Teach you how to calculate differential pressure liquid level measurement range

According to the density of the liquid being measured

The highest and lowest height of the liquid level

Calculation: Maximum pressure P+ of the positive pressure chamber

Maximum pressure P- of the negative pressure chamber

The measuring range of the differential pressure level gauge is P-~P+

The measuring range is ΔP=P+-P-.

How to migrate and set up differential pressure level gauge

Under normal circumstances, differential pressure transmitters generally do not need to be migrated. However, in the actual application process, due to the need to consider maintenance, installation and other aspects to facilitate operations during use, there will be a level difference between the pressure point and the instrument. Therefore, in order for the differential pressure transmitter to measure accurately, migration processing must be performed.

The zero point migration of differential pressure transmitters is generally divided into three categories: positive migration, negative migration, and no migration. The zero point migration referred to here is to implement a numerical change safeguard measure based on the starting point of measurement of the differential pressure transmitter in order to ensure the effectiveness of the basic quantity of the instrument during measurement. The changed values ​​are only the upper and lower limit values. It does not change its measurement range.

The figure above shows the three types of zero point migration of the differential pressure transmitter. From left to right, they are no migration, negative migration and positive migration.

If the liquid level of a water storage tank is 0-100mm, the differential pressure is 0 when the liquid level is zero, and the differential pressure is 9.8kpa when the liquid level is 100mm. Therefore, if the installation position and the pressure point are in a horizontal position, there is no migration, and the differential pressure transmitter is set to a range of 0 to 9.8kpa.

If the installation position of the differential pressure transmitter and the pressure tapping point are not at the same horizontal position, the calculated pressure difference is less than zero, which is negative migration, and the differential pressure is greater than zero, which is positive migration.

When the calculated differential pressure is less than zero, △P=ρgH-△hρog=9.8-△hρog<0. When the liquid level is zero, the output signal is lower than 4mA, and when the liquid level is 100mm, the output signal is lower than 20mA. Therefore, by performing negative migration on the differential pressure transmitter, the standard signal output of 4-20mA can be restored. At this time, the lower limit of the measuring range of the differential pressure transmitter is no longer 0 but -△hρog, and the upper limit of the measuring range is 9.8-△hρog, that is, the measuring range of the differential pressure transmitter is set to -△hρog~9.8-△hρog.

When the calculated differential pressure is greater than zero, △P=ρgH+△hρog=9.8+△hρog>0. When the liquid level is zero, the output signal is higher than 4mA, and when the liquid level is 100mm, the output signal is higher than 20mA. Therefore, when the differential pressure transmitter performs positive migration, it can restore the standard signal output of 4-20mA. At this time, the lower limit of the measurement range of the differential pressure transmitter is not 0 but △hρog, and the upper limit of the measurement range is △P=9.8+△hρog, that is, the measurement range of the differential pressure transmitter is set to △hρog~9.8+△hρog.

When the differential pressure transmitter performs positive and negative migration, it only changes the upper and lower limit values, and its range does not change. It can be seen from the above that if the differential pressure transmitter needs to perform positive migration but does not perform it, the output signal will be high, and if the differential pressure transmitter needs to perform negative migration but does not perform it, the output signal will be low.