What are the Working Principles of Transmitters

The role of the transmitter is to detect process parameters and transmit the measured values in the form of a specific signal for display and regulation. Its role in automatic detection and regulation systems converts various process parameters such as temperature, pressure, flow, level, composition, and other physical quantities into a uniform standard signal. It is then transmitted to regulators and indicator recorders for regulation, indication, and recording.

The transmitters we will discuss today contain four main types. They are pressure transmitters, level transmitters, differential pressure flow transmitters, and temperature transmitters.

Before we can understand how these transmitters work, we need to know some basic concepts and meanings.

1. What do we need to know before understanding the principle of transmitters?

1) Pressure is the force acting vertically on a unit area. (In physics, it is called pressure)

2) Standard atmospheric pressure is the pressure acting on the surface of a division area at sea level at 45° latitude with a temperature of 0°C. The pressure working on the surface of the division area is called a standard atmospheric pressure (1 atm). The height of the equivalent liquid column is expressed as 760mmHg.

3) absolute pressure is the absolute zero pressure as the reference pressure. Absolute zero pressure refers to the state of a sealed container without any gas molecules present inside. Therefore, the pressure gauge used to measure the absolute pressure is called the absolute pressure gauge.

4) Gauge pressure is the pressure at which atmospheric pressure is used as a reference for zero pressure. A Gauge pressure gauge is called a pressure gauge for short.

5)Vacuum is a kind of gauge pressure called negative pressure. p true = p absolute – p atmospheric <0

6) Level is the (relative) position of the surface layer of material stored in a container of a certain volume. It is a collective term for liquid, material, and boundary levels.

7) Liquid level is the relative height of the surface of the liquid in a container.

Material level refers to the relative height of the surface of the solid blocks, particles, powders, and other accumulations.

Boundary level refers to the relative position of the interface of two mutually insoluble substances.

8) Reynolds number is a dimensionless parameter that characterizes the ratio of fluid inertial force to viscous force (Re)

Re < 2000 for the laminar state (flow when the flow line is parallel to the axis of the tube)

Re>4000 for turbulent flow (flow in a pipe with relatively chaotic flow lines)

Once these basic terms are understood, we can better understand the transmitters and how they work. It also helps us to use them in a better way.

2. What’s the working principle of a pressure transmitter

Pressure transmitters are commonly used sensors in industrial practice. It is widely used in various industrial automation environments. It is involved in many industries such as water conservancy and hydropower, rail transportation, intelligent buildings, production automation, aerospace, military, petrochemicals, oil wells, electric power, ships, machine tools, pipelines, etc.

It converts the physical pressure parameters such as gas and liquid sensed by the pressure measuring element sensor into a standard electrical signal (e.g., 4~20mADC, etc.). In turn, it can be supplied to secondary instruments such as indicator alarms, recorders, and regulators for measurement, indication, and process regulation.


The differential pressure transmitter is used to prevent the medium in the pipeline from entering the transmitter directly, and the pressure-sensing diaphragm is connected to the transmitter by a capillary tube filled with fluid. It measures the level, flow, and pressure of a liquid, gas, or steam and then converts it into a 4 to 20mA DC signal output. The differential pressure transmitter is used to measure the differential pressure of a medium in a process pipeline or tank and to convert the measured differential pressure value into a current signal output through data conversion and open square.

3. What’s the working principle of a level transmitter?

The level transmitter is a technological extension and development of the pressure transmitter. Based on the principle that the pressure generated by liquids of different specific gravity at different heights is linearly related, this enables the accurate measurement and transmission of the volume, liquid height, and weight of water, oil, and pastes.

The working principle of the liquid level transmitter is when the two pressures of the measured medium are passed into the high and low-pressure chambers, they act on both sides of the isolation diaphragm of the δ element (i.e., sensitive element). They are transferred to both sides of the measuring diaphragm using the isolation diaphragm and the filling fluid inside the element.

The common types of liquid level transmitters are divided into four classes.

1)Single flange level transmitter

Single flange level transmitters with a flat flange and flange level transmitters with an insertion barrel are used to measure the height of the level of the measured medium by its gravity. The single flange level transmitter is suitable for viscous, easily crystallized media. It is generally used in open vessel or equipment installations.

Single flange level transmitter

2)Double flange level transmitter

A double flange level transmitter is a kind of differential pressure transmitter. It provides a reliable measurement method for avoiding direct contact between the measured medium and the isolation diaphragm of the transmitter. It requires the isolation of the high-temperature medium from the transmitter. It is suitable for measuring media that have a corrosive effect on the sensitive original of the transmitter, as well as suspended liquids or high viscosity media. It can also be used where the measured medium will solidify or crystallize due to changes in ambient or process temperature, where replacement of the measured medium requires rigorous purging of the measuring head, etc. A double flange level transmitter is mainly used to measure the level of sealed pressure vessels.

Double flange level transmitter

3)Throw-in level transmitter

The throw-in level transmitter has a rod type and a range type. The principle is based on the principle that the static pressure of the measured liquid is proportional to the height of the liquid, and the piezoresistive effect of the diffusion silicon or ceramic sensitive element is used, thus converting the static pressure into an electrical signal. It is temperature compensated and linearly corrected to convert the static pressure into a 4-20mADC standard current signal output. This transmitter is easy and simple to install and is highly adaptable. From water and oil to pasty media with high viscosity, it can be measured with high accuracy and is not affected by blistering or sedimentation of the measured medium. Therefore it is generally used for the installation of open containers or equipment.

Throw-in level transmitter

4)RF capacitive level transmitter

The principle is when the probe level and the conductive liquid constitute a capacitor, in which the metal core of the probe level line is one pole of the capacitor, the conductive fluid is the other pole of the capacitor, and the middle is the highly stable polytetrafluoroethylene. In other words, the insulating outer layer of the probe-level wire acts as a medium between the two poles, and as the liquid level changes, the area of the fluid surrounding the probe-level wire changes, thus changing the relative place of the two poles that make up the capacitor. When the liquid level rises, the capacitance increases and the RF circuit converts this change into an analog signal output. The RF capacitive level transmitter is suitable for measuring high-temperature pressure vessels, and the measured value is not affected by the temperature of the liquid being measured, the specific gravity, and the shape and pressure of the ship. It is mainly used for liquid measurement of acids and bases, chlorides, organic solvents, liquid CO2, ammonia, PVC powders, grey materials, oil-water interfaces, Chinese medicine, etc.

4. What’s the working principle of a flow transmitter?

The flow transmitter is a differential pressure flow meter, an instrument for measuring flow rates. It uses the principle that there is a relationship between the differential pressure generated when a fluid flows through a throttling device and the flow rate by measuring the differential pressure to achieve flow determination. The throttling device is a partial contraction element installed in the pipe, the most commonly used are orifice plates, nozzles, and venturi tubes.

Flow transmitter

The flow transmitter consists of a primary device and a secondary device. The primary device is called the flow measurement element, which is installed in the pipeline of the measured fluid, generating a pressure difference proportional to the flow rate (flow rate) for the secondary device to display the flow rate. The secondary equipment is known as a display meter. It receives the differential pressure signal generated by the measuring element and converts it into a corresponding flow rate for display. The primary device of the flow transmitter is often a throttling device or dynamic pressure measuring device (Pitot tube, equalizer tube, etc.). Secondary devices for various mechanical, electronic, and combined differential pressure meters with flow display instrumentation. The differential pressure-sensitive element of the flow transmitter is primarily elastic. As the differential pressure and flow is a square root relationship, the flow display instrument is equipped with the open square device to make the flow scale linearization. Most instruments also have a flow accumulation device to show the cumulative flow for economic accounting.

5. What’s the working principle of the temperature transmitter

Temperature transmitters are devices that convert physical measurement signals or standard electrical signals into standardized electrical outputs or can be output in a communication protocol. For example, a temperature transmitter is an instrument that converts a temperature variable into a standardized output signal that can be transmitted. It is mainly used to measure and control industrial process temperature parameters. A current transmitter is a device that converts the primary circuit AC to be measured into a constant current loop standard signal that is continuously delivered to the receiving device.

Temperature transmitter2

The temperature transmitter uses a thermocouple or RTD as the temperature measuring element, and the output signal from the temperature measuring element is sent to the transmitter module. It is converted into a 4-20mA current signal, a 0-5V/0-10V voltage signal, and an RS485 digital signal output with a linear relationship to the temperature after being processed by circuits such as voltage stabilization and filtering operational amplification, non-linear correction, V/I conversion, constant current and reverse protection.

The transmitter with sensor usually consists of two parts: the sensor and the signal converter. The sensor is mainly a thermocouple or RTD. The signal converter consists primarily of a measuring unit, signal processing, and conversion unit (as RTD and thermocouple indexing meters for the industry are standardized, the signal converter is also called a transmitter when it is a stand-alone product).

Some transmitters have additional display units, and some have a Fieldbus function, as shown below.

A transmitter is called a temperature transmitter. It consists of two sensors that measure the temperature difference, with a continuous function between the output signal and the temperature difference.

There is a continuous function between the transmitter output signal and the temperature variable (usually a linear function). For example, the early production of transmitters had a linear function between the output signal and the temperature sensor’s resistance value (or voltage value).

The standardized output signals were mainly DC signals from 0mA to 10mA and 4mA to 20mA (or 1V to 5V). However, we do not exclude other standardized output signals with special provisions. Temperature transmitters are available in two-wire, three-wire, and four-wire versions according to their power supply wiring.

Temperature Transmitters are available in the electric unit combination meter range and the miniaturized modular, multi-functional intelligent type. The former are available without sensors. The latter two types of transmitters can be formed with thermocouples or RTDs as transmitters with sensors.

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