully unroll - it only expands enough to rotate the needle. The gauges are probably measuring the air reservoir pressure and brake line pressure.

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Yes, that's right.

Yeah, the needle didn't move, but I gained three inches on my.......

I'll try it on my compressor some day. The gauge I mean.

Ok, I though that might be it. Thanks. Mark

Mark

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