How to determine K type thermocouple and the accuracy of the measurement result

What's the problem I K type thermocouple? How do I know if my measurements are all right?

we three chang expert solves the problems of these is not so simple.

q:

we K type thermocouple diagnosis in 25 oc 1 K type thermocouple. 0 mV。 It has a long TC cable, don't show the continuity, but the resistance of 130 ohms. Can you explain why this happens and how to solve this problem?

a: one of the problems associated with long extension cord is noise. The best choice is to not send low level TC signals for long distances, but the launcher electronic components directly on the top of the thermowell. Today, in most industrial applications using monolithic transmitter on cost competitive, and in fact is longer than the thermocouple wire is connected to the control room at a lower cost.

in the figure shows the transmitter with K type thermocouple. In this case, the hot junction is process temperature ( T意义) E, and generate millivolt voltage in 25 & deg; C, when the temperature is 25 m to 1. 0 mV。 The transducer detects the millivolt voltage ( V） The E ( T意义) And cold junction reference temperature (E T ref) The difference between the known millivolts. The difference is converted to the process of the transmitter temperature readings. These details are usually hidden from the user, because the reference junction blocks ( With T ref) , voltmeter and equation solver combination into a single product.

using known reference temperature ( T ref) Measure the voltage V is used to calculate temperature ( T意义) The configuration of the K type thermocouple ( 镍铬合金, 镍铝镍) 。

today, wireless intelligent transmitter also can be used in the design of wireless. If the transmission distance is very long, I will consider these for your application.

graph: wireless and intelligent temperature transmitter

a: K type thermocouple is a dc voltage source, the voltage by the head of the hot junction and by applying the measurement instrument lead the cold junction temperature difference between the decision. Measuring terminal net potential or total resistance of the circuit may interfere with the normal operation of continuity tester, and can produce fuzzy or no results.

all ohmmeter contains dc voltage source. Set to add or subtract the thermoelectric potential, and generate greater than or less than the results of the actual circuit resistance. The error is very small. This technique should be a method to test the feasibility of thermocouple circuit. Thermocouple wire resistance per unit length is higher than the same specifications of the copper wire.

ohmmeter by current source ( Usually a fixed voltage) And the scope of current limiting resistor connected in series and the connection on the terminal of the voltmeter. Connect the terminals to the unknown resistance leads to the unknown resistance of terminals on the ratio of voltage, R= E（ Measurement) / I（ Known) ） 。 In the installation type K thermocouple ( TC） , under the condition of unknown TC will be the series circuit resistance, temperature difference of heat knot minus half cold junction voltage. The voltage is measured by the ohmmeter and therefore indicate the resistance will be affected. Are used to connect to the thermocouple is ohmmeter terminals on a measurement, pay attention to reading. Then repeat the test connection plus the minus sign and the second reading. Together, the two readings and the sum divided by 2. The result will be resistance.

a: please make sure from the node to join T/C how much is the length. I assume that the T/C sensor cable to connect to the 3 line, only to connect the two wires on the sensor. I also assume that all three wire are connected to the junction box of the terminal, and the three wire on the terminal connected to the final instrument.

T/C total resistance depends on the size of the internal thread, usually between # 20 AWG # to 30 AWG, and its total length. 300 mm long, for example, # 22 AWG TC line of total resistance is zero. 9238× 300 = 0. The total resistance of 922047244 ohms. T/C cable resistance to rice of multiplying the total length of cable for the unit to extend the resistance of the cable size.