<VV> CHT data (No Corvair)

[John]

Larry you have do not have a complete understanding of the physics
involved. Thermocouples measure differences in temperature, not absolute
temperature.
Thermocouples produce a voltage that is proportional to the difference
in temperature between the hot junction and the reference junction.
Because thermocouples measure relative temperature differences, cold
junction compensation is required if the system is to report absolute
temperatures. Cold junction compensation simply means knowing
the absolute temperature of the cold junction and adjusting the reported
temperature value accordingly.

Confused?  Each thermocouple instrument or gauge actually has two
thermocouple junctions, one real (the temperature probe) and a second
one (sometimes real but usually electrically simulated).  Consider a
simple thermocouple circuit, two lengths of dissimilar wires twisted
together at each end and fused together (welded usually).  Assume one
end is in an ice bath mixture and the other is the temperature sensor. 
When the sensor end is heated a current will flow in the circuit.
Electrons will flow from the heated end to the cold end.  Measure the
current flow and knowing the type of wires, say Iron and Constantan, you
can determine the temperature of the probe relative to the ice bath. 
Since the ice bath is at 32F you also know absolute the absolute
temperature of the probe end.

In a practical instrument you can not have a reference ice bath.  You
create an electronic equivalent cold junction.  The absolute accuracy
depends on the accuracy of the electronic circuit simulating the Iron -
Constantan or other junction characteristics.

VDO gauges are simple inexpensive thermocouple devices.  They have the
reference cold junction inside the gauge.  The absolute gauge
calibration assumes some reference temperature, most likely the 70F
Roger quotes.  It's the equivalent of the ice bath but is much less a
known temperature reference.  In fact it varies as much as the interior
temperature of the automobile.  There are other wiring complications,
but as long as the connector junction metal pairs are of the same type
and are at the same temperature the errors cancel.

Anyone interested in a more depth understanding can send me their e-mail
address for a thermocouple article with circuit diagrams and charts of
thermocouple characteristics.

John Dozsa


> Date: Sat, 23 Oct 2004 22:51:42 -0700
> To: "Roger Gault" <r.gault@sbcglobal.net>,
>    "Corvair List"
>   <virtualvairs@corvair.org>
> From: Larry Forman <Larry@Forman.net>
> 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