What do rocket engines, food buffet lines, brick kilns, and ice cream freezers have in common? They all require precise temperature measurements for optimal performance.
Every industry requires temperature probes to measure the temperature of various processes and environments. Choosing the correct type of temperature probe sensor is critical to ensure product consistency, personal safety, and equipment reliability.
This post aims to provide information about the most common temperature probe types, their features, and how to select the right temperature sensor for your needs.
What is a Temperature Probe Sensor?
Temperature probe sensors measure temperature changes. Unlike liquid thermometers that rely on the expansion of mercury or another liquid to measure temperature, a temperature probe measures the change in the resistance of a small electrical current due to temperature changes.
Every industry uses them for monitoring and control purposes. Some examples include:
- The pharmaceutical sector monitors the storage and transport of temperature-sensitive drugs to ensure their efficacy
- HVAC systems need them to maintain a room’s temperature
- Food and beverage processes require accurate temperature control from storage to production, sometimes using industrial immersion heaters, and follow Hazard Analysis and Critical Control Points (HACCP) guidelines.
- Automotive manufacturers rely on sensor probes to monitor engine and passenger cabin temperature controls
- Medical devices incorporate sensors for monitoring the temperature of a patient’s blood or tissues.
Temperature probe sensors ensure safety, prevent problems, and optimize processes across various industries.
How Does a Temperature Sensor Work?
Temperature probe sensors measure various properties, including the intensity of a sample’s heat, its rate of heat flow, or its ability to conduct heat. Temperature sensors provide readings, either digital or analog, via electrical signals.
Temp sensors have two different metals such as copper, nickel, or platinum. Each metal generates a different electrical voltage or resistance when a temperature change occurs. Measuring the voltage provides the temperature reading. For example, when the temperature increases, the voltage also increases.
There are various temperature-sensing probes, both contact and non-contact.
Types of Temperature Probe Sensors
There are four primary temperature probe sensors, each with its unique features and sub-sets of probes.
- Thermocouple – A thermocouple measures the voltage generated by the junction of two dissimilar metals, such as copper and constantan. They connect at two different points called the “cold junction” and “hot junction.” The cold junction remains at a set temperature, and the voltage differences between the two wires determine the temperature. It’s a simple, cost-effective temperature probe sensor with an extensive temperature range of -200 °C to 1750 °C (-328°F to 3182°F ).
- Resistance Temperature Detector (RTD): An RTD is a type of temperature probe sensor that is more accurate, stable, and linear than a thermocouple. They consist of tightly wound coils of high-purity conducting metals like platinum, copper, or nickel. Platinum RTDs are the most accurate and expensive. Their electrical resistance is similar to a thermistor temperature sensor.
- Thermometer – Thermometers typically measure temperature based on a liquid’s or gas’s expansion and contraction. Bi-metal thermometers use a spring that reacts to the movement of the metal inside the probe’s tip. An Infrared (IR) thermometer is a non-contact temperature probe sensor. It measures the temperature of a surface or object by the infrared radiation it emits. It is the best choice for temperature measurements in applications where contact with the object is impossible or undesirable.
- Thermistors – Thermistor probes use the change in resistance of a semiconductor material (oxides of nickel or manganese/cobalt coated in glass) to measure temperature. They have fast response times and high accuracy. We’ll discuss these in more detail below.
What is a Thermistor?
The word Thermistor is a contraction of two words, Thermal and Resistor. They are electrical resistors that are temperature sensitive.
A thermistor senses temperature changes using sintered semiconductor material that exhibits a significant difference in resistance in proportion to a slight change in temperature. It measures the resistance or voltage drop of DC (Direct Current) passing through it.
There are two types, Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). The resistance in PTC thermistors increases proportionally to the temperature increase. NTCs are the opposite.
NTC Thermistors vs. Resistance Temperature Detectors (RTDs)
The main difference between a thermistor and an RTD is the temperature range. The RTD is the only option if an application requires temperatures above 130°C (266°F). For applications below that temperature level, thermistors are more accurate.
Which Type of Sensor Is Best For You
There are several considerations when purchasing a temperature probe sensor for your application.
Probe Accuracy – Accuracy is critical if you need to record temperatures. Even a few degrees can make a big difference in processes like sterilization.
Working Environment – You’ll want to consider the environment of the probe’s placement. Will it be immersed in a fermentation tank or attached to the inside of a conveyor oven?
Initial and Replacement Cost – Temperature probe sensors can vary widely in price. Why pay for a platinum RTD when a lower-cost model works?
Choose a thermometer sensor that meets the temperature and accuracy requirements of the specific process.
How to Choose the Right Temperature Probe Sensor
Some temperature probe sensors measure the air temperature, while others monitor liquid or solid substances. So which is suitable for your application?
Features to Consider for Temperature Sensors
There are a few characteristics that can help you choose the best type of temperature probe for your application.
Accuracy – Probe accuracy is critical for most applications. Check the manufacturer’s specs for performance if a single degree makes a crucial difference to your process.
Working Environment – Consider your sample’s properties, like its moisture level. Will the sensor probe contact liquids, such as when monitoring over-the-side immersion heaters? Will it contact extreme cold or heat?
Temperature Range – Choose an accurate temperature sensor that can handle the heat range of your application. The sensor may fail or deliver erroneous information outside its rated capacity.
Response Time – Response time is a measure of how quickly a sensor responds to changes in temperature. It can vary depending on the sensor’s size and type. For example, thermocouples have a rapid response time, while thermistors are slower.
Response Range – The response range is the minimum and maximum at which a sensor can respond to a change in temperature. For example, a response range of 0 to 100°C means the sensor will be inaccurate outside that range.
Cost – Consider the cost vs. the process application, as temperature probe sensors can vary widely in price. Sometimes you need the best for greater accuracy, responsiveness, and longevity. Other applications need not be as precise.
Weighing these factors will help you choose the suitable sensor type for your needs.
Tips On Using and Caring for Temperature Probe Sensors
As with any equipment, it’s essential to maintain temperature probe sensors to ensure longevity and reduce costs. Here are a few tips to keep them working longer:
- Use the right probe type for the sample
- Make sure the probe inserts fully into the sample
- Be careful not to kink or break the wires of the sensor
- Avoid extreme temperatures outside of the sensor’s range
- Keep it clean
- Store in a clean, dry location away from chemicals
- Replace the sensor if it shows signs of wear and tear
- Avoid having it in contact with high voltages, currents, or strong electromagnetic fields
Trouble Shooting Heater Sensors
If temperature sensors are not working or giving false readings, you’ll find some troubleshooting tips in our Heater preventative maintenance article. Usually, a connection has come loose in the control panel, or the sensor may need replacing.
Temperature Probe Sensor Applications
A quality temperature sensor can provide accurate and reliable results in a wide temperature range while withstanding temperature variations. Install probes designed for the specific environment, such as using plastic-covered probes instead of metal for corrosive gases or fluids.
This article, “Get immersed in immersion heaters,” can help you choose the right immersion heater and guide you to the suitable probe for your application.
Safe, evenly distributed heat for your temperature-sensitive fluids with Powerblanket's Immersion heaters.
