Re: Intercooler Outlet Temperaturesensor ; What is it ?

["Scott Mo." <scottmo@teleport.com>]

Eric,

> I thought thermistor's resistance decreased with increasing temps?  >They react opposite of normal resistors to heat.

This is true, the typical thermistor is of the Negative Temperature
Coefficient (NTC) design, (resistance goes down with increasing
temperature) my reference book describes thermistors as follows:

"A thermally sensitive sold-state semiconducting device made by
sintering mixtures of the oxide powders of various metals. As its
temperature is increased, the electrical resistance (typically)
decreases. Negative temperature coeficient (TC) types show an
exponential decrease in resitance as temperature increases. Positive TC
types show an increase with increasing temperature. Also called a
thermal resistor."

Another reference book refers to the thermistors as NTC devices and the
PTC types as "semiconductor temperature transducers".
 
It further explains that the "semiconductor temperature transducers"
physically look like 1/4 watt resistors and their nominal resistance
output ranges between 10 ohms and 10K ohms. This appears to match the
one used in the intercooler outlet on the Audi 1986-91 5000/200 and in
the Audi V8. 

Using the term "thermistor" for a PTC type device does not appear to be
the normal convention.

"Thermocouples" on the other hand, are a different animal altogether,
and output a voltage dependent on temperature. They are typically made
by using two wires of different alloys and welding the ends together to
form a thermocouple junction.

A basic description of how thermocouples work and how they are used with
some hand held Digital Multimeters can be found at:

http://www.tek.com/Measurement/cgi-bin/frameit.pl?http://www.tek.com/Measurement/App_Notes/dmm916/eng/

I worked with the engineer who wrote this article during my stint at
Tektronix as the product mananger for the handheld Digital Multimeters.

HTH
-- 
Scott Mo.
1989 200TQ
1988 5000TQ
1966 VW Beetle
http://www.teleport.com/~scottmo