> Date:    Thu, 25 Feb 2010 20:56:22 -0600
> From:    Tom Hargrave<thargrav@HIWAAY.NET>
> Subject: Re: external temperature guage
>
> You can shorten or lengthen the probe wires with no change in the =
> measurement, but you have to do an excellent job soldering the ends =
> together so as to not add any appreciable resistance to the circuit.
>

Well, yes and no.  If the sensor is a Thermistor, it depends on the
temp/resistance curve of the individual sensor.  In general, the
resistance is so high (100's of kOhms at ambient), and the
temperature/resistance curve is so steep, that adding lead wire is not a
problem.

If it's a thermocouple (TC), that is a problem unless you use the same
type of TC wire as the extension (most of these tend to be Type K).  The
TC gauge has to do an ambient temp correction (TC is a differential
measurement device, measuring only the *difference* between the
temperature at the junction - i.e. probe - and the measurement point,
i.e. where the TC wire transitions to straight copper for both leads -
or at the meter input terminals), which is measured at the gauge itself
(usually by a thermistor in fact - for cheaper gauges).  So if you
attach a copper lead under the van, and the temp under the van is 25�F
higher than the temperature at the temp gauge, the gauge will read 25�F
too low.

Just use a regular DVM and check the resistance of the probe.  A TC will
be basically zero ohms (it should also have one red lead and one other
color - whereas thermistors/2-wire RTDs have no + or -), whereas if its
a thermistor it'll be in the 10's to 1000's of kOhms at ambient.  For
the thermistor, the higher the resistance measured, the less the lead
wire will affect the calibration.

There's an off-chance that it uses an RTD, but in these cases it's
typically like a 1K ohm nickel probe, so you'll see a pretty stable
resistance of some fixed value (whatever the nominal RTD resistance
is).  Then just grab the probe tightly in your hand and watch the
resistance change.  It'll go up with an RTD, and down if it's a thermistor.

Keith Hughes
'86 Westy Tiico (Marvin)