Severe climates with high humidity and cold temperatures. Reliable level measurement in dairies

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Application Detail

Hydrostatic level measurement using a pressure sensor is a standard procedure in many dairy applications. Nevertheless, problems with drift or unstable measurements are not uncommon. These usually occur in areas prone to condensation, such as storage tanks, where cold media are measured in a humid environment. Because condensation can potentially lead to the complete failure of the measurement sensor, many users look for the cause of the problem and a technical solution.

Analyses have shown that these failures are often due to how the measuring principle is applied. In hydrostatic
level measurement, a pressure sensor located at the lowest point in the tank measures both the pressure of the fluid column above it and air pressure. To obtain the correct level, the measurement result must compensate for the air pressure. Otherwise, air pressure fl uctuations, which usually range up to approx. 50 mbar, would lead to measurement result variations of up to 16 % in a tank filled three meters high. To avoid this inaccuracy, the air pressure must be compensated for in the result.

Methods of avoiding measurement errors

Two different models of climate-resistant sensors are available on the market that avoid this type of measurement error:

  1. Pressure measuring cells with equalizing capillaries that compensate the atmospheric pressure
  2. Sensors with a permeable double membrane as a humidity barrier
In a so-called relative pressure measuring cell with equalizing capillaries, a thin tube conducts the atmospheric pressure from the environment to the rear of the measuring membrane. Because the air pressure now acts on both sides of the membrane, the resulting measuring signal represents only the hydrostatic pressure of the medium being measured in the container (Fig. 1). In the other procedure, which features a closed relative pressure measuring cell and an integrated permeable double membrane, separate pressure membranes are used for the atmospheric pressure and the hydrostatic pressure. Both the equalizing capillary method and the double membrane method have a decided disadvantage: The diffusion of water vapor cannot be prevented in the long term. As mentioned, when operating the measuring cell in a tank with a cold medium surrounded by warm humid air, the water vapor condenses (Fig. 2). This occurs when the temperature drops below the dew point at the measuring cell, which is the coldest point in the system. Humidity that has entered the sensor results in sensor drift or in fl uctuating readings and ultimately leads to the destruction of the measuring equipment.