How to calibrate the thermocouple temperature transmitter

How to calibrate the thermocouple transmitter? How do you simulate the thermal power puppet? When will the zero reference side be used? What if your measurement temperature changes?
Answer a
Most people use thermocouple simulator to calibrate the temperature transmitter. You can usually set them to generate a variety of thermocouple types. Just make sure that the thermocouple lead used to connect the simulator to the transmitter is the correct type of wire.

The "calibration" thermocouple is another matter -because it is actually effective or invalid. You can pull it up in the bathtub, although few people really do so. However, if it is critical, most people spend time putting the thermocouple in the bath trough or drying block, or at least in contrast to another thermocouple or other methods to cross -check the readings.

zero -degree connection point is slightly more complicated. Basically, two different metals are connected at any time to generate slight hecy signals. This is the thermocouple -two different metals are welded together, generating different voltages according to the temperature of the node. When you run the thermocouple circuit , you try to use the same metal as the thermocouple in the entire circuit -that is, the thermoelectric puppet line that runs with the thermocouple is Special thermocouple wiring terminal. This eliminates any extra nodes -the same metal is always connected to itself. However, at some point, you must connect to a device with copper -wiring terminals (transmitters, indicators, etc.). Unfortunately, this will generate another thermocouple knot in the place where copper contact wires.

In order to solve this problem, most devices have a built -in cold -end compensation circuit, which can automatically induce the temperature of the wiring box and minus the impact from the reading. Almost every transmitter and read -out device are built into the standard function -only older devices will lack it.

Answer 2
Compared with the noise and errors from the sensor and signal wiring and connection, the correctly calibrated intelligent temperature transmitter's error can usually be ignored. The use of Class 1 special level rather than Class 2 standard level thermocouple and extended lead can increase accuracy by 50%. Due to the large span and unable to personalize the calibration, using the thermocouple input card instead of the intelligent transmitter will introduce a large error.

Thermocouple (TC) drift can change between 1 to 20, and the repetitiveness can be changed between 1 and 8, which depends on the TC type and application conditions. For key operations that require high precision, the required sensor calibration frequency is problematic. Although the dry block caliber is faster than the wet batch and can cover a higher temperature range, removing the sensor from the process will destroy the operation. Compared with the simple transmitter calibration, the required time is still considerable. The best way is to perform a single -point temperature check to compensate for the offset caused by drift and manufacturing tolerance.

In the application of the distillation tower, when calibration or replacement of the thermal power puppet, the operator is confused and annoyed by the poor tower performance. It turns out that the operation has returned to a temperature setting point, which effectively compensates the offset in thermocouple measurement. Even after being aware of the new setting point because of the more accurate thermocouple, it takes several months to several years to find the best setting point.

Temperature is the key to chromatographic column control because it is an inference. This is also critical to reactor control, because the response rate determines process capacity and selectivity, and process efficiency and product quality are greatly affected by temperature. In these applications with a working temperature below 400, the resistance temperature detector (RTD temperature sensor ) is a better choice. Table 1 compares the performance of the thermocouple and RTD temperature sensor .

Table 1: The precision, accuracy, signal, size and linearity of temperature sensor
The insertion length of the staircase hot suit tube should be greater than 5 times the diameter of the tip (L/D & GT; 5) to maximize the heat from the heat suite tip to The heat conduction error of the pipe or equipment is connected, and the insertion length should be less than 20 times the diameter (L/D & LT; 20) less than the tip to maximize the vibration of the tail flow. The length should be calculated by the supplier to confirm the heat conduction error and vibration damage. Step type heat suite can reduce errors and damage, and provide faster response. The spring loading the grounding thermocouple is shown in Figure 1. The cycle gap between the inner wall of the protective cover and the inner wall of the heating sleeve can provide the fastest response to maximize the error of the tip temperature of the tip temperature of the tip of the actual process temperature.

Figure 1: Spring for the spring of the sheath TC or RTD temperature sensor loads the compressed joint
Thermal suite material must have corrosion resistance and has armal conductivity under possible circumstances to maximize the conduction error or response time to maximize the most important. The tapered tip of the heat suite must be close to the central line of the pipeline, and the cone -shaped part of the thermal sleeve must be completely across the equipment wall, including any baffle. For chromatography columns, the maximum and most symmetrical temperature changes should be used to increase and reduce the position of controlling traffic. Simulation can help find this, but the wise approach is to use multiple connections to confirm the best position through on -site testing. The tip of the heat suite must be seen. On the other side, the tip is at the gas phase due to the decline of the liquid level of the descending pipe.

For TCs with a degree of 600 degrees Celsius, ensure that the sheet material is compatible with TC types. For TCs higher than the temperature limit of the sheet, please use the best thermal conductivity and design ceramic material to minimize the measurement lag time. For TCs with restrictions on the temperature restrictions of the cover with gaseous pollutants or restore conditions, the primary (external) and secondary (internal) protective tubes that may have been blown to prevent TC components from polluting and providing faster response.

small diameter pipeline (for example, less than 12 inches) thermocouple sleeve is in the pipeline elbow to maximize the insertion length of the pipeline center. If solid wear is a problem, you can install the heat suite in the elbow facing downstream, but it requires a longer length to reduce the noise generated by the vortex. If the pipeline is half -filled, the installation should ensure that the narrow diameter of the stepped thermocouple sleeve is in the liquid instead of the steam.

The position of the thermocouple sleeve must be sufficient downstream at the side exit of the flowing convergence or the side exit of the heat exchanger to achieve the re -mixing of the flow. Due to the increase in transportation delay, the position should not be too close to the lower reaches. This is the stay time of the piston flow, that is, the pipe volume between the exit or joint and the sensor position is divided by pipeline flow (volume/flow). For the length of the 25x pipeline diameter (L/D = 25), the increase in the dead area of the circuit in a few seconds is not as harmful as the signal -to -noise ratio of the signal -to -noise ratio caused by poor uniformity. For the temperature -device, in order to prevent water droplets from generating noise, the thermocouple sleeve must provide a stay of more than 0.3 seconds. For high gas speed, this may be much more distance from liquid heat exchangers.

In order to obtain higher reliability and better diagnosis, dual isolation sensing elements can be used, but more effective solutions are redundant installation of thermocouple sleeve and transmitter. The selection of three completely redundant intermediate signals provides the best reliability and minimum drift, noise, repetitive and slow response. The measurement from the middle signal selection is effective for any type of measurement. Also gain quite a lot of knowledge, which can avoid comparing each measurement value with the intermediate value.

Suppose there is no pollution or changes in raw materials, the drift in the sensor is displayed as the output of different average controllers under the same productivity. The repeatability of the sensor shows excessive changes in the temperature controller output. For a very strict control of the controller gain, assuming the controller is adjusted correctly and the valve has a smooth and consistent response, the sensor in the sensor output is the most obvious.

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