DP Level Transmitter-Differential Pressure Tank Level Measurement

Applications of DP level transmitters

The differential pressure liquid level transmitter can prevent the measured medium from directly contacting the sensor diaphragm of the transmitter. It is suitable for the following situations:

1.  When the measured medium has a corrosive effect on the transmitter connector and sensitive components;
2. When the high temperature measured medium needs to be isolated from the transmitter;
3. When there are solid suspended solids or high viscosity in the measured medium, it is easy to block the transmitter connector and the pressure chamber;
4. When the measured medium is easily cured or crystallized by the pressure tube;
5. When changing the measured medium needs to be rinsed and not to be mixed;
6. Hygienic conditions must be maintained to prevent pollution. .
7. Changing the measured medium requires strict purification of the measuring head
8. Suspended liquid or high viscosity medium
9. Measurement of sealed pressure vessel.

Read more What is a diaphragm seal?

Differential pressure type level transmitter working principle

The differential pressure level transmitter is an instrument that uses the pressure generated by the liquid column to measure the height of the liquid level. When the liquid level changes, the pressure received by the diaphragm at the high-pressure side flange will change accordingly. The differential pressure value calculated by the transmitter will also change accordingly. There is a linear relationship between them.

Normally, the high-pressure side (H side) and the low-pressure side (L side) cannot be installed reversely. Generally, the H side is installed at the lower part of the equipment, and the L side is installed at the higher part of the equipment. Before installation, the flange surface of the equipment should be cleaned and the gasket should not be placed off-center to prevent the diaphragm from being punctured or deformed, causing distortion of the transmitter.

Differential pressure transmitter level measurement calculation

When using a Differential pressure (DP) level transmitter to measure the liquid level as shown in the figure below.

The measured liquid density in the figure is ρ.
The working medium density in the capillary of the double flange differential pressure transmitter is ρ0.
The measuring range of the measured liquid level is H.
The center distance of the sampling tube of the measured liquid level is h.

It can be seen from the figure that the maximum measurement range of the liquid level △ P = P + — P- = H × ρ × g – h × ρ0 × g.

It can be seen from the formula that the dual-flange differential pressure transmitter should perform negative migration. The migration amount S is h × ρ0 × g. And the installation position of the double flange differential pressure transmitter has no effect on the migration amount and the measurement result.

The dual-flange differential pressure transmitter requires negative migration.

When the measured liquid level is 0, the pressure difference between the positive and negative measurement chambers of the remote differential pressure transmitter is the largest. The output current of the double flange differential pressure transmitter is 4mA.

As the measured liquid level rises, the pressure difference between the positive and negative measurement chambers of the transmitter gradually decreases.

When the measured liquid level rises to the highest Hmax. The pressure difference between the positive and negative measurement chambers of the transmitter is the smallest. The output current of the double flange differential pressure transmitter is 20mA.

Read more about: Differential Pressure Transmitter Installation Guide

DP level transmitter calibration

The differential pressure liquid level transmitter has been calibrated in the range, accuracy, linearity and other parameters according to customer requirements at the factory. And mark the range, accuracy, etc. on the nameplate of the differential pressure liquid level transmitter. As long as the density and other parameters of the measured medium meet the requirements of the nameplate, generally no adjustment is required.

If the customer needs to adjust the range or zero position, please adjust according to the following methods. Assuming that the range of the differential pressure liquid level transmitter is 0 ~ 10 meters:

Extended reading: How to calibrate HART pressure transmitters