The temperature probes that could be used for this purpose are as versatile as your applications. In measurement and control technology and in the control of processes and systems, correctly recorded temperatures are crucial for the trouble-free operation of the systems. In heating systems, temperature probes ensure the correct recording of the outside or room temperature or the correct temperature in buffer storage tanks or on heat pumps.
In this guide, we have compiled information on what you should pay attention to when selecting, purchasing, installing and operating your temperature probe.
What is a temperature probe?
A temperature probe is an electronic component that measures the temperature and transmits it as an electrical signal to the display, the control unit or the heating system. Based on the measurement task, the specification of the probe must fit exactly so that the measurement technology also functions reliably and stably.
The following factors must be taken into account
- In which medium is the measurement to be made?
- How is the measurement to be performed (penetration measurement, surface measurement, etc.)?
- How should the temperature probe be installed in the process, i.e. which process connection is required?
- In which temperature range should the measurement take place?
- How fast should the measurement take place?
- Are there any other influencing factors, such as steam, vibration, etc., that affect the temperature probe?
How does a temperature probe work?
The functionality of temperature probes is primarily dependent on the temperature probes installed. They determine the way in which the physical measurand temperature is converted into a signal. Basically, a distinction can be made between resistance-based temperature sensors and thermocouples.
Resistance-based temperature probes
Temperature probes with resistance-based temperature sensors indicate the temperature by a change in the resistance value. Therefore, these sensors are also called Resistance Temperature Detector. (RTD)
RTDs are differentiated depending on whether the resistance rises (cold conductor = PTC) or falls (hot conductor = NTC) with increasing temperature.
Classic PTC thermistors are, for example, platinum sensors such as Pt100 or Pt1000, but also nickel sensors or KTY sensors. These sensors consist of precious metals such as platinum. They increase their resistance with increasing temperature. The advantage of these sensors is that their characteristic curves are linear and standardized. This means that these sensors can be replaced regardless of the manufacturer. In addition, these sensors measure very accurately depending on the class and resistance value. However, they may be somewhat more expensive than NTC sensors. RTDs have a longer response time and the self-heating of the sensor must be taken into account so that this does not produce a falsified measurement result.
At Testo Sensor we offer the following PTCs (temperature sensors) depending on the temperature probe:
- Pt100 Sensors: Pt100 Class A, Pt100 Class B, Pt100 1/3 Class B, Pt100 1/10 Class B
- Pt1000 Sensors: Pt1000 Class A, Pt1000 Class B
- Pt500 Sensor: Pt500 Class B
- Ni Sensors: Ni1000
In addition to sensors made of platinum and nickel, there are also KTY sensors. These are measuring resistors made of silicon. They are characterized by a particularly fast response, a low tolerance as well as a high long-term stability, but have a limited measuring range of -50 °C and 150 °C only.
We offer the following KTY sensors:
- KTY sensors: KTY 81-210, KTY 81-110
Unlike PTC thermistors, NTC thermistors reduce their electrical resistance as the temperature rises. Thermistors consist of metal oxides or semiconductors. Due to their individual composition, these sensors do not have a standardized characteristic curve according to a (DIN) standard. Therefore, if you want to use such a sensor, you must know exactly the characteristic curve of the sensor. Only if the characteristic curve of the sensor (and thus of the temperature probe) matches the characteristic curve stored in the measuring instrument can the measurement be carried out accurately and without interference.
You can find our guide to thermocouples here.
What are the different types of temperature probes?
Temperature probes are as different as the applications in which they are used. For this reason, there are many different types. On the one hand, the type differs according to the type of measurement: temperature probes with a handle are used for insertion or random sample measurements. If, on the other hand, the temperature probe is to be firmly inserted into the process, then screw-in probes or immersion probes with cable, connector or measuring head are often used. The materials used, such as the cables, may determine the temperature ranges in which the probe can be installed. At Testo Sensor we offer the following temperature probes:
You will also find an extensive selection of types for thermocouples:
In each type we have different versions, so that you can really find your temperature probe for most measuring tasks. However, if there are any open requests or you cannot find the type you need, please contact us.
In which applications are temperature probes used?
Temperature probes are as different in their design and type as the fields of application in which they are used. For example, in measurement and control technology and building automation and in the control of processes and systems, the temperature probes measure the temperatures correctly and thus enable the systems to work without malfunctions. In heating systems, temperature probes ensure the correct recording of the outside or room temperature or the correct temperature in buffer storage tanks or on heat pumps. They are used to control solar systems and underfloor heating systems. Temperature probes are also used in different branches of industry, such as the plastics industry, the food industry, and in the production of medicines in the pharmaceutical industry.
What should I pay attention to when selecting the right temperature probe?
The correct selection of the temperature probe depends on your measuring task and the associated requirements for the temperature probe. We often hear questions like:
- Is the probe also compatible?
- What type of sensor should I choose?
- Can I change my temperature probe?
- Is this the right sensor?
- Which cable do I need or also can I replace the cable?
Answering the following questions may help you make your selection:
What type do you need? Select the required type for your temperature probe by means of the categories. You will find an overview of all types here.
What is the correct sensor? Make sure whether and if so which type of sensor is specified by your measuring device / measuring point. Especially for NTC sensors, please check carefully whether the characteristic curve of the sensor matches the characteristic curve stored in your measuring device. If necessary, you will find this information in the operating instructions. The characteristic curves of our sensors can be found here in the overview or also in the operating instructions, which you can download for each sensor type.
Does the sensor fit their measuring range? In the operating instructions you will also find the measuring ranges in which the sensor may be used. Please note that the cables, connectors or housings also have a temperature range in which they may be used. If you are unsure about choosing the right sensor, we have compiled important information about the sensor here. Feel free to contact us if you have any questions.
What is the correct connection type for your temperature probe? Pt sensors and Ni sensors are also available as three- and four-wire versions. Please check your measuring device whether you need a three-wire or a four-wire version. The distance between the measuring point and the measuring device should also play a role in your considerations.
Two-wire circuits In two-wire circuits, the intrinsic resistance of the connecting lead adds to the resistance value of the measuring resistor (thermistor or Pt) and thus falsifies the measurement result. If the resistance value of the measuring resistor is large compared to the intrinsic resistance of the connecting cable, this intrinsic resistance can be neglected. However, for measuring resistors with a lower resistance value (e.g. PT100), the intrinsic lead resistance can significantly reduce the measuring accuracy. For this reason, we recommend the two-wire technique in conjunction with low-resistance measuring resistors only if you can use comparatively short connecting leads, i.e. if you only have to overcome short distances between the measuring point and the measuring device.
Three-wire circuits For applications that require very accurate temperature measurement, we recommend that you use a three-wire or even four-wire circuit. In the three-wire circuit, the lead resistance of each of the three connected cables is determined using the third connected cable (all connecting wires must be exactly the same length) and then subtracted from the measured total resistance (measuring resistance + lead resistance), so that effectively only the resistance value of the measuring resistor is used for temperature determination. This subtraction can be performed automatically in the measurement electronics, for example.
Four-wire circuits The safest and most accurate measurement is achieved with the help of a four-wire circuit. Here, an additional cable is connected on each side of the measuring resistor. Two cables are used for the voltage supply, the other two are used to determine the line resistance (if necessary also asymmetrically, i.e. also with connecting wires of different lengths) and to subtract it from the measuring resistor value. This subtraction can be performed automatically in the measurement electronics, for example.
How do you ensure the correct mechanical connection of the temperature probe to the process? Correct mechanical connection of the temperature probe to the process ensures efficient, error-free and trouble-free measurement.
Thread size / process connection
For Screw-in probes, please make sure to select the correct thread size and use the thread according to the instruction manual.
Sleeve length or neck tube length
For temperature probes with variable sleeve length or neck tube length, please measure in advance what length you need. Each probe requires a minimum immersion depth to accept and correctly measure the temperature of the medium being measured. As a rule of thumb, the following advice can serve you: optimally, the immersion depth should correspond to 10 times the thermowell diameter, but at least a 5-fold value should be used.
Please consider which diameter you need: When measuring temperature in solids or when the probe is mounted in an immersion sleeve, the air surrounding the probe slows the conduction of heat from the medium to the probe. This also slows down the temperature measurement and, in extreme cases, leads to incorrect readings. For this reason, please ensure that the temperature probe is coupled to your measurement medium in the best possible way in terms of heat transfer, i.e. select the sleeve diameter so that the air gap is as small as possible.
More detailed information on mounting and immersion depths is provided in the operating instructions, which you can download for each probe.
Please also pay attention to whether you have certain requirements at the measuring point or in your process regarding temperature, pressure, IP protection, steam or similar and compare your specification with our data sheets. If you have any questions or are unsure, please feel free to contact us.
What do I have to consider when mounting the temperature probe?
If you are wondering: Where should I mount the probe?
Or how can I mount the temperature probe?
Then take a look at the operating instructions. Because the mounting is as different as the probes themselves and also differs depending on the probe design, we have stored in the operating instructions of the respective probe exactly where and how the probe is to be mounted and what you must pay attention to during mounting. You will find the operating instructions with each probe or in our download area.
What accessories do you need for your temperature probe?
Please check the delivery details for each temperature probe to see what is included and consider whether you need any accessories when placing your order.
We offer the following accessories:
Replacing a defective probe
When replacing a defective temperature probe, please pay close attention to which temperature sensor you require. Sensors such as PT100, PT1000, KTY, NTC or thermocouples are not compatible with each other. Therefore, please check the technical data of the respective system which sensor type is required. If the required information is not apparent in the operating instructions of the measuring instrument / device, the manufacturer of the machine or system should be contacted in case of doubt.
Once you have found the correct sensor, please also test the mechanical connection. The temperature can only be measured correctly if the temperature probe can be attached to the mounting location in accordance with the regulations. Otherwise, machines and equipment can easily malfunction.
Please also make sure that you order thermal paste and mounting accessories such as compression fittings, cutting ring screw fittings or bayonet nipples if these also need to be replaced.
Frequently asked questions about temperature sensors
How do I check the function of a temperature probe?
With resistance temperature probes you can measure the resistance with a multimeter, because these probes have a defined ohmic resistance. You should be able to find this value in the technical data or in the operating instructions of the probe.
The situation is different with thermocouples: If you cannot find a cable break or technical defect, then you need special multimeter with appropriate measuring input (usually designed as type NiCr-Ni of type K). Alternatively, you can replace the probe as a test. or a trial replacement of the probe.
When is a temperature transmitter needed?
Transmitters are used when the temperatures of distant measuring points are to be recorded. In this case, the cable length does not have a negative effect on the measurement result. In addition, transmitters are necessary when the evaluation electronics only allow standard signals (e.g. 0 - 10 V or 0 - 20 mA) at the input.
How can I tell that my temperature probe is defective?
The temperature probe shows strange values? The temperature probe does not show any values? This behavior may indicate that your temperature probe is defective or that it has not been connected correctly.
To find out if there is a defect, you can simply measure the resistance value of resistance-based temperature probes and compare it with the specification on the probe itself or in the user manual. You can use a standard multimeter to do this.
If you are using a thermocouple instead of a resistance-based temperature probe, the situation is somewhat different. Here you need a multimeter with an appropriate connector for thermocouples, because thermocouples do not have a resistance signal, but a voltage signal.
Alternatively, you can test on a trial basis whether a replacement will remedy the situation.
How do I connect the temperature probe correctly?
When connecting your temperature probe, please make sure that you follow our wiring diagrams. You will find the wiring diagrams in the operating instructions for your temperature probe.
Which extension cable can be used for temperature probes?
When selecting your extension cable, pay attention to the environment in which the cable will be used. Not every cable can be used for every application and in every temperature range.
Therefore, please make sure that the technical data of the connection cable, especially regarding temperature and IP protection class, match your application. Detailed information can be found in our data sheets.
Temperature ranges and IP protection classes of our cables:
PFA: -50 °C to +260 °C - IP 67
PVC -30 °C to +105 °C - IP 67
Silicone: -50 °C to +180 °C- IP 67
Glass fibre: -50 °C to +400 °C - IP 20
For very long connecting cables, please note the influence of the intrinsic resistance value of the cable on the measuring accuracy. For probes with Pt or Ni measuring elements, we recommend the use of a three- or four-wire circuit in this case.
For thermocouples you have the possibility to use thermocouple leads or compensating leads. Please pay attention to their measuring range and that the cable fits to your thermocouple.
How can I protect a temperature probe from certain influences?
Select a temperature probe that is well suited to your measuring task. Depending on the medium in which you want to measure, you will need a different probe design. We have specified exactly which material we use in our operating instructions so that you can check for yourself whether the probe is suitable. Please pay attention to the measuring range and other environmental conditions such as dust, water, steam, solar or UV radiation.
We offer immersion sleeves in our accessories, which can protect the probe. In addition, especially for outdoor probes, you will also find a sun shield and a ball throw protection to shield your outdoor probes from environmental influences and to obtain the correct measurement result.
For easier orientation, we have always indicated suitable accessories in the respective operating instructions of the temperature probe.