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Thermocouple test easy to ignore the problem
Dec 24, 2017

Thermocouple test easy to ignore the problem

In the course of verifying the thermocouple, the test personnel can find out easily that it is easy to deal with the inaccuracy of the measured data caused by the common problems such as the unsatisfactory terminal, short circuit of thermocouple or deviation from the geometric center. The result is easy to ignore the problem.

First, the length of the thermocouple

JJG351-1996 "working with metal thermocouple" test procedures clearly stipulated in the 

thermocouple length of not less than 750mm, the reason why the provisions of the length of the 

thermocouple, because taking into account the thermocouple after leaving the temperature

 measurement zone should have a wide enough temperature Gradient region. Thermocouple thermo-emf 

is also generated in this area, to effectively prevent the hot end of the thermocouple 

(measurement side) to the cold side of the heat (terminal), the most basic method is the cold end 

of the thermocouple to have enough Away from the hot side. Generally speaking, because the 

thermocouple length is not enough to bring the error is negative, the correction value is positive

. The shorter the length, the greater the error, therefore, in the furnace before the test need to

 determine the length of the thermocouple.

Second, the thermocouple wire bending

Thermocouple wire is thin and soft, easily deformed, when the dual wire folding, twisting and other

 plastic deformation of the thermocouple even wire stress, it changes the thermocouple 

thermoelectric properties, so that deformation thermocouple measurement accuracy affected. 

Therefore, the thermocouple wire must be straightened before the test.

Third, the thermocouple wire is contaminated

The thermocouple wire is contaminated, or even oxidation, will make the thermocouple wire surface

 is not bright, dark dark, then the thermal electrode thermoelectric properties are very unstable,

 the accuracy of measurement data is poor, therefore, to clean the pollution Of the electrode, 

eliminating the pollution layer.

        Fourth, the impact of response time

Contact the basic principle of temperature measurement components and the measured object to 

achieve thermal equilibrium. Therefore, in the temperature need to maintain a certain time in order

 to achieve the two thermal equilibrium. The length of the holding time, with the temperature 

response of the thermal response time. The thermal response time depends on the structure of the 

sensor and measurement conditions, the difference is great. Therefore, in the daily verification 

process according to different types of thermocouple select the appropriate heating rate, heat 

balance time.

Fifth, the impact of insulation resistance

Thermocouple at high temperatures, the insulation resistance decreases rapidly with increasing 

temperature, it will produce leakage current, the current through the insulation resistance has 

decreased insulation into the instrument, the instrument indicates the instability or measurement

 error. Therefore, the thermocouple furnace installed before they do not ignore the insulation 

resistance of the test, only when the test procedures to meet the requirements in order to carry

 out temperature tolerance test.

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