Meter maintenance - Temperature meter testing and commissioning
1, thermocouple detection method:
When detecting the thermocouple in the control room, the influence of the ambient temperature, that is, the cold end temperature, must be taken into account. The specific calculation method is as follows: the actual temperature millivolt = the thermocouple millivolt at the control room end + the ambient temperature millivolt in the control room, and then check the thermocouple index table to obtain the actual temperature value. The ambient temperature of the control room can be measured by shorting the compensation wire (or short contact block).
Cold end temperature correction method: Because the indexing relationship of various thermocouples is obtained when the cold end temperature is 0 ° C, if the hot end of the thermocouple is t ° C, the cold end temperature is t0 ° C (t0>0 ° C), it can not be measured E(t, t0) to check the indexing table to obtain t, which must be corrected according to the following formula:
E (t, 0) = E (t, t0) + E (t0, 0)
Where: E(t,0) - thermoelectric potential when the cold end is 0℃ and the hot end is t℃;
E(t,t0) - thermoelectric potential when the cold end is t0℃ and the hot end is t℃;
E(t0,0) - The correction value to be added when the cold end is t0 ° C.
The thermocouple mainly has broken lines, poor contact and other faults, the specific discrimination method is to measure the resistance of the thermocouple, the normal is on, the resistance value is only a few ohms, in addition, you can measure the millivolt value of the thermocouple, compared with the normal value, you can determine whether the contact is poor.
The function of the compensation wire is to extend the cold end (reference end) of the thermocouple to a place where the temperature is constant and the display instrument is connected to a wire, not cold end compensation. Thermocouples with different index numbers must use different compensation wires, and the polarity cannot be connected wrong.
Commonly used thermocouple compensation wire
Thermocouple Index number Millivolt value at 100℃ Thermocouple material Compensation wire material (color) (Model)
Type K (WRKK) 4.095 mv Nobelium - nickel silicon copper - Constantan (red - Blue) (SC)
Type S (WRSK) 0.645mv Platinum rhodium 10 - Platinum copper - Copper nickel (Red - Green) (KC)
2, thermal resistance detection method:
When detecting thermal resistance in the control room, the simple method is as follows: thermal resistance value =A, B end resistance value - B, C end resistance
Value, and then check the thermal resistance index number.
The exact method is as follows: Rt=R0(1+At+Bt2) (0-850 ℃)
Rt=R0[1+At+Bt2+C(t-100)t3)] (-200 - 0℃)
Among them: A = B = 3.90802 x 10-3 ℃ - 1-5.802 x 10-7 ℃ - 2 - C = 4.27350 x 10 to 12 ℃ - 4
Rt, R0: refers to the resistance value at t℃ and 0℃.
When measuring the resistance of the thermal resistance in the control room, the calculation method is: R = R12-R23.
In order to eliminate the influence of various conductors such as the thermal resistance lead line, the three-wire system or four-wire system is generally used.
The main faults are short lines and short circuits, which can be judged by measuring the resistance value.
The commonly used thermal resistance is platinum resistance (PT100), the resistance value is 100 ohms at 0℃, and the resistance value is 138.5 ohms at 100℃.
3, integrated temperature transmitter
The integrated temperature transmitter is installed in the junction box, which converts the millivolt signal or resistance value into 4-20mA current signal and sends it to the control room. The temperature transmitter can adjust the zero point and range, give the signal through the signal generator, and measure the current value with a multimeter.