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Guide screw-in thermocouple active

Active screw-in thermocouples are used in process measurement technology. The screw-in thread and the correct sealing geometry (O-ring) allow the temperature probes to be inserted into the process permanently, pressure proof and vibration resistant.

In this guide, we have compiled information on what you need to consider when selecting, purchasing, installing and operating your active screw-in thermocouple.

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What is an active screw-in thermocouple?

Thermocouples are thermoelectric sensors with two wires made of different materials. These are connected to each other at one point (measuring point) and one can measure a thermoelectric voltage at their ends (reference junction), which also changes when the temperature changes. This thermoelectric effect is also called Seebeck effect.

Thermocouples measure the temperature difference between the measuring point and the reference junction, i.e. the temperature at the reference junction must be known.

Like platinum sensors, thermocouples are also standardized and can be immersed regardless of the manufacturer. They also have a very fast response time and can be used at very high or very low temperatures. They are inexpensive and come in many different types. Thermocouples also have disadvantages: they are more sensitive than resistance-based temperature probes. Their characteristic curve is not linear. The measuring principle should be known to the user in order to avoid measuring errors.

Active screw-in thermocouples provide the measured values not as a thermoelectric voltage, but as a linear output signal.
Depending on the transmitter used, the measured value is output either as a current in milliamperes (mA) or as a voltage in volts (v).
For this purpose, the probe either has a temperature transmitter built in or connected, which converts the thermoelectric voltage.
We offer convenient sets consisting of a Hat rail transmitter with a 4-20 mA output and the matching passive screw-in thermocouple.

The use of a transmitter has further advantages:

  • longer distances can be bridged
  • the output signal is amplified
  • the output signal is standardized and can be used on many devices
  • depending on the transmitter type, malfunctions such as cable breakage or incorrect connection of the probe can be detected if necessary

There is also a disadvantage: active probes need a supply voltage or current to convert the signal.

Please consider the following points when selecting your active screw-in thermocouple

  • What is the measurement range, i.e. in what temperature range will the measurement take place?
  • What is the ambient temperature in which it will operate? (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? 


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What are the different types of screw-in thermocouples?

For screw-in thermocouples with cable we offer the following different process connections / threads.

Screw-in thermocouples with metric thread:

For 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 probes with fine thread:

The screw-in thermocouples with fine threads are offered with a fixed mounting length of 17mm.

You can choose between thermocouple type J or type K for all screw-in thermocouples with cable.

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What should I pay attention to when selecting my active screw-in thermocouple?

The measurement system consists of two components: the transmitter and the thermocouple.

For the complete system, please check that the measurement accuracy, tolerance and response time of the entire measurement system match your specifications.

The Hat rail transmitter fits into your control cabinet due to its slim design and standardized dimensions. If you need a different type, e.g. in the housing, please contact us.

Furthermore you should check which signal you need for further processing: 0 to 10 V or 4 to 20 mA.

The right choice of your thermocouple depends on your measuring task and the associated requirements. We often hear questions like:

  • Is the probe also compatible with?
  • Which thermocouple should I choose?
  • Can I change my thermocouple?
  • Which cable do I need or can I replace the cable?

Answering the following questions can help you make your selection:

  1. What type do you need?

    Select the required type in the category.
    We offer screw-in thermocouples with G1/2 ", G1/4 " and G3/8 " and fine thread M4, M6, M8.

  2. What is the right thermocouple for me?

    The choice of thermocouple depends on the one hand on which measuring device you want to use the thermocouple. Please check which thermocouple is to be used according to the technical data or the operating instructions of the measuring device. The different types of thermocouples are not compatible with each other and also have different characteristic curves. You can find an overview of the characteristic curves of our thermocouples here.
  3.  Does the thermocouple fit your measuring range?

    In addition to the measuring device, please also check in which measuring range and with which accuracy / tolerance you want to use the thermocouple. Information on the measuring range and tolerance can also be found in the characteristic curves and in the operating instructions for each probe. With thermocouples, it is particularly important to adhere to the measuring range, because if the limit is exceeded, the metal alloy can be irreversibly damaged and then the thermocouple will no longer measure within the specified tolerance range.

  4. How do I connect it? Should I use a thermocouple cable or a compensating cable?

    Thermocouples are connected to the reference junction. By using thermocouple cables or compensating cables, the reference junction may be physically separated from the measuring junction, because often the transducer cannot be operated in close proximity to the measuring junction because it is located in machinery or similar hot, dirty environments. Thus, it is necessary to transmit the measurement signal over a certain distance.
    When do I use a compensating cable?
    Compensating cables are made of most cost-efficient substitute materials, which are not identical with the metal pair of the thermocouple. Therefore, they behave exactly the same as the thermocouple only within a certain measuring range. Compensating cables are used primarily for cost reasons due to their limited measuring range, as they are less expensive than thermocouple cables.
    When do I use a thermocouple cable?
    A thermocouple cable is made of the same material as a thermocouple, therefore the thermocouple cable also has the same physical properties and also the same application range as the thermocouple itself. Connecting the two wires at one end creates a thermocouple.
    What do I have to pay attention to when insulating the compensating cable or the thermocouple cable? 

    In addition to the type of cable (i.e. whether a thermocouple cable or a compensating cable is used), the range of application of the cable also depends on the insulation material used for the cable and its properties. PVC, PFA, silicone or glass fiber are used. In addition to the area of application, the cables differ in other properties such as resistance to dust and moisture (IP class) and resistance to aggressive media. You will find the following cables in our product range:

    Glass fibre thermocouple cable Type J / application range: -50 °C to +400 °C / IP 20
    Glass fibre thermocouple cable Type K / Application range: -50 °C to +400 °C / IP 20
    PVC thermocouple cable type J / Application range: -30 °C to +90 °C / IP 67
    PVC thermocouple cable Type K / Application range: -30 °C to +105 °C / IP 67
    Silicone thermocouple cable type J / Application range: -50 °C to +180 °C / IP 67
    Silicone thermocouple cable Type K / Application range: -50 °C to +180 °C / IP 67

     

  5. Do I need a connector and what size should the connector be?

    Thermocouples are either connected directly to the measuring device. However, it is much more common for thermocouples to be connected to the measuring device via a plug connection. At Testo Sensor, you will find the following connectors on offer:
    • Mini TC connector
    • Standard TE connector
    • Mini TC connector
    • Standard TC coupling
  1. How do you ensure the correct mechanical connection to the process?

    Correct mechanical connection of the probe to the process ensures efficient, error-free and trouble-free measurement.
    What length of protection sleeve do I need for my thermocouple?
    For thermocouples with variable sheath length, please measure in advance what 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. 
    Which sheath diameter should my thermocouple have?
    Please consider what 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. Therefore, please make sure that the thermocouple is coupled to your measuring 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.

    We have provided more detailed information on mounting and immersion depths in the operating instructions, which you can download for each probe.

What environmental influences is the thermocouple subject to?

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, feel free to contact us.

What do I have to consider when mounting the screw-in thermocouple?

Due to the heat dissipation to the environment, measuring errors can occur. To keep these as small as possible, we recommend to immerse the protection sleeve of your temperature probe as deep as possible into the medium to be measured during installation. The optimum installation depth should be 10-15 times the Ø of the protection sleeve or, when using an immersion sleeve, the Ø of the immersion sleeve. When installing in pipelines whose Ø does not have a sufficiently deep installation depth, you should install the probe either at an angle or in a pipe elbow. Make sure that you have sufficient space so that the probe can be removed again. 1) Installation with sufficient installation depth 2) Installation at an angle with small pipe Ø 3) Not like this: Minimum installation depth not reached

Installation by using an immersion sleeve (4): Please ensure that the Ø and length of the immersion sleeve are selected to suit the installation situation so that the minimum immersion depth can be achieved. Please also pay attention to the correct process connection. Since the probe is not inserted directly into the medium, but via the immersion sleeve, the response times are somewhat slower. The probe should be selected in such a way that the protection sleeve touches the bottom of the immersion sleeve and that the air cushion around the protection sleeve is as small as possible. The use of thermal paste can improve the response times.

Please lay the cable in such a way that no water can penetrate the probe and with reserve loop (4). This allows you to extend the probe without disconnecting the electrical connection.

Which accessories do I need?

When ordering, please consider whether you need any accessories. Please check the delivery details for each probe to see what is included in the scope of delivery.

We offer the following accessories:

  • Cables
  • Connectors
  • immersion sleeves

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How do I configure my active thermocouple?

On the detail page you can configure your screw-in thermocouple exactly:

First step: select your transmitter
First, please select your temperature transmitter. This should match the input signal of your evaluation electronics.
In our online store you can currently find the Hat rail transmitter with a 4-20 mA output.
If you need another transmitter e.g. with 10V output or another transmitter type e.g. head transmitter, please feel free to ask us - we will gladly advise you about your possibilities.

In the next step you configure your thermocouple.
Please choose the appropriate diameter and mounting length for your screw-in thermocouple.

For the cable material, please pay attention to the permissible operating conditions and operating temperature for the temperature measurement. Currently we offer the following function and measuring ranges:

  • PVC cable (permissible measuring range: -30 °C to +105 °C | IP 54)
  • Silicone cable (permissible measuring range: -30 °C to +105 °C | IP 65)
  • PFA cable (permissible measuring range: -30 °C to +105 °C | IP 54)
  • Glass fibre cable (permissible measuring range: -30 °C to +105 °C | IP 20)

In addition to the cable material, you can also select the required cable length and your required connector.

As with our passive temperature probes, bend protection can be selected as an option, depending on the type.

The exact information can be found in detail in our data sheets and operating instructions in our online store.

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