Calculate a thermistor B constant from two resistance-temperature points, or find resistance at a target temperature and temperature from resistance.

For an NTC thermistor: enter resistance at two different temperatures. Resistance must fall as temperature rises.

Temperature must be above absolute zero.

Temperature must be above absolute zero.

B Value Formula

The following formula is used to calculate the B value.

B = ln(R1 / R2) / (1 / T1 - 1 / T2)

Variables:

  • B is the B value (kelvin)
  • R1 is the resistance at temperature T1 (ohms)
  • T1 is the temperature T1 (kelvin)
  • R2 is the resistance at temperature T2 (ohms)
  • T2 is the temperature T2 (kelvin)

To calculate the B value, you need to measure the resistance of the thermistor at two different known temperatures. Then, apply the formula where B is the B value, R1 is the resistance at temperature T1, T1 is the first temperature, R2 is the resistance at temperature T2, and T2 is the second temperature.

What is a B Value?

The B value describes the temperature sensitivity of an NTC thermistor over a specified temperature interval. The constant-B approximation uses R2 = R1 exp[B(1/T2 โˆ’ 1/T1)], or T2 = 1/[1/T1 + ln(R2/R1)/B]. It is distinct from the B coefficient in the full Steinhartโ€“Hart equation. Check the component datasheet range and minimize measurement self-heating.

How to Calculate B Value?

The following steps outline how to calculate the B Value.

  1. First, measure the resistance of the thermistor at a known temperature T1 (R1).
  2. Next, measure the resistance of the thermistor at a second known temperature T2 (R2).
  3. Use B = ln(R1 / R2) / (1/T1 โˆ’ 1/T2), with both temperatures in kelvin and resistances in the same unit.
  4. Finally, select B constant from two points, enter all four values with their units, and select Calculate. The other solve modes estimate resistance or temperature from a positive B constant and one reference point.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Resistance at temperature T1 (R1) = 10000 ohms

Temperature T1 = 298.15 kelvin (25ยฐC)

Resistance at temperature T2 (R2) = 2500 ohms

Temperature T2 = 353.15 kelvin (80ยฐC). These example points give B โ‰ˆ 2653.91 K. Resistance falls as temperature rises, as required by this positive-B NTC model.