Are thermistors better than other sensing elements?
Yes and no. Compared to other common sensing elements such as RTDs, analog transmitters, semiconductors, and thermocouples, thermistors have advantages and disadvantages. Which is best will depend on the requirements of your application. System compatibility, operating temperature, price, physical size, and accuracy will all impact the sensing element that is best for the job.
What are the advantages of thermistors?
Thermistors are typically lower in cost and less susceptible to electrical interference than other options. They are very accurate and can be expected to operate for many years without issue. They do not need calibration or regular maintenance. Thermistors can also be used with extended lead runs and are very accurate.
What is the typical accuracy of thermistors?
The standard thermistors used by ACI are accurate to within +/- 0.36˚F / 0.2˚C accuracy within the range of +32 to +158 ˚F (0 to +70 ˚C). Because thermistors are non-linear by nature the accuracy is specified as +/- ˚ F or ˚C rather than as a percentage. ACI offers a high-accuracy thermistor in the AN and CP curves (10K type II & III) that is +/- 0.18 ˚F / 0.1 ˚C.
What are the disadvantages of thermistors?
Thermistors are less accurate than RTDs and due to their non-linearity are difficult to convert to analog or other signals. Thermistors may not be effective at extremely low or high temperatures outside of the -40 to +302˚F range.
What is the difference between the various 10K thermistors?
The only difference is the resistance curves. They all will output 10K ohms at 77˚F but will have different resistance values at temperatures other than 77˚F.
What are the maintenance or service requirements for thermistors?
None. A thermistor will follow a defined, predictable resistance based on its component materials. Since these materials do not change, neither does the resistance output. There’s nothing to calibrate or service. Under normal conditions, a thermistor has an indefinite life span.
What would cause a thermistor to fail?
As mentioned above, a thermistor does not need calibration. They either work or not, like a light bulb. If a thermistor fails, it’s likely due to one of the following causes:
- Moisture infiltration has caused a short
- Too much current was applied to the thermistor, causing it to overheat and fail
- Loose wire connection, or damaged, cut, or crimped lead wires
What are the causes of an inaccurate reading from a thermistor?
If a thermistor is not reading properly, it’s likely due to:
- Self-heating from improper airflow, or too much current being applied to the thermistor
- Input error (for example: controller is programmed for a 20K thermistor, but the actual sensor is 10K
- Sharing AC power in same cable jacket can cause interference that affects the resistance. ACI recommends separate cable using VAC in the same conduit as a thermistor
Stay tuned for Part 3 of this series where we will answer questions about identifying and choosing the correct thermistor.