Sprint95m Posted July 20, 2018 Posted July 20, 2018 How is a capillary temperature gauge operated? Is it triggered by the water expanding/contracting as it heats/cools? Thanks, Ian.
Nick Jones Posted July 20, 2018 Posted July 20, 2018 Yes, also called a "filled system". The gauge is actually a bourdon type pressure gauge calibrated in degrees and operated by the increasing pressure as the fill fluid expands as it warms up. Simple, effective and reliable.
JohnD Posted July 20, 2018 Posted July 20, 2018 This appeared a while ago in TRaction, hope you can read it. John
Sprint95m Posted July 21, 2018 Author Posted July 21, 2018 Yes, thanks Nick and John for your replies. I might go for this type of gauge on my Dolomite Sprint, given that the feedback I have read suggests that the senders currently available available are quite hit and miss. The one thing putting me off the idea is the probably that fitting such a gauge could make removing the instrument panel more complicated? Also I do prefer the appearance of the original Dolomite gauges! Ian.
oldtuckunder Posted July 21, 2018 Posted July 21, 2018 From what I have read the way to go about setting up the electronic (non capillary) type, is to get a couple of senders and bench test them with the gauge and a pan of water that you can bring the temperature up in, and thus find the sender that works well with the gauge. I have also read that its a good idea when doing it out of the car to use a length of wire connecting the sender and the gauge of about the right length & gauge as that in the car between the two. Alan
Nick Jones Posted July 21, 2018 Posted July 21, 2018 I like mine, which is in a combined oil-pressure/temperature gauge unit. It is accurate, responsive and calibrated in ºC. It is a bit of a pain to work with though as the whole thing has to be removed as a unit and the capillary is somewhat fragile and needs to be treated with respect. I've also never managed to get it properly backlit as the one I have is designed for external lighting and a thin (<3mm panel mount). Originally MG I think? Nick
JohnD Posted July 21, 2018 Posted July 21, 2018 (edited) Alan, as the sensors work on resistance, and its change with temperature, then the length of the wire between the gauge and the pan of hot water could offset the reading, as any wire has a resistance. https://chemandy.com/calculators/round-wire-resistance-calculator.htm But it is very low! This online calculator suggests that the Ohms in a meter of typical wiring wire (0.5mm^2)are 0.03. So you would need to be heating the pan in the kitchen while the car was in the garage (10 meters of wire?) to get near the change in resistance of a typical sensor. See below John Edited July 21, 2018 by JohnD
oldtuckunder Posted July 21, 2018 Posted July 21, 2018 Well from that I'm not sure John? given that I'd assume 2 meters of cable that's around 0.06 resistance, and given that that chart shows that less than 0.2 covers a range of 70-100 (30)deg, then a 2 meter length of cable = 30 / (0.2/0.06) = 9 deg C. Ok wouldn't throw figures totally out of the ball park, but still a measurable difference. However given my knowledge of electronics and maths is limited, this could all be bollocks Alan
JohnD Posted July 21, 2018 Posted July 21, 2018 You're right, Alan, it does add up. So bring your picnic stove to the car, and use the minimum length of cable. That won't invalidate the result. Or else do the whole thing on the bench:
Sprint95m Posted July 22, 2018 Author Posted July 22, 2018 Thanks, this is interesting. Hadn't thought about the wire length. I am planning buying a new sensor from Chris Witor and had thought about doing a bench test (or trying to at any rate!) in order to calibrate the gauge. On Sprints the sensor is situated in the inlet manifold but I am considering relocating this to the cylinder head transfer housing (I have a Sprintspeed transfer housing plate with a tapped hole for this purpose). Thanks, Ian.
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