Source: Sensors Mag Online article
by Ashley Johnson, content writer for Ametherm Inc., A thermistor probe is otherwise identified as a bead thermistor manufactured as a bare bead encapsulated with epoxy or glass that is usually potted in a metal or plastic housing with two high temperature insulated wires. The following article will explain its function, needs and differences in use between the housings.
NTC Thermistor probes have been used as far back as 1940 to measure and analyze temperature, but today they are indispensable to everything from home appliances to electric vehicles that require li-ion batteries. They have long been employed to measure liquid levels and temperature in industries that extend from automotive electronics to medical technology to green energy. Their applications are endless, and their purpose is significant.
NTC thermistor temperature sensors come in all types of housings including closed end metal tubing, threaded metal tubing, and housings with a threaded tip and hex screw (shown here).
A thermistor probe is otherwise identified as a bead thermistor manufactured as a bare bead encapsulated with epoxy or glass that is usually potted in a metal or plastic housing with two high temperature insulated wires. These devices can be found in all the places that we live and work, from the moment we wake up and make coffee until the end of the day when we arrive home and relax in front of the television. They are indispensable to the cars we drive to work, the computers necessary to do our jobs, and the appliances we use to prepare food at the end of the day.
Understanding the Impact of Thermistor Probes
Because of their prevalence, it is important to understand how thermistor probes operate, in what capacity, and what their potential is. Part of this understanding includes considering the electrical properties of probes that abide by certain operating principles, including electrical resistance, capacitance, ultrasonics, radioactivity, and pressure and thermal conductance.
Given their pervasiveness and range of applications, and despite the large amount of thermistor sizes and shapes, it is impossible to manufacture a universal thermistor probe. The type of thermistor used in automobiles to measure coolant temperature is vastly different from the type of thermistor used to measure temperature for wind turbines.
Thermistor Probes in Automotive Innovation
Without thermistor probes many automobiles, appliances, industrial equipment, and anything that requires electricity essentially would not function properly, or at all. Engineers depend on these devices as a design consideration for any product that generates energy, hot or cold, and requires temperature measurement, control, and accuracy.
In 1966, automotive vehicles used around two thermistors per unit to report to various gauges and electronic control modules. In the 1970s that number grew to 15, in the 1980s to 40 thermistors, and in the 90s there were 60 thermistors in vehicles. Today, automotive vehicles use between 80 to 100 thermistors.
This exponential increase in thermistors has mainly been fueled over time by the complexity and refinement of technology in automotive vehicles. As automotive technology has increased and improved, so has the demand for thermistors. The need to detect, regulate, and control temperature in vehicles can be seen in car seat heaters and air conditioners, oil temperature gauges, defrosters for windows and mirrors, li-ion batteries in electric vehicles, and even just the heating and cooling systems that regulate and control cabin temperature of vehicles.
Additionally, more features and processes require sensing to improve vehicle performance, driver convenience, automotive safety, and reduce or control emissions. Vehicles today exhibit more artificial intelligence, part of which is the need to sense and regulate temperature.
As an example, thermostats that in the past were used to measure coolant temperature and control cabin climate temperature now have been replaced with a thermistor, which has a faster response time as well as being more accurate. Another indispensable application of thermistors in automobiles is the ability to detect oil levels and viscosity and then alert the driver or operator when to change it.
Green energy, medical technology and other applications for thermistor probes
In the medical field thermistor temperature sensing probes are critical because of these devices’ high sensitivity, high accuracy, and interchangeability. A good example is a respirator. Air entering a patient’s lungs must be exactly 37 °C, the same as body temperature. A thermistor senses the air temperature and sends a signal to the respirator to regulate and compensate as needed.
In our homes, thermistors are everywhere in microwave ovens, toasters, dishwashers, air conditioning units, portable heaters, washers and dryers, bidets, hot tubs, saunas, refrigerators, electric kettles, to name a few. Without a thermistor to sense heat, a microwave oven is prone to overheating. Same with portable heaters, which also use thermistors, could possibly get too hot and start a fire.
NTC Thermistor probes are heavily used in green energy applications that include wind turbines and solar panels to monitor and control temperature in colder climates by the ocean or that receive snow and ice.
Green energy is an area where the use of thermistors is making great strides. Wind turbines are a great example where the use of thermistor probes is essential. Wind turbines in colder countries or environments are prone to freezing. A thermistor probe used on the fan blade with two very long leads regulates and senses the air temperature to control the on/off actions of the wind turbine’s heater so that the blades won’t freeze. If the blades freeze, this creates inefficiencies and makes the entire unit inoperable.
How to select and integrate thermistor probes into different applications
Like the limitless applications requiring thermistor probes, so too exist the endless methods to mount them, adhere them, and apply them. Some thermistors can get very sophisticated in the ways that they are housed and the methods in which they are mounted.
When talking about a probe, this is referring to the metal or plastic assembly housing that covers the bead thermistor for different applications and functions. These probes come in all shapes, sizes, and mounting options. Some probes are long and narrow with a stainless-steel housing that are used in vehicles for measuring oil or water temperature. Other thermistor probes integrate a threaded tube that are used in engines or medical equipment to detect and monitor temperature. Probes with a design that features a ring lug are often used together with heatsinks or lithium-ion batteries.
Each thermistor probe starts out as a chip that is then encapsulated in either glass or epoxy. Depending on the application, this bead thermistor is then potted in a metal or plastic housing that is customized according to very specific requirements.
When used with an application such as a lithium-ion battery, a thermistor probe with a ring lug can be easily mounted securely to the battery terminal. This thermistor probe can then effectively monitor the battery temperature, so it does not rise above a certain degree.
In the chart below, we illustrate different thermistors and the housing with which they are compatible.
NTC Thermistor versus housing compatibility chart.
About the author
Ashley Johnson is the content writer for Ametherm Inc., based in Carson City, which produces NTC thermistor temperature sensors and inrush current limiters. She has been a writer, marketing communications professional, and editor in the tech industry for more than 10 years.