The Verification Method and Problems of Temperature Compensation of Conductivity Meter

Measuring method and problem of temperature compensation for conductivity meter. Users of the conductivity meter all know this. The conductivity of the solution is closely related to the temperature. As the temperature changes, the electrolyte's ionization degree, solubility, ion migration velocity, solution viscosity, etc. will change and the conductivity will change. As the temperature increases, the conductivity increases. The temperature compensation function of the conductivity meter at the moment is to overcome the temperature effect.

First, what is the temperature compensation function of the conductivity meter:

The conductivity of the solution at the actual temperature is converted to the conductivity at the reference temperature (usually 25°C), making the conductivity of the solution comparable at different temperatures. The conductivity meters used in the market today have Temperature compensation function to meet the needs of various industries to compare or control indicators. This article uses the conductivity meter, the warming function required during the verification process, a brief analysis and discussion.

It is also necessary to add this verification item during the verification process. There are two methods to achieve the temperature compensation of the conductivity meter. One is that the KMR before temperature compensation is a fixed value, and the other is that the KMV after temperature compensation is a fixed value. The two methods are based on the same principle, and the specific verification steps are based on The different designs of the instrument can also be divided into two methods. During the verification process, we also found that the temperature setting will affect the conductivity cell constant. Analysis shows that the temperature compensation of the conductivity meter is essentially the same as that of the conductivity cell constant. When the temperature compensation of the instrument is missing or there is a fault, the cell constant can be used. The compensation to achieve conductivity temperature compensation.

Second, the temperature compensation test methods and issues For the conductivity of greater than 1 × 10-4S · cm-1 strong electrolyte, there is a linear relationship between conductivity and temperature:

KT = K0 [1α (T-T0)] (1); In the verification process, as long as the conductivity value at different temperatures is measured, the temperature coefficient α of the instrument can be obtained by Equation (5) in JJG376-2007. In order to achieve the conductivity meter temperature compensation coefficient test.

Set the conductivity meter constant Kcell to 1.00cm-1, input the conductivity value of a signal (such as 50μS·cm-1), and adjust the temperature sensor analog resistance so that the temperature indication is 25°C and 15°C (35°C). , and then read the corresponding conductivity meter measured values ​​KMR and KMV.

problem:

1) The domestic conductivity meter is manual temperature compensation. The temperature coefficient cannot be set. The default value is 2.00%/°C. For this type of instrument, when the temperature is set to 25°C, the measured conductivity is KMR, and the measured conductivity value at other temperatures is the compensated conductivity value KMV, which can achieve temperature compensation. Verification.

2) For different conductivity meters, the verification steps of temperature compensation are not the same. For example, the DDS-307 produced by sacco in Anhui Province is taken as an example: the DDS-307 conductivity meter at later stage (new type) is used to adjust the temperature indication. , Conductivity changes significantly, is defined as type I (DDS-308, foreign conductivity meter such as con5, etc. also belong to this type). Early production of the DDS-307 conductivity meter, when the temperature indication was adjusted, there was no change in conductivity. To facilitate differentiation, we defined it as Type II (most of the digitally-displayed DDS-11A/12A also belong to this class).

For Type I instruments, the error of the temperature coefficient can be measured according to the method described in JJG376-2007. First set the conductivity cell constant and then adjust the temperature indication. For type II instruments, the temperature indication has no effect on the conductivity value, and does not indicate that the temperature sensor analog resistor has failed because if the instrument is adjusted to the “check” state, it is found that the conductivity cell constant also occurs when the temperature indication is adjusted. In the changes, when the temperature indication was adjusted to 15°C and 35°C, the cell constants changed to about 1.200cm-1 and about 0.800cm-1, respectively.

For the verification of this kind of instrument temperature compensation, the temperature should be adjusted to the target temperature (15 °C or 35 °C), and then adjust the conductivity cell constant to 1.00cm-1, and then read the corresponding conductivity values, according to the formula (3 ) The temperature coefficient of the instrument can be found. However, the data obtained from this type of instrument has a large deviation from the standard value (α=2.0%) calculated according to equation (3), and the temperature compensation coefficient values ​​obtained at different temperatures vary greatly. This kind of instrument has a similar deviation, not because of the failure of a certain instrument, and this issue is for further study.

The relationship between temperature compensation and conductivity cell constant compensation Some of the domestic conductivity meters mentioned earlier, the temperature setting has a significant effect on the cell constant, for the instrument I (such as the new DDS-307), when the temperature indication is adjusted to 15 °C and 35 At °C, the conductivity cell constants changed to about 1.250 cm-1 and about 0.833 cm-1, respectively. This phenomenon can also be observed in type II instruments (values ​​differ from those of instrument I and are for further study). This shows that the temperature compensation and the conductivity cell constant compensation are the same, and the two are intrinsically linked.