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increasing length of thermistor probe?

A

Albert Porch III

Jan 1, 1970
0
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?

Thanks,
AP
 
R

Rene Tschaggelar

Jan 1, 1970
0
Albert said:
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?

The accuracy is likely to be affected. A better answer
can be given upon more details provided. What is the
resistence of the sensor, is it a two wire or a four
wire measurement?

Rene
 
R

Rich Grise

Jan 1, 1970
0
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.

Thanks,
Rich
 
J

joseph2k

Jan 1, 1970
0
Albert said:
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?

Thanks,
AP

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.
 
P

Phil Allison

Jan 1, 1970
0
"Albert Porch III"
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?


** Only way to find out is to try it.

Some RG59 co-ax would be the go as an extension.

See if you can measure the probe's resistance - if more than 100 ohms your
are fine to try.



....... Phil
 
S

Spehro Pefhany

Jan 1, 1970
0
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.


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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