Thermocouple is affected by the measurement environment, atmosphere, operating temperature, and pollution of insulating materials and protective tube materials during temperature measurement. After a period of use,its thermoelectric properties will change, especially in high temperature and corrosive atmospheres.There are several common problems:
Indicated value of the thermometer is zero;
Indicated value of the thermometer is unstable;
Indicated value of the thermometer is larger or smaller than the actual value;
Change of thermoelectric potential is not sensitive.
1. Problems & Treatments
(1) .The possible reasons for the extreme instability of thermocouple thermal electromotive force are as follows:
A. Poor contact between thermocouple terminal and thermode;
B. Instrument connections are not secure. There are intermittent short circuit, grounding, disconnection and poor welding in the measurement circuit;
C. The thermocouple swings due to loose installation or external vibration makes the measured value unstable;
D. There are conductive liquids, moist dust, and metallic liquids inside the junction box of the thermocouple. The junction box is used for connecting thermocouple and compensation wire, and its outlet hole and cover are sealed with gaskets to prevent dirt from falling in and affecting the reliability of the wiring. When there are pollutants, the inside of the thermocouple junction box must be carefully cleaned and dried in a drying oven before use;
E. The thermal electrode is oxidized during use, and the crystal grains will become larger at high temperature. At the same time, deformation stress will be generated by external force. The pollution and corrosion of the thermal electrode by the environment will affect the stability of the thermocouple.
(2) . Treatments:
A. Check whether the contact between the terminal and the thermode is firm and reliable, whether the fixing screw at the terminal is oxidized, deformed or loose, and re-fix the screw and the terminal;
B. Use a multimeter to test the loop resistance, and it should meet the requirements of 15Ω or specified. At the same time, check the thermocouple, compensation wire, eliminate the grounding point, weld or replace a new compensation wire, thermal electrode, and tighten the fixing screws;
C. Take vibration reduction measures, install the thermocouple firmly, and the insertion depth of the thermocouple should not be less than 10 times the diameter of the protective sleeve itself. For metal protection tube thermocouple, the insertion depth should be 15-20 times the diameter. For non-metal protection tube thermocouple, the insertion depth should be 10-15 times the diameter;
D. Check junction box, clean and dry carefully.
(3) .After the inspection, the thermocouple is installed, and the temperature jump still occurs when measuring the temperature.It may be due to the negligence of the staff, and the artificial fault between the thermocouple, the compensation wire and the temperature measuring instrument can also cause temperature instability. The specific reasons &treatments are as follow:
A. Improper configuration of compensation wire and thermocouple;
B. The thermocouple is inconsistent with the graduation number of the temperature measuring instrument;
C. Wrong connection of compensation wire. If the two compensation wires of the thermocouple are reversed, the temperature will jump to a negative number, and the instrument cannot reflect the real temperature at this time. The compensation wire is equivalent to thermocouple within a certain temperature range (0-100°C), so its current also flows from the positive pole to the negative pole through the reference terminal, so when the thermocouple is connected, the positive pole of the compensation wire , The negative pole should correspond to the positive and negative poles of the thermocouple. When the positive and negative poles are connected oppositely, not only can’t play a compensation role, but it will offset part of the thermoelectric potential of the thermocouple, so that the indicated temperature of the instrument is low; at the same time, various compensation wires can only be used with the corresponding type of thermocouple.All kinds of thermocouple and the matching compensation wires must be consistent within the specified temperature range;
D. The measurement loop is grounded, and the additional ground potential is connected in series.
2. Reason for the temperature jump caused by adding an additional ground potential in series
The thermal electromotive force consists of two parts, one is the contact electromotive force of two conductors, and the other is the thermoelectric electromotive force of a single conductor.
(1). Contact electromotive force. When the homogeneous conductors of two different materials, A and B, are in contact, because the number of free electrons per unit volume inside the two is different(the electron density is different), the rate of electron diffusion in the two directions is different. Assuming that the free electron density of conductor A is greater than that of conductor B, the number of electrons diffused from conductor A to conductor B is more than the number of electrons diffused from conductor B to conductor A, so conductor A loses electrons and becomes positively charged, and conductor B gets negatively charged electrons. So an electric field from A to B is formed on the contact interface between conductors A and B. The direction of the electric field is opposite to the direction of diffusion, which will cause electron transfer in the opposite direction and hinder the continuation of diffusion; when the diffusion force and the electric field force balance each other, it is in a state of dynamic equilibrium. In this state, a potential difference is generated at the contact between the conductors A and B, which is called the contact electromotive force. Denote by eAB(t).
(2). Thermoelectric force. For conductor A or B, place its two ends in different temperature fields t, t0 ( t > t0 ). Inside the conductor, the free electrons at the hot end have greater kinetic energy and move to the cold end, so that the hot end loses electrons with a positive charge, and the cold end gains electrons with a negative charge. Electrostatic field at the cold end. The electric field prevents the electrons from continuing to run from the hot end to the cold end and makes the electrons move in the opposite direction, finally reaching a state of dynamic equilibrium. In this way, a potential difference is generated at both ends of the conductor, and the potential difference is called thermoelectric force, expressed by eA( t, t0 ). The thermocouple circuit diagram is shown infigure 1.
Figure 1
The total electromotive force of the circuit. Conductors A and B are connected end to end to form a loop. If the electron density of conductor A is greater than that of conductor B, and the temperature of the two junctions is not equal, there are four potentials in the thermocouple loop, two contact electromotive forces and two A thermoelectric electromotive force. The total electromotive force of the thermocouple is EAB ( t，t0 ) = eAB ( t)-eAB ( t0).
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