Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question dated 2020-09-03

When measuring liquid levels and the lower pressure tapping is connected via a pressure transfer tube, the pressure to be measured is transmitted to the transmitter location through the medium inside the pressure transfer tube. It is certainly possible for the installation height of the transmitter to be higher than that of the lower pressure tapping. When using differential pressure transmitters to measure liquid levels, in many cases the container is under pressure; if there is a pressure of 0.1 MPa inside the container, this can generate a water column with a height of 10 meters. If the installation height of the differential pressure transmitter is higher than h, the distance between the upper and lower pressure tapping points of the container is h, and the medium inside the container is water, then the range of the differential pressure transmitter △p = -ρgh to ρg(h-h) ； The range for a differential pressure transmitter installed at the same level as the lower pressure tapping is △p=0～ρgh ； Therefore, a negative migration of the differential pressure transmitter is required when it is above the lower pressure tapping; the claim that a liquid level below the transmitter cannot be measured once it is above the lower pressure tapping is incorrect. In many low-temperature media (liquid nitrogen at -196°C), the diaphragm of transmitters cannot withstand such low temperatures. So how can a differential pressure transmitter be used to measure the liquid level in a liquid nitrogen tank? In this case, we wait until the liquid nitrogen coming out of the suction port has vaporized before connecting it to the differential pressure transmitter; as a result, the transmitter can be installed at any height. The high-pressure side transmits pressure to the diaphragm on the positive-pressure side of the transmitter through the vaporized nitrogen, so the installation height has no impact on the measurement.

When it is above the lower process connection, the transmitter needs to be shifted positive; such a shift corresponds to changing the lower limit of measurement, which reduces the measurement range and violates the requirements for accurate measurement.

When using a differential pressure transmitter to measure liquid level, if the installation height of the transmitter is higher than that of the lower pressure tapping point, the medium in the pressure guiding tube will leak away. Calculating the liquid level based on the pressure measured by the pressure transmitter will result in pressure changes, which in turn lead to changes in the liquid level.

The main goal is to keep the transmitter diaphragm chamber under positive pressure; if this pressure is higher than that at the lower pressure tapping point, it can easily cause bulging of the diaphragm.

When measuring liquid levels and the lower pressure tapping is connected via a pressure transfer tube, the pressure to be measured is transmitted to the transmitter location through the medium inside the pressure transfer tube. It is certainly possible for the installation height of the transmitter to be higher than that of the lower pressure tapping. When using differential pressure transmitters to measure liquid levels, in many cases the container is under pressure; if there is a pressure of 0.1 MPa inside the container, this can generate a water column with a height of 10 meters. If the installation height of the differential pressure transmitter is higher than h, the distance between the upper and lower pressure tapping points of the container is h, and the medium inside the container is water, then the range of the differential pressure transmitter △p = -ρgh to ρg(h-h) ； The range for a differential pressure transmitter installed at the same level as the lower pressure tapping is △p=0～ρgh ； Therefore, a negative migration of the differential pressure transmitter is required when it is above the lower pressure tapping; the claim that a liquid level below the transmitter cannot be measured once it is above the lower pressure tapping is incorrect. In many low-temperature media (liquid nitrogen at -196°C), the diaphragm of transmitters cannot withstand such low temperatures. So how can a differential pressure transmitter be used to measure the liquid level in a liquid nitrogen tank? In this case, we wait until the liquid nitrogen coming out of the suction port has vaporized before connecting it to the differential pressure transmitter; as a result, the transmitter can be installed at any height. The high-pressure side transmits pressure to the diaphragm on the positive-pressure side of the transmitter through the vaporized nitrogen, so the installation height has no impact on the measurement.

When using a differential pressure transmitter to measure liquid level, if the installation height of the transmitter is higher than that of the lower pressure tapping point, the medium in the pressure guiding tube will leak away. Calculating the liquid level based on the pressure measured by the pressure transmitter will result in pressure changes, which in turn lead to changes in the liquid level.

When it is above the lower process connection, the transmitter needs to be shifted positive; such a shift corresponds to changing the lower limit of measurement, which reduces the measurement range and violates the requirements for accurate measurement.

When it is above the lower process connection, the transmitter needs to be shifted positive; such a shift corresponds to changing the lower limit of measurement, which reduces the measurement range and violates the requirements for accurate measurement.