Liquid Column Static Pressure Affecting Pressure Sensor Accuracy

The liquid column static pressure will affect the measurement accuracy of the pressure sensor in the actual production process. It is common for the pressure sensor installation below the pressure tapping point. When measuring steam and liquid medium, the output value P_i of the pressure sensor is the sum of the actual working pressure P_g and the liquid column static pressure P_s due to the liquid column static pressure, which produces an additional error. For example, the installation position of the pressure sensor is 10m below the measuring point. If the pressure sensor indication is 5.04MPa, it including the liquid column static pressure value. Therefore, the difference that subtracting the liquid column static pressure value is the actual working pressure value.

At the production site, we usually use an approximate correction method, that is  P_g=P_i-P_s. In the formula, P_g is the actual working pressure; P_i is the pressure sensor output; P_s is the liquid column static pressure, that is, the pressure value to be corrected. Since P_s = ρ × H, where ρ is the water density (which is under a certain condition), H is the liquid column height, that is, the vertical distance between the pressure sensor and the measured point.

Why is it an approximate correction? Because inside of the pressure guiding tube is all steam condensed water, P_s=ρ×H is calculated according to the water when the gravity acceleration is 980.665 cm/s² and the density is lg/cm³. At this time, P_s is water column and can be converted by 10m water column = lat (engineering atmospheric pressure) = 98066.5Pa. Such an approximation can meet measurement requirements in most applications. However, for the occasion in which the pressure of the measured medium is low and the measurement accuracy requirement is high, if the approximate correction method is used, the measurement error will be large. At this point, the water temperature in the pressure guiding tube should be considered and determine the water density ρ.

If corrected according to theoretical calculations, the vertical distance between the pressure sensor and the sampling point should be accurately measured; it also needs to determine the steam condensate density in the pressure guiding tube; In addition, for accurate measurements, local gravity acceleration problems are also need to be considered. This is very complicated and troublesome.

• How to correct the error produced by liquid column static pressure for pressure sensors

The discussion purpose is to understand the principle of liquid column static pressure and its influence on the measurement results in theory. In fact, at the production site, it is very simple and convenient to correct the influence of the liquid column static pressure. Since during design and manufacture for the pressure sensor, the migration device has been considered for the user, so that the above problem can be easily handled. The specific method is as follows (take the sensor with 4-20mA output as an example)

1. When the shutdown process pressure is zero, due to the liquid column static pressure, the pressure sensor will have an output but will not be 4 mA, and there will be an output value, which is the liquid column static pressure value. At this time, the sensor output value can be adjusted to zero (4mA) by adjusting the positive migration of the pressure sensor, which is equivalent to subtracting the liquid column static pressure value. The adjusted output value of the sensor is the actual working pressure value.
2. When the process cannot be stopped, we can close the root valve of the pressure sensor, and then unscrew the thread or unload the process pressure through the sewage valve. Migration method to adjust the pressure sensor as above.

• Experience sharing

1. When measuring the lower pressure and the measurement accuracy requirement is high, if the condensate temperature changes greatly, the sensor can be adjusted several times a year according to the season.
2. For the effects of local gravitational acceleration, this problem has been solved by sensor migration adjustment. The reason is that the pressure sensor installed in the field should be calibrated (or verified) according to the relevant regulations. For the standard device, that is, the piston pressure gauge whose dedicated weights have been gravity acceleration correction for user's location when it is sent to the upper-level measurement department for inspection. Even the pressure sensor is calibrated with a precision pressure gauge, it is still comply with the standard traceability.

The problem of the liquid column static pressure can be solved only by the adjustment for the pressure sensor migration. It is an example that simplify a complicated problem.