Electronic – Alternative to NTC Thermistor

It's possible the manufacturer has no idea. The beta, as mkeith comments, is given, however the two temperatures at which beta is specified is not. It should be something like 25/50, 25/85 or 25/100.

I suggest you take a good sample of the instrument and adjust the input resistance with a pot until it reads 25, 50, 75, and 100°C. Read the pot with a digital ohmmeter (the first number should be 10K). Then you will be able to match a proper thermistor datasheet.

Or you could just buy some other no-spec sheet piece of 'stuff' that has the string '3950' in the description and hope it works well enough for long enough.

A reasonable and moderately educated guess would be that it's \$\beta_{25/50} = 3950K\$ but definitely confirm it.

That would imply it would read exactly 50.0°C at 3.5882 K, ideally.

Let's have a voltage divider with 10VDC input

^{simulate this circuit – Schematic created using CircuitLab}

If R1 is the NTC and R2 is an ordinary resistor, Vout increases when the temperature increases. If R2 is the NTC and R1 is an ordinary resistor, Vout decreases when the temperature increases.

Generally Vout = Vin*R2/(R1+R2) if we think the output current =0. If there's some substantial load, the formula is more complex.

The hard part: How we solve the temperature if we know the Vout and the other resistor? It's quite complex calculation and we must have the curve how the resistance of the NTC depends on the temperature

NOTE: Vout = 0V does not mean 0 degrees and Vout changes not 1V per a degree. The latter is called also "Vout has non-linear dependence on the themperature"

In the past very complex circuits were developed to make the dependence between the voltage and the temperature simpler. An example:

^{simulate this circuit}

The selection of the components requires a strong math. But it's possible to achieve the following:

• the voltmeter scale has zero degrees in the wanted position
• the needle in the voltmeter turns nearly linearly as the temperature changes. This means the scale has near equally wide degrees.
• the calibration for an individual NTC is possible by having some resistors as trimpots.
• By having R1 as a trimpot, the aging of the battery could be compensated (needs also a switch and a fixed resistor in place of the NTC during the calibration)

This is a typical bridge for resistor-like physical sensors. The exact calculations are beyond the scope of this answer.

The operational amplifier circuits increase the possiblities and reduce a little the complexity of the calculations. The bridge can be replaced by a differential amplifier.

A computer (input via an ADC) moves all difficulties to the software. It does not remove the need of strong math, but in software the temperature-ohm-dependency can be taken into the account exactly. This is the used way also in the digital multimeters that have a temperature measuring range for a specific probe.

Your other resistor: nearly 5 kOhm, for example 4,7 kOhm is a good choice because

• it makes the voltage change big per a degree near 25 degrees and the dependence is quite linear, too, but not exactly linear
• at 25 degrees you have 5 volts room to both directions

Read other articles in this site (there are several) and find some application notes and circuit diagrams.

ADDENDUM due the comment: Self-heating is possible to compensate or reduce to be non-disturbing.

Reduction: Switch the Vin off for most of the time or make the thermal contact to the ambience more effective

Compensation: by hard math. If the self heating does not cause substantial heat generating or heat consuming extra processes, you can assume a known heat source in a linear medium. This requires also the time been taken into the account.

I recommend the reduction. Measure once per a second a 10 ms period and keep the Vin off for 990 ms. The self heating is reduced 99%.