# How to Apply and Select Pressure & Differential Pressure Transmitters

Pressure transmitters are ubiquitous sensors in industrial practice. It is widely used in various industrial automation environments, involving water conservancy and hydropower, railway transportation, intelligent construction, production automation, aerospace, military, petrochemical, oil well, electric power, ship, machine tool, pipeline, and many other industries.

## 1. Difference between pressure transmitters and differential pressure transmitters

A differential pressure transmitter measures the difference in pressure between two (containers) of gas or liquid; it is a relative quantity.

The pressure transmitter measures the magnitude of a single (vessel) gas or liquid; it is an absolute quantity.

Differential pressure transmitters are for gas or liquid pressure. Therefore, pressure transmitters are generally referred to as gas and fluid pressure. The pressure is also called pressure sensors (force sensors).

Differential pressure transmitter is the measurement of two (container) gas or liquid pressure difference is a relative amount; pressure transmitter is the measurement of a single (container) gas or liquid pressure size, is an absolute amount. In terms of use, differential pressure transmitters have two input ports for introducing two samples to be measured to obtain their differential pressure; pressure transmitters have only one port.

## 2. Working principle

The measuring principle of the pressure transmitter is as follows. The medium differential pressure (pressure) is transmitted through the isolation diaphragm and silicon oil to the measuring diaphragm located in the center of the eighth chamber. It acts as an elastic element and deforms with differential pressure on both sides.

The displacement of the measuring diaphragm is proportional to the differential pressure and is very large at 0.1 mm. Therefore, removing the measuring diaphragm causes a change in the capacitance of the differential capacitor formed by the pressure-sensitive diaphragm and the two fixed electrodes. This change in capacitance is converted by the measuring circuit (capacitance and current conversion circuit) into a DC signal, converted into a 4-20 mA DC output by an operational amplifier circuit, algebraically with the zero signal.

## 3. Differences in use

Pressure transmitters are field instruments in direct contact with the measured medium and often work in high temperature, low temperature, corrosion, vibration, and shock environments. The main parameter measured by pressure (differential pressure) transmitters is pressure or differential pressure, but they can indirectly count several parameters.

In addition to measuring pressure, pressure transmitters can also measure the liquid level in equipment. When calculating the fluid level in an atmospheric pressure vessel, one pressure transmitter is required.

When measuring the level of the container under pressure, consider using a differential pressure transmitter by measuring the lower limit of the tank that is 1, the upper limit of 1, their output signal for subtraction. You can measure the level, which is generally used differential pressure transmitter. In the case of a constant level and pressure value in the container, it can also be used to measure the density of the medium.

The differential pressure transmitter can measure the differential pressure value of the two measured pressure can also be used with a variety of throttling elements to measure the flow of medium fluid (orifice flowmeter). It can also directly measure the level of the container under pressure and atmospheric pressure, container level, pressure, negative pressure, etc.

## 4. Selection of models

The type of instrument must be selected to meet the process requirements. We can only ensure the everyday work of the device and safe production. The selection of pressure (differential pressure) transmitters is mainly based on the following situations.

### 4.2 Characteristics of the medium to be measured

We take the nature of the medium to be measured as an indicator. We save money and facilitate installation and maintenance as a reference.

The temperature, viscosity, corrosiveness, flammability, and explosiveness of the medium to be measured and whether these properties have special requirements for the instrument (spring tube material). We can directly use a standard pressure transmitter if the measurement is of relatively clean fluid.

Suppose the measured medium is easy to crystallize or viscous. In that case, we use external diaphragms or chemical seals together, which will effectively prevent the medium from blocking the pressure measurement hole, thus playing a role in protecting the pressure transmitter and extending its service life.

### 4.3 the choice of the measurement range

Public pressure (differential pressure) transmitters have a specific range adjustable range. They will use the content set in 1/3 to 2/3 section to guarantee the accuracy, for the micro differential pressure transmitter is even more critical. Therefore, when measuring tremendous pressure values, we choose a transmitter with a pressure range about 1.5 times larger than the immense value.

There are peaks and continuous irregular up and down fluctuations in many systems, especially in water pressure measurement and processing, which can destroy the pressure sensor. So we can use a buffer to reduce the pressure burr, but this reduces the sensor’s response. So when we choose a transmitter, we have to take complete account of the pressure range, accuracy, and stability.

### 4.4 Accuracy class.

We have to measure the pressure range and the vast absolute error allowed in the process. We first select the field. Then we calculate the huge quoted permitted error in the instrument according to the total error. Finally, we have to remove the ± sign and %, the value of which is the accuracy. If this level is unavailable, rely on the accuracy level (too high precision).

Pressure (differential pressure) transmitters are marked with different levels of accuracy in various countries.

For example, in China, the United States, and other countries, they are marked by the sensor’s accuracy in the part of the linearity is excellent, which is what we usually call the measurement range between 10% a 90% accuracy.

On the other hand, the European standard accuracy is bad linearity. We usually call the measurement of the inverse of 0-10% and 90% to 100% of the accuracy. For example, if the accuracy of the European standard is 1%, the accuracy of the bar in China is 0.5%.

### 4.5 Output signals

At present, the output signal of the pressure transmitter on the market now has a variety. Outputs are mainly 4 to 20 mA, 0 to 2O mA, 0 to 10 V, 0 to 5 V, etc. However, the two more commonly used are 4 20 mA and 0 to l0 V. Among these output signals listed above, only 4 to 20 mA is a two-wire system (including ground or shield), while the others are all three-wire systems.

### 4.6 Medium temperature

As the signal of the pressure transmitter is converted through the electronic circuit part. So, in general. The temperature of the measuring medium of the pressure transmitter is 30 to +100 ℃.

We generally use a condensation bend to cool the medium if the temperature is too high. Compared to let the manufacturer specially produced a high-temperature resistant pressure transmitter. It will reduce the cost considerably compared to having the manufacturer make a particular pressure transmitter resistant to high temperatures.

## 5. Conclusion

Among the various types of instruments, transmitters are standard. Transmitters are broadly divided into pressure and differential pressure transmitters, commonly used to measure pressure, differential pressure, vacuum, liquid level, flow, and density. Although, unlike sensors, transmitters can convert non-electricity into measurable electricity, they also have a specific amplification role.

In today’s modernized and developed industry, pressure transmitters play an essential role in industrial control systems, realizing the pressure of the design displayed on the control screen and participating in crucial logic calculations and parameter alarm functions.

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