Thermistor used as temperature sensor

[John Popelish]

Almost all of that tolerance is in the absolute resistance of the
thermistor - a good single point calibration ( in a well stirred ice
bath where the ice has been made from distilled/de-ionised water) can
get you to accuracies of some millidegrees.

You can buy trimmed "interchangeable" thermistors which offer
unadjustaed accuracies of +/-0.2C (about +/-0.8% tolerance on
resistance). Farnell offered Betatherm parts, and Newark has parts from
YSI - including at least one offering +/-0.05C accuracy. Thermometrics
and Fenwall are also in the business.

The Steenhart-Hart fitting function is usually cllaimed to be good
enough to get millidegree accuracy over a respectable temperature
range.

That kind of function models the resistance versus temperature of the
thermistor pretty well (I other functions that I like better) but they
get pretty ugly when expressing temperature versus the output of a
divider.

 

[[email protected]]

[email protected] wrote...

Steinhart-Hart

Oops.! Sorry. Thanks. I should have checked ....

 

[John B]

Without knowledge of the application requirements, this is a ridiculous
statement.

NTCs are cheap and easy to interface. They are not especially accurate but
that may not be a
problem.

OK, I give in. All I want to do is measure the temperature of an airflow and
maintain it at 70C +/- 1C. On playing further with my Excel spreadsheet,
I've found that by using a 100k (25C) thermistor, a 14.3k ballast resistor
at the GND end of the thermistor and measuring across the fixed resistor,
the transfer characteristic is almost linear between 50C and 90C. That's
good enough for what I need.

For a heater I intend to bolt 4 off aluminium clad 150W/500R resistors to a
Marston FC-C1 fan assisted heatsink. The resistors will be run from 240VAC
controlled by an opto-isolated solid state switch with zero crossing
detection and should dissipate ~115W each.

Thanks to all for their interest.

 

[John Popelish]

OK, I give in. All I want to do is measure the temperature of an airflow and
maintain it at 70C +/- 1C. On playing further with my Excel spreadsheet,
I've found that by using a 100k (25C) thermistor, a 14.3k ballast resistor
at the GND end of the thermistor and measuring across the fixed resistor,
the transfer characteristic is almost linear between 50C and 90C. That's
good enough for what I need.

For a heater I intend to bolt 4 off aluminium clad 150W/500R resistors to a
Marston FC-C1 fan assisted heatsink. The resistors will be run from 240VAC
controlled by an opto-isolated solid state switch with zero crossing
detection and should dissipate ~115W each.

Thanks to all for their interest.

For the largest, most linear signal right around one temperature, use
a series resistor equal to the resistance of the thermistor at that
temperature, only (about 15k for the thermistor you described,
earlier. Linearity doesn't matter much when you will hold one
temperature.

 

[Pooh Bear]

Without knowledge of the application requirements, this is a ridiculous statement.

NTCs are cheap and easy to interface. They are not especially accurate but that may not be a
problem.

*They are not especially accurate*

Exactly. They are a waste of space IMHO.

Graham