<VV> CHT data (long - possibly boring)

[Roger Gault]

Hi, Larry,
Yes, the VDO gauges are thermocouples.  Iron-Constantan thermocouples, I'm
almost certain.  And yes, TCs do put out a voltage relatively proportional
to the temperature at the junction.  But the TCs have a nasty family secret.
They bring one or two of their relatives along with them.  There are always
at least two in the circuit.

In a classic laboratory TC setup, there is an easily recognized "reference
junction".  The easiest setup to imagine is a copper-constantan setup.  You
have a copper wire from the meter to the reference junction, a constantan
wire to the measuring junction, and a copper wire back to the other side of
the meter.  Note that the two junctions are reversed in polarity with
respect to each other.  If they are at the same temperature, the voltages
they are producing oppose each other and the resulting voltage is zero.  In
this setup, the reference TC would be put in an ice bath to maintain it at
32F, and the measurement  TC would but where the experimenter wanted to read
temperature.  So now, we have the voltage at 32F opposing the voltage at the
measuring junction.  The meter reads the difference in the two voltages,
which is related to the difference in temperatures.  We can go into the
graphs of voltage vs. temperature and find out what the temperature is.  If
we're lazy (or not too particular about accuracy), we can just divide the
measured voltage by the average volts/degree figure for that kind of TC and
add 32 degrees.

In the VDO setup, there are three thermocouples; the Iron-Constantan one
that VDO made for you, and a Copper-Iron and a Copper-Constantan one that
you built when you connected the VDO TC to the copper cable.  Even if you
ran Iron and Constantan wire all the way to the gauge, you'd still have
these, because the gauge guts are Copper.  It's a little harder to see, but
the combined TCs in the connector act just like the reference junction
discussed above.

VDO doesn't tell us what they assume as a reference temperature, but when I
measured mine using 200F water and 95F air (Texas summer) I got about 140 on
the gauge.  So my gauge is calibrated for a 65F connector.  I suspect VDO
targeted 70F.

By the way, that's why, according to your gauge, your engine doesn't seem
run any hotter in the afternoon than it does in the morning, even though
it's hotter in the afternoon.  What you're really measuring is the
temperature rise of the engine over ambient air, which is pretty much
constant for a given load condition.  (Yes, there is a density difference,
so it might cool slightly more efficiently when the air is cold - give me a
break. <grin>).

I looked for a better explanation, but I couldn't find the article I was
looking for - web based information glut.  The following one might help.
http://www.allaboutcircuits.com/vol_1/chpt_9/5.html

Roger Gault

----- Original Message ----- 
From: "Larry Forman" <Larry@Forman.net>
To: "Roger Gault" <r.gault@sbcglobal.net>; "Corvair List"
<virtualvairs@corvair.org>
Sent: Sunday, October 24, 2004 12:51 AM
Subject: Re: <VV> CHT data


> At 09:59 PM 10/23/2004 -0500, Roger Gault wrote:
> >Remember that the VDO gauges are TC gauges, not Thermistors.  So, the VDO
> >actually reads the temperature difference between the ring and the
connector
> >near the ring (about a foot away as I remember).  When I checked mine, it
> >looked like the VDO gauge reading assumes that the connector is at about
> >70F.  So, you might look at where the connectors are, and what removing
the
> >shrouds did to the temperature in that area.  That might explain some of
the
> >wierdness.
> >
> >Roger Gault
>
> Hi Roger,
> They are WHAT??  It is my understanding these are thermocouples and as
> such, they generate a small voltage proportional to temperature RIGHT AT
> the junction of the thermocouple.  Yes, there are different thermocouple
> junctions with different curves for temperature, but none of these are
> related to the difference between the junction where the voltage is
> generated and the temperature of the connector.  If you can provide some
> additional reference for your theory, I would like to learn more about it,
> but I believe you are mistaken.  And to do this right, you should use
> thermocouple wiring for the best accuracy, but none of this would account
> for the temperature difference you are seeing, at least not related to the
> connector temps.  Everything is just related to the temperatures of the
> thermocouple junction, as I understand it.
>
> -- Larry