Manufacturing Tolerances

Interchangeable temperature tolerances:

Interchangeable NTC thermistor temperature tolerances are expressed in ± °C temperature tolerances over a range of temperatures. For example, North Star Sensors T1A temperature tolerance is accurate within ± 0.1 °C over the temperature range of 0 °C to 70 °C. They are called interchangeable because one part is equivalent to another within that tolerance and can be used interchangeably without calibrating each thermistor individually. Temperature tolerances can go from ±0.05 °C to ±2.0°C, and temperature ranges can go anywhere from -40 °C to 150 °C.

When designing a product that will use thermistors, it is important to determine the broadest temperature tolerance and minimum temperature range where precision is required. For example a medical product may only need to be accurate to ±0.05 °C within a 34 °C to 40 °C range, and it would provide no benefit and add unnecessary cost to have the part specified to within ±0.05 °C over a 0 °C to 70 °C range. Likewise, in some applications such as equipment temperature monitoring, an accuracy of ±0.5 °C over a 0 °C to 70 °C range may work perfectly for the application and to specify the accuracy to within ±0.1 °C would be a waste.

Consideration should be given to overall system accuracy. Overall system accuracy is the summation of the thermistor tolerance plus the monitoring system tolerance. As an example, a ± 0.1 °C  paired with a system accuracy of ± 0.05 °C gives an overall system accuracy of ± 0.15 °C.

Point matched tolerances:

Point matched thermistors are specified within a certain % resistance at a single temperature point such as 25 °C. To determine the percent resistance tolerance at another temperature point, add the manufacturing tolerance to the specified point matched tolerance using the provided tables.

Here is an example to determine the percent resistance tolerance at 100 °C for a Curve 44 thermistor with a ± 1% resistance at 25 °C: 

± 1% [Tolerance at 25 °C] +  ± 3% [Manufacturing tolerance adder] =  ± 4% Resistance tolerance.

The tables containing the manufacturing tolerance adders in 5 °C increments for each curve for point matched tolerances can be found on the following page:

North Star Sensors’ Temperature Curves

North Star Sensors’  1 °C Resistance vs. Temperature Ratio Tables:

The 1 °C tables for the NTC values for each curve at each temperature next to the resistance ratio values can be found on the following page:

North Star Sensors’ Temperature Curves

To determine the resistance of a thermistor at a temperature point other than 25 °C, multiply the given ratio by the resistance at 25 °C.

NIST Traceability and Uncertainty of Temperature Standards:

North Star Sensors’ thermistors are manufactured utilizing standards with calibrations that are either traceable to the National Institute of Standards and Technology (NIST) or are derived from the ITS-90 (International Temperature Scale of 1990) fixed points:   TPHg (triple point of mercury), TPH2O (triple point of water), MPGa (melting point of gallium), FPSn (freezing point of tin), and/or FPZn (freezing point of zinc). 

In house, North Star Sensors has a triple point of water cell, melting point of gallium system, SPRT, and a set of fixed standard resistances. 

Our triple point of water cell is the same type of temperature standard used by NIST to calibrate standard platinum resistance thermometers (SPRT’s) and other thermometers requiring extremely low uncertainties.  The triple point of water is a defining temperature point for the ITS-90 with a value of 0.01 °C and an uncertainty of + 0.00000 °C / − 0.00015 °C.  

The melting point of gallium system, which provides another ITS-90 defining temperature point with a value of 29.7646  °C and an uncertainty of ± 0.001 °C .                                                                                  

The standard platinum resistance thermometer (SPRT), which is considered by NIST and other national laboratories to be a defining instrument for the ITS-90.  North Star Sensors’ SPRT is calibrated by a nationally recognized metrology laboratory and verified by in-house TPH2O and MPGa standards.  The expanded uncertainty *, U, of North Star Sensor’s primary SPRT calibration system is ± 0.003 °C with a coverage factor k = 2. 

In addition, North Star Sensors has a set of super stable temperature probes, which are secondary temperature standards used to calibrate the production temperature controlled baths to an expanded uncertainty of ± 0.01 °C. North Star Sensors has a set of resistance standards with expanded uncertainties of ± 2 ppm.  These standards are used to calibrate resistance measuring instruments to an expanded uncertainty of ± 0.02 %, which corresponds to an equivalent temperature expanded uncertainty of ± 0.007 °C.

Therefore, the North Star Sensors test and measurement system used to manufacture thermistors has a total expanded uncertainty of ± 0.012 °C and is traceable to NIST.

Note:  Each expanded uncertainty listed on this page was obtained by multiplying the combined standard uncertainty by the coverage factor k = 2, as recommended by NIST.