Differential Pressure Transmitters

Differential pressure transmitters are transmitters that measure the difference in pressure across the transmitter. Output standard signals (such as 4~20mA, 0~5V).

Differential pressure transmitters are mainly used to measure differential pressure, pressure and negative pressure of liquids, gases and steam. It is also possible to measure liquid levels in open or pressurized vessels. Equipped with a throttling device, it can measure the flow of liquid, gas or steam, and convert the measured parameters into a 4-20mA DC signal output. Density can also be measured with a differential pressure transmitter.

RFQ

Featured Industrial Differential Pressure Transmitters

**SI-804DP Small Differential Pressure Sensor**

**SI-3151DP Differential Pressure Transmitter – Capacitive**

**SI-3151 DLT Differential Pressure Level Transmitter**

**SI-8051DG Monocrystalline Silicon High Accuracy Differential Pressure Transducer**

**SI-D2000 Differential Pressure Gauge**

**8051FY Micro Differential Pressure – Air Pressure Transmitter**

differential pressure transmitter working principle

A differential pressure transmitter is a transmitter that measures the difference in pressure at both ends of the transmitter and outputs a standard signal (such as 4~20mA, 0~5V). Differential pressure transmitters are different from ordinary pressure transmitters in that they have two pressure interfaces.

Differential pressure transmitters are generally divided into positive pressure end and negative pressure end. Generally, the pressure at the positive pressure end of the differential pressure transmitter should be greater than the pressure at the negative pressure section before measurement can be made.

The differential pressure from the pressure-conducting pipes on both sides acts directly on the isolation diaphragms on both sides of the transmitter sensor. The sealing liquid in the diaphragm is conducted to the measuring element.

The measuring element converts the measured differential pressure signal into a corresponding electrical signal and passes it to the converter. After amplification and other processing, it becomes a standard electrical signal output.

differential pressure transmitter working principle

Types of differential pressure transmitters

Capacitive Differential Pressure Transmitter

This transmitter uses a capacitive sensor. The process pressure passes through isolating diaphragms on both sides or on one side. Then the filling fluid acts on the tensioned measuring diaphragm in the delta element (ie, the sensitive element). The measuring diaphragm and the capacitive plates on the insulators on both sides each form a capacitor.

When the pressure on both sides is inconsistent, the measuring diaphragm will be displaced. Its displacement is proportional to the pressure difference. This displacement is converted into differential capacitance formed on the capacitor plates. The differential capacitance is converted into a 4-20mADC two-wire current signal by an electronic circuit.

Diffused silicon differential pressure transmitter

The diffused silicon differential pressure transmitter uses diffused silicon sensing technology. By applying the measured pressure to the silicon chips on both sides respectively, the tiny resistance changes inside the silicon chip are converted into electrical signal output.

Differential pressure transmitters also include measures such as temperature compensation and linear correction to ensure the accuracy and stability of the output signal.

Monocrystalline silicon differential pressure transmitter

The sensor used in the monocrystalline silicon differential pressure transmitter utilizes the piezoresistive effect of monocrystalline silicon.

When single crystal silicon material is subjected to axial stress, its resistivity will change. This phenomenon is called the piezoresistive effect.

Taking advantage of this phenomenon, four mutually symmetrical strain resistors were produced on a single crystal silicon wafer using photolithography and etching processes. Form a Wheatstone bridge.

When the monocrystalline silicon wafer senses pressure changes, the resistance of the two pairs of arms of the bridge changes. causing the bridge output voltage to change. Thus achieving pressure to voltage conversion.

Capacitive Pressure Transmitter-Differential Pressure Transmitter Structure

The output signal of the differential pressure transmitter can be a voltage signal or a current signal.

Normally, its output signal is a standard signal, such as 4~20mA, 1~5V, 0~20mA, 0~5V, RS485, etc. The intelligent type can be configured with 4~20mA+Hart protocol. These signals can be controlled, adjusted and monitored by PLC, DCS and other equipment.

Calibration of conventional differential pressure transmitters:

First adjust the damping to zero state, first adjust the zero point. Then add the full pressure and adjust the full range so that the output is 20mA.

On-site adjustment is all about fast. Here we introduce the quick adjustment method of zero point and range.

When adjusting the zero point, it has almost no impact on the full scale. However, when adjusting the full scale, it has an impact on the zero point. Without migration, its impact is about 1/5 of the range adjustment amount, that is, the range is adjusted upward by 1mA. The zero point will move upward by about 0.2mA. ,vice versa.

For example: if the input full-scale pressure is 100Kpa, the reading is 19.900mA.
Adjust the range potentiometer so that the output is 19.900+(20.000-19.900)×1.25=20.025mA.
When the range increases by 0.125mA, the zero point increases by 1/5×0.125=0.025.
Adjust the zero point potentiometer so that the output is 20.000mA.

After the zero point and full scale adjustment are normal, check the middle scales to see if they are out of tolerance. Make fine adjustments if necessary. Then adjust migration, linearity, and damping.

Calibration of intelligent differential pressure transmitter

It is not possible to calibrate the smart transmitter using the above conventional method, because this is determined by the structural principle of the HART transmitter.

Because the intelligent transmitter is between the input pressure source and the generated 4-20mA current signal. In addition to machinery and circuits, there is also a microprocessor chip that operates on the input data, so the adjustment is different from the conventional method.

In fact, manufacturers also have instructions for the calibration of smart transmitters. For example, for ABB transmitters, calibration can be divided into: “setting the range”, “re-ranging” and “fine-tuning”.

The “setting range” operation mainly completes the configuration work through the digital setting of LRV and URV. The “re-ranging” operation requires connecting the transmitter to a standard pressure source. Guided by a series of instructions, the variable The transmitter directly senses the actual pressure and sets the value.

The initial and final settings of the measuring range depend directly on the actual pressure input value. But it should be noted that although the analog output of the transmitter has the correct relationship with the input value used, the digital reading of the process value will show a slightly different value, which can be calibrated through the fine-tuning item.

Since each part needs to be adjusted individually or jointly, the actual calibration can be carried out according to the following steps:

First make a 4-20mA fine adjustment to calibrate the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required.
Make full fine-tuning again to make the 4-20mA and digital reading consistent with the actual applied pressure signal. Therefore, a pressure signal source is required.
Finally do the re-ranging. Adjust the analog output 4-20mA to match the external pressure signal source. Its function is exactly the same as the zero adjustment (Z) and range adjustment (R) switches on the transmitter shell.

The installation position of the differential pressure transmitter on the process pipeline is related to the medium being measured. In order to obtain better installation results, the following conditions should be considered:
- ① Prevent the differential pressure transmitter from direct contact with corrosive or overheated measured media;
- ②Prevent debris from depositing and clogging in the pressure pipe;
- ③The length of the pressure pipes on both sides of the positive and negative pressure transmitter should be as consistent as possible;
- ④The liquid column pressure heads in the pressure pipes on both sides of the positive and negative pressure transmitters should be balanced;
- ⑤The pressure pipe should be installed where the temperature gradient and temperature fluctuation are minimal.
When measuring liquid flow, the differential pressure transmitter should be installed next to or below the pipe being measured so that bubbles can be discharged into the pipe;
When measuring gas flow, the differential pressure transmitter should be installed next to or above the pipe being measured so that the accumulated liquid can easily flow into the pipe;
When measuring steam flow, the differential pressure transmitter should be installed below the pipe to be measured so that the condensed water can fill the pressure pipe.

Special attention should be paid to preventing the temperature of the differential pressure transmitter from contacting the medium exceeding the limit temperature of the transmitter when measuring steam or other high-temperature media.

Industrial differential pressure transmitters

Differential Pressure Transmitter Industrial Applications

Differential pressure transmitters are widely used in petroleum, chemical, electric power, metallurgy and other industries. This product is mainly used to measure differential pressure, pressure and negative pressure of liquids, gases and steam. It is also possible to measure liquid levels in open or pressurized vessels. Equipped with a throttling device, the flow rate of liquid, gas or steam can be measured. And convert the measured parameters into 4~20mA DC signal output. And passed to the secondary instrument or regulating unit for detection or control.

Measure pressure

The differential pressure transmitter has two pressure holes. Just plug one of them up. It can be used to measure pressure.

Measure differential pressure

Connect the pressure source to the high pressure and low pressure pressure holes of the differential pressure transmitter. You can measure differential pressure.

Measure liquid level

Pressure transmitters and differential pressure transmitters can be used to measure the level of liquids such as water or fuel in vented or sealed tanks. And in many special working conditions, it has advantages that other liquid level transmitters cannot match. For example, measurement of viscos , high-temperature, corrosive liquids or gases in vacuum tanks.
Read More about: Tank Level Measurement with Pressure Transmitters/DP Transmitters

Measure flow

The differential pressure generated by the throttling member is used to measure the pipeline flow. This is our differential pressure flow meter.

Differential Pressure Flow Meters, also called DP flow meters. Differential pressure flow meters consist of flow sensors and pressure/differential transmitters. Flow sensors are the important part of differential pressure flow meters. Like: Orifice plate, Venturi tube, Wedge, V-cone and Averaging Pitot Tubes.

Differential Pressure Transmitter Industrial Applications

Measure density

The principle of the differential pressure density meter can calculate the density value based on the differential pressure value of the medium at a certain vertical distance. It can also automatically perform temperature compensation, with high accuracy, good reliability, and simple installation and use.

It is a two-wire density transmitter. It is mainly used for industrial process control.

The online density meter generates a corresponding 4-20mA signal according to the concentration and density, and can be remotely calibrated and monitored through digital communication.

Differential Pressure Transmitters – Guide and Selection

Featured Industrial Differential Pressure Transmitters

differential pressure transmitter working principle

Types of differential pressure transmitters

Differential Pressure Transmitter Industrial Applications

More Pressure Measurement Solutions

Featured Industrial Differential Pressure Transmitters

differential pressure transmitter working principle

Types of differential pressure transmitters

differential pressure transmitter signal output

Differential pressure transmitter calibration

differential pressure transmitter installation

Differential Pressure Transmitter Industrial Applications

More Pressure Measurement Solutions