I have a small digital thermometer that has a 12 foot length probe with thermistor at end. Since the area where I need to see temperature readings from outdoors is much greater than this, I have been wondering about increasing the probe length to between 50-100 feet. Can this be done and what affect would it have on the accuracy?
I've used thermocouples with "long" leads - like 4 feet; but I don't really know how they would work with really long leads, like 100' - but it seems to me that if you use the right wire (and it is, in fact, a thermocouple), in a twisted pair, it shouldn't make that much difference; but I'm no authority there - I'll need somebody to corroroborate this or correct me.
It is not too bad for medium to high R value thermistors (a few kOhm to several 10's of kOhm). The resistance of the wire will produce an offset component and the resistance combines with the temperature coeffecient of the wire to produce a scaling error. This can be calibrated out. Thermocouples using thermocouple wire can be remoted to 100' as well. Good "cold" junction compensation is required though.
Gegen dummheit kampfen die Gotter Selbst, vergebens.
Actually most of the inexpensive thermistor thermometers use the thermistor resistance to charge and discharge a capacitor (comparing that with a precision reference resistor) and there will be some effect from cable capacitance, particularly at low (measured) temperatures.
Thermocouples can be run to essentially unlimited length, and there is no special need for cold junction compensation based on lenght.. the important factor is the input current and impedance of the thermocouple meter, which places an upper limit on the loop resistance of the sensor for a given level of accuracy.
Best regards, Spehro Pefhany
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