The temperature probes that could be used 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 need to consider when selecting, purchasing, installing and operating your screw-in probe.
What is a screw-in temperature probe?
Screw-in temperature probes are used in process measurement technology. Due to the screw-in thread and the correct sealing geometry (O-ring), the temperature probes can be inserted into the process permanently, pressure proof and vibration resistant. There they measure the temperature in gaseous or liquid media. Temperatures of up to 400 °C can be recorded in pipelines, containers or plants.
Screw-in temperature probes can be individually adapted to your application.
For this purpose, we offer different threads, such as G1/2 ", G1/4 " and G3/8 " and fine threads M4, M6, M8.
Screw-in probes in PVC, silicone, PFA and glass fibre cable are also available.
Please note the following points for your selection
- What is the measuring range, i.e. in which temperature range should the measurement take place?
- In which ambient temperature will be worked? (The heads, housings or connecting cables, just like the sensors used, also have a temperature range in which they may be used).
- Which other environmental conditions prevail (humidity, steam, pressure, vibration)?
- In which medium shall be measured?
- How should the temperature probe be installed in the process, i.e. which process connection is required?
- How much space do you have for the installation of the probe?
- How much process safety do you need (e.g. should you use a double thermocouple)?
What are the different types of screw-in temperature probes?
You can find Screw-in probes with cable. Screw-in probes with head or housing can be offered on request.
For screw-in temperature probes with cable we offer the following different process connections / threads.
Screw-in probe with metric thread:
With these probes, you can determine which mounting length you need and thus determine the immersion depth for these probes. We offer the following mounting lengths (in mm):
50, 100, 150, 200, 250, 300, 400, 500
Screw-in probe with fine thread:
We offer the Screw-in probes with the fine threads with the fixed mounting length of 17mm.
You can choose from the following sensors for all Screw-in probes with cable:
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
Thermocouple type J, Thermocouple type K
Other sensors may be available on request.
What should I pay attention to when selecting the right screw-in temperature probe?
The right choice of 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:
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What type do you need? How should the sensor be inserted into the process?
Screw-in probes can either be inserted directly into the process by connecting them to the existing connection using the screw thread. In this case, please make sure to select the correct thread.
Which threads we offer you can find here: Screw-in thread
For measurements in particularly aggressive media or in closed circuits (e.g. with pressure), you can also insert your Screw-in probe into the process by using an immersion sleeve. The immersion sleeve provides additional protection and also makes it easier to change the temperature probe later. We offer an immersion sleeve with internal thread suitable for Screw-in probes with G1/2 " thread.
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What is the right sensor?
In our online store you will find the following sensors for Screw-in probes: Pt100 class A, Pt100 class B, Pt100 1/3 class B, Pt100 1/10 class B, Pt1000 class A, Pt1000 class B, Pt500 class B
Please pay attention if and if yes which kind of sensor your measuring device / measuring point specifies and please check exactly if the characteristic curve of the sensor fits to the characteristic curve which is 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.
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Does the sensor fit to your 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.
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What is the correct connection type for your screw-in temperature probe?
You can order the Pt sensors from us as two-, three- or four-wire versions. Please check at your measuring device / at your measuring point 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 lead, 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 cover short distances between the measuring point and the measuring instrument.
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 total measured resistance (measurement resistance + lead resistance), so that effectively only the resistance value of the measurement resistance 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 using 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 measured resistance value. This subtraction can be carried out automatically in the measurement electronics, for example.
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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 operating instructions.
Mounting length
For temperature sensors with metric screw-in threads (as G1/2", G1/4" and G3/8") you can choose the mounting length. Please measure in advance which mounting length you need. Each probe requires a minimum immersion depth in order to accept and correctly measure the temperature of the medium to be 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.
More detailed information on installation and immersion depths can be found in the operating instructions, which can be downloaded for each probe.
Environmental influences
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.
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What do I have to consider when mounting the temperature probe?
Please check whether you need accessories for the correct mounting of your Screw-in probe. If necessary, thermal paste can improve the conductivity and thus the response time. Maybe you also need a suitable immersion sleeve?
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Which accessories do you need for your temperature probe?
Please check the delivery details for each temperature probe to see what is included in the scope of delivery and consider whether you need 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 can I tell if my temperature probe is defective?
The temperature probe shows strange values? The temperature probe does not show any values? This behavior may be a sign 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 cables or compensating cables. Please pay attention to their measuring range and that the cable fits to your thermocouple.
