RE temp Gauges

[Nic Webb <nic@webb.demon.co.uk>]

Yet another opinion on temp gauges.

We use various temp sender units at work for coolant temperature, they
usually have a stated tolerance of 8 degrees celcius ( sorry about the
units), but this can vary upon the price, Having measured some they may
actually be more accurate. This error is the spread of many sensors and
not the repeatable accuracy of one sensor, so as some one said if you
use your gauge as as refernce device you should be OK.

One important point is to have a good earth for the sensor, most use the
engine earth, so make sure that is good and clean, as a slight voltage
drop there can cause irregular readings on the gauge particularly when
heavy loads are turned on. ie headlamps.

As to where you put the sensor, well the sump drain plug is no use
unless your after bulk oil temperature, (only really usefull for
monitoring oil temp after a cold soak). I originally had one in the
sump, but moved it into the top of the external oil filter housing, so
the sensor is measuring the flowing oil into the engine. This way you
can see engine temp changes straight away.

The cylinder head temp gauge (Thermocouple) is in theory a much more
accurate type of measurement, but the principle of them is to measure
the voltage between 2 sensors. To make things easy the one junction
(sensor) is held at freezing point 0 degrees C or 32 deg F, as a
referenc,. the other junction (sensor) is used to make the measurements.
The reference junction can be elctronically controlled, but in the case
of the VDO gauge it can't be. Therefore I think the VDO gauge has its
reference junction inside the gauge. Thus the displayed temp will vary
with engine temp and dash board temp. A change in 50deg ambient will
cause the gauge to alter by 50 deg.

I hope I explained that Clearly.

Finally, for making accurate checks or callibrating other gauges an
electronic thermocouple gauge is very accurate and can be used to
measure oil, air, water, heads, exhaust, tyres and even food quickly.
These are coming down in price so may make a good investment.

A second finally, and at last you might say, but to check the
callibration of an existing gauge, wire it up in the kitchen and place
the measuring tip of the sensor in pure boiling water, or steam (make
sure to keep the water of the connections, and you fingers). As water
normally boils at 100 deg C / 212 deg F????? you can check the reading
on the gauge. (unless you live up a tall mountain).

Nic Webb
Team Roobarb & Custard
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