Thermistors: Specifying the thermistor
The next step is selecting a thermistor. Remember, we are talking about NTC thermistors.
There are no hard and fast rules for thermistor selection, but here are some pointers.
- Many thermistor families come in a range of R25 values.Typical values are 1K, 2K, 5K, 10K, 20K, 50K and 100K.
- The higher the temperatures you want to measure, the higher the R25 value will need to be to get optimum performance, and visa versa. For example, if you are interested in temperatures around 0°C, a 1K or 2K thermistor will be appropriate. Around room temperature a 10K unit will work well and for boiling water look at a 100K. The rule of thumb is that the thermistor resistance at the temperature of main interest should be about the same as the feed resistor Rfeed.
- Thermistors come in many shapes and sizes. Some are designed for temperature measurement, while others are designed for other functions like circuit protection. (A common circuit protection use is inrush current limiting in power supplies). Obviously, you want one that’s intended mainly for temperature sensing or measurement. You also want one you can “live with” in terms of mounting, encapsulation etc. Some companies offer thermistors already made up into probes (click here, if you are online, to search), while others sell the bare component. Betatherm is a typical company that specializes in quality thermistors for temperature measurement, but there are many others. Temperatures.com provides many more suppliers plus a lot of additional information about temperature measurement.
| We are in no way associated with any of the above referenced resources and take no responsibility for their accuracy. |
- Thermistors come in many accuracy grades. Usually they are specified in terms of the R25 tolerance, with typical specs being 0.5%, 1%, 2% or sometimes 5%. As a first approximation the temperature accuracy of the thermistor at 25°C will be half the percentage tolerance, meaning a 2% thermistor will be about 1°C accurate at 25°C. That compares very well with semiconductor temperature sensors. For example, the popular LM35 has a 2.5°C accuracy for the low grade part, and 1°C for the premium part. Betatherm and several other specialist companies have thermistors with an initial accuracy of 0.05°C at 25°C
| Don’t equate the thermistor accuracy with the final measurement accuracy you will achieve! There are many other sources of error that cannot be controlled as easily as just paying out a few more dollars for the thermistor. The main benefit of buying a very high accuracy thermistor is interchangeability. |
| Once you have selected a thermistor, look up its Beta value and R25 value and enter them into the spreadsheet. |
Don’t worry if entering these numbers causes errors in the sheet. That will all get sorted out later.
Once you have entered your thermistor specification, you can move on to the next section.