FAQ (Frequently Asked Questions)
Thermocouple/Compensating Cable
What is the impact when the sleeve temperature increases and the connection part of the thermocouple and compensating cable has a high temperature?
The connection part (sleeve or terminal box) sometimes has a high temperature depending on the environment where the thermocouple is installed. What is the impact?
The following two impacts are predicted in terms of an increase in the temperature of the sleeve part.
- (1)
When the measured temperature is close to the temperature of the sleeve, the performance of the compensating cable is better produced than that of the sheathed thermocouple.
- (2)
When the surrounding heat exceeds over the heat resistance of epoxy resin within the sleeve, disconnection and an decrease in insulation resistance may be caused.
These phenomena are explained below.
(1) Impact on Extension Lead Wire
Normally, the connection part for the sheathed thermocouple and compensating cable is called “sleeve,” where both conducting wires are connected within a pipe of the outer diameter of φ8 mm and length of 32 mm with epoxy resin filled up within the pipe for insulation and fixing. Since, with a standard product, the heat resistant temperature of this epoxy resin is 80℃, the heat resistant temperature of this part is 80℃. (Since the material of the campensating cable connected is the same as that of the thermocouple, even if the compensation junction temperature is at 80℃ or higher, its characteristics are within the tolerance. Also, the insulation coating material has the heat resistance of 80℃ or higher.)
However, if a product whose sheath length is extremely short (50 mm for example) is inserted to a measurement place or temperature tank for calibration (oil bath, etc.), the temperature of the sleeve gradually increases not only by heat conduction but also by radiant heat from the liquid surface. In this case, the thermo-electromotive force generated from the thermocouple is output with the thermo-electromotive force of the compensating cable added and unexpected errors may be generated which are not caused under the normal use conditions.
Figure-1 Status on Insertion to Oil Bath
Since it is originally assumed that the sleeve is used under approx. atmospheric temperature and there is no big difference between the temperture of the sleeve and that of the terminal part of the measurement instrument, only the performance of the sheathed thermocouple is output and displayed/recorded on the instrument. However, as for an extremely short sheathed thermocouple, the output from the compensating cable is added because the sleeve has a high temperature as well and a result different from the original performance of the product is obtained. The diagram below shows the status.
Figure-2 Addition of Thermo-electromotive Force (Intermediate Temperature Laws)
When t1 (measurement target temperature in this diagram) =150℃, t2 = 60℃ under the condition where the temperature of the sleeve exceeds over 60℃ and if t0 = 20℃, V1 shares the thermo-electromotive force of (150-60)℃ as an output from the sheathed thermocouple, and V2 shares the thermo-electromotive force of (60-20)℃ as an output from the compensating cable; therefore, if the product is used in this state, the thermo-electromotive force of the compensating cable is added and an error different from the characteristics of the sheathed thermocouple is generated.
From the above, when the high-precision measurement using a sheathed thermocouple is preferred, the temperature of the target measured must be considered and attention must be paid so that the sheath length is designed as long as possible and that the sleeve has the normal room temperature.
If a product with a short sheath is mandatory, we process the edge of a long sheath (200 mm at least) to make a heat junction and implement a product inspection. Then we process the product into the necessary length. Therefore, when short sheathed thermocouples are inspected for acceptance, sufficient attention must be paid.
- a）
Since insufficient insertion length may occur, insert the product as deeper as possible.
- b）
When the measurement temperature is high, the temperature of the sleeve increases due to the radiant heat from the measurement target (test tank); therefore, it is necessary to have insulation between the test tank and the sleeve, have cooling, etc.
- c）
If the insertion length/method of the standard resistance thermometer sensor used as the criteria is not the same as the sample provided, the temperature distribution of the test tank may be influenced.
It must be understood that measurement errors are always generated if sufficient considerations above are not provided.
In addition, (2) Comparison Method, 11.2.4 Thermo-electromotive Force Characteristics in Japanese Industrial Standard “JIS C 1605-1995 Sheathed Thermocouple” states “The temperature measuring part of the standard thermometer and the temperature measuring part of the sheathed thermocouple are placed close to each other and are inserted deep enough to compare the temperature of the both. At this time, they must have situation where measurement errors caused by radiation or heat conduction from a heat source are not generated.”
Furthermore, 10.4 Compensating Cables of “JIS Z 8704-1993 Temperature Measurement Method-Electrical Method” states “Sincethe property of the thermo-electromotive force of the compensating cable and that of the thermo-electromotive force of the thermocouple used do not match completely, some measurement errors will be generated depending on the temperature of the compensation junction.”
(2) Impact by Epoxy Resin
Our most basic sheathed thermocouple is Type T35, where a compensating cable is connected to the terminal of a sheathed thermocouple. The sheathed thermocouple has a sleeve part with epoxy adhesive materials filled up inside to fix the humidity proof seal at the end of the sheath and the compensating cable when the thermocouple wire within the sheath is reconnected to the compensating cable.
Figure-3 T35 Type Sheathed Thermocouple
We adopt epoxy resin with the heat resistant of 80℃ with good fluidity for our standard product and also adopt compensating cables with the heat resistant of 100℃ or lower as a standard item. Therefore, if the product is exposed to the temperature of 100℃ or higher, various troubles may occur and unexpected states may be generated. Attention must be paid. The following are expected troubles.
- (1)
If the temperature becomes high, the epoxy resin is expanded and the internal element wire is pulled and cut.
- (2)
If the temperature becomes higher, the epoxy resin is carbonized, the insulation performance cannot be retained, and the short-circuit status occurs.
- (3)
Since the guaranteed temperature of the thermo-electromotive force of the compensating cable is 100℃ or lower, if the temperature reaches over 100℃, the error becomes larger.
In addition, if it is known that the temperature becomes 100℃ or higher, we can design the product adaptable to the temperature. We can support the temperature of up to 200℃ by selecting epoxy resin with high heat resistant temperature and a low heat expansion rate, and by using not a compensating cable but a coated thermocouple wire.
We recorded the changes in the status of the temperature sensor after we filled up the standard epoxy resin and retained it at 80℃, 100℃ and 150℃. At 150℃ or higher, it is assumed that the epoxy resin changes in color to brown or black, which can produce a bad impact.
Figure-4 Epoxy Resin after Exposed in High Temperature
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