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Old 08-28-2007, 10:34 AM   #25
ride5000
Scooby Guru
 
Member#: 32792
Join Date: Feb 2003
Chapter/Region: NESIC
Location: lincoln, ri
Vehicle:
2003 GGA MBP
12.9 / 105+

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yes, you're missing something too.

a thermISTOR presents a varying RESISTANCE wrt temperature. they may have negative or positive temperature coefficient characteristics. positive means resistance goes up proportionally with temp, and negative means resistance goes down inversely proportionally with temp. basically every conductor in the world exhibits varying resistivity wrt temperature. those elements specifically used for measuring temperature have very carefully chosen characteristics to improve linearity. in order to "read" a temperature from a thermistor you have to read the resistance. a resistance DOES NOT GENERATE A VOLTAGE--it is a passive element that MUST have current flow through it in order to determine the value of resistance.

a thermocouple relies on the seebeck effect which basically states that a temperature gradient along ANY conductor will generate an electrical potential across it... aka VOLTAGE. under normal circumstances this potential is minuscule and is partially offset by attempts to measure it. by using two dissimilar metals the electrical potential can be increased which improves resolution. this is typically called a thermocouple since it is made of two dissimilar metallic conductors COUPLED together. it is important to note that the thermocouple does not "measure" the absolute temperature at the junction itself since there is no temperature gradient there. it actually measures the gradient along the lengths of the connection leads. this is why you must use the proper wire to extend thermocouple leads. you can literally take a thermocouple and measure the voltage across its leads with a garden-variety multimeter. however this voltage must be compensated for in order to arrive at a more accurate temperature. in the old days it used to be performed by placing the "local" ends of the thermocouple wires in an ice bath which will self-maintain at 32*F... a known temperature. then the difference in temps across the lengths of the TCs could be calculated. these days this task is typically performed by a thermocouple interface chip which carries out the "ice bath" compensations by electronically measuring its own internal temperature and using that as a reference.

the bottom line:

thermistors present a changing RESISTANCE wrt temperature and must have a metering circuit which MEASURES RESISTANCE by using ohm's law, which requires imposing a VOLTAGE and a CURRENT through that unknown resistance to arrive at a value.

thermocouples present a changing VOLTAGE SOURCE wrt temperature and must have a metering circuit which MEASURES VOLTAGE. although not strictly necessary, thermocouple "adapters" perform a linearization of the voltage--temperature characteristic and correct for the local junction temperature. typical adapters will output a certain VOLTAGE per DEGREE which can then be logged with some device that MEASURES VOLTAGE.

the OEM ECU is expecting a PURE RESISTANCE between pins B16 and B19 and will use a varying voltage and current to determine it. it will not be useful AT ALL in measuring a voltage. go take a DMM, set it to measure resistance, and put the leads across a battery. tell me if the numbers make any sense whatsoever.

what is necessary--as i've said before--is to use an ecu input that is expecting an analog voltage between 0 and 5vdc. this has already been done by some folks who have used the fuel level sensor input, for example, to directly log a wideband uego controller's 0-5vdc analog output.

is that helpful enough for you?
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