The solutionconductivity meterIt is closely related to temperature, because when the temperature changes, the ionization degree, solubility, ion migration rate, solution viscosity, and conductivity of the electrolyte will also change. As the temperature increases, the conductivity also increases. At this moment, the temperature compensation function of the conductivity meter is to overcome the influence of temperature. Temperature has a significant impact on the conductivity of a solution. As the temperature of the solution increases, the hydration of ions weakens. The viscosity of the solution decreases, the resistance to ion movement decreases, and the directional movement of ions accelerates, thereby increasing the conductivity of the solution; On the contrary, as the temperature of the solution decreases, the conductivity decreases. In industry, the conductivity of a solution at a temperature of 25 ℃ is used as the conductivity of the solution, so values measured at other temperatures must be converted to the value at 25 ℃. Different substances and temperatures have different corresponding values.
Because different solutions have different temperature coefficients, it is generally impossible for instruments to compensate for the effects of temperature, and the larger the difference between temperature and 25 ℃, the greater the error.
The commonly used temperature compensation method is to add a temperature compensation formula to the conductivity calculation. The following formula is an empirical formula for temperature compensation:
Gt= G25[ 1+ a( t- 25)]
In the formula: Gt is the conductivity of the solution at a temperature of t ℃; G25 is the conductivity of the solution at a temperature of 25 ℃; A is the temperature compensation coefficient of the solution; T is the actual temperature of the solution.
Figure 2
Gt is the actual conductivity of the solution, which must be converted to the conductivity of the solution at 25 ℃, and G25 is the obtained conductivity value. Different solutions have different temperature compensation coefficients, and a varies from 0.014 to 0.027. Because Gt and t can be measured by instruments, as long as the temperature coefficient is determined, the conductivity G25 of the solution at 25 ℃ can be calculated.
The above empirical formula is only applicable to a limited range, for example, there are different empirical formulas for pure water, so temperature compensation is impossible. The instrument not only performs temperature compensation, but also temperature control. The conductivity cell is designed as a measurement cell with temperature control function, as shown in Figure 2.
Heating controller at the entrance of the conductivity cell. The core component of this temperature controller initially used a semiconductor cooling block, which has a small volume. By changing the direction of the current, the cold and hot surfaces can be changed, and both cooling and heating can be achieved. Changing the magnitude of the current can change the power of cooling or heating (based on the working surface), making control extremely convenient. However, after long-term operation on site, it was found that its disadvantages are short lifespan and poor reliability. Based on long-term on-site operating experience, the temperature of water samples is generally below 25 ℃. Therefore, a heating element with simple working mode, long service life, and high reliability is selected for temperature control, and the temperature controller also has a stabilizing effect. When controlling the operation of the heating element, first use a temperature electrode with conductivity to collect temperature signals and perform temperature compensation calculation. At the same time, based on the comparison with the reference temperature of 25 ℃, control the temperature controller to heat the conductivity cell. If the water sample temperature is higher than 25 ℃, the heating element will not work and only perform temperature compensation calculation. Through practice, it has been proven that the quality difference between using delayed feedback control and simple switch quantity control is not significant. Therefore, switch quantity control is adopted, which not only ensures control, but also simplifies the corresponding circuit and flow path, and improves the reliability of operation. Minimize errors caused by temperature effects as much as possible.
