Just testing a Gunsons Gastester Pro that I recently acquired. Set to 2.00% in fresh air as per instructions and then tested in exhaust of 1998 Peugeot 106 1.4 injection and reading came out as -0.60%? Should be less than 1% according to my Haynes manual but how can it be less than zero?
Borrowed a friends Gastester and got similar result - am I missing something here?
Anyone had experience with these?
Thanks.
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Caspar
I'm not familiar with a Gastester; but if you've got 2% CO in the fresh air round your way - I'd worry?
With the cat hot on a '98 car; I'd expect it to read 0.00 CO - or 0.25 as a max.
If both read the same it looks like a calibration error.
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I think I can remember seeing the same sort of readings on a similar tester even when calibrated correctly.
However just check that the calibration has not drifted by checking the free air 2% reading at the end of your test. Mine was never very stable and I think that the sensor can be battery voltage dependent if you rev the car the car under test and if the alternator regulation is poor. If you find that is the cause, use a battery supply from a different vehicle or static well charged battery.
My tester was always accurate when checked against MoT for levels around the 2% figure for non cat vehicles
I cannot remember the exact physics of the sensor but I believe it is to do with the thermal conductivity of the 'air(mix)' which will vary with the relative proportion of gases. Hopefully NC will be along with a more quantitative description of the physics.
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pmh (was peter)
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>>Hopefully NC will be along with a more quantitative description of the physics.
Way beyond me I'm afraid. Dissociation, chemical rate kinetics, all foreign and slightly scary subjects for me!
My guess is that the sensor is being comletely fooled by reading the gases downstream of the cat. On a pre-cat car, a certain level of CO would be accompanied by an amount of HC and NOx - i.e., the composition of the mixture as a whole would vary in a known way. This known relationship is changed by the reactions in the cat. Having said that, if the CO is anywhere near 1% as suggested by Haynes, there's something sadly amiss, I would expect 0.0% or certainly less than 0.3%
Number_Cruncher
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My Gunson Gastester works very well with pre cat cars as long as you don't leave it turned on for too long. After about 20 minutes the readings start to drift. On post cat cars it has readings from -60 to -80%, so I have assumed that it was not designed to work with a cat.
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They were designed for pre-cat cars(adjustable idle),were never particularly accurate and were not designed to be that accurate due to the simple technnology used-totally different to a proper gas analyzer or even a hot-wire machine.
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Isn't the Gunson a hot-wire machine?
The claimed accuracy was only .5% anyway, altho mine was consistently better than that cw the MoT test machine.
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pmh (was peter)
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If I remember rightly-it was a long time ago-this tester does not measure CO but CO2-a much easier job due to greater concentrations in the exhaust and then back-calculates the CO from that.
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JC2 is right,there is a direct correlation between CO and CO2,which is what the gastester measures,using a hotwire system.I still use mine to set up carbs,it can give a negative reading,but once the mixture is set correctly the MOT confirms it is pretty accurate.
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Because the Gunsons measures CO2, not CO, it probably won't give accurate reults on cars with cats because the cat will have converted all, or most, of the CO to CO2 anyway.
They were a useful guide to mixture settings, in the days before cats, I've got one somewhere in the garage!
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Small jc please-big JC is someone else!!
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