Characteristics of various transmitters

Characteristics of various transmitters

Sensors and transmitters play a vital role in the field of instrumentation and industrial automation. Unlike sensors, transmitters can not only convert non-electrical quantities into measurable quantities, but also generally have a certain amplification effect. This article briefly introduces the characteristics of various transmitters for users to choose from.

1. Integrated temperature transmitter

Integrated temperature transmitters are generally composed of a temperature probe (thermocouple or RTD sensor) and a two-wire solid-state electronic unit. The temperature probe is directly installed in the junction box in the form of a solid module to form an integrated transmitter. Integrated temperature transmitters are generally divided into two types: RTD and thermocouple.

Thermistor temperature transmitter is composed of reference unit, R/V conversion unit, linear circuit, reverse connection protection, current limiting protection, V/I conversion unit, etc. After the temperature measuring thermal resistor signal is converted and amplified, the linear circuit compensates for the nonlinear relationship between temperature and resistance, and after the V/I conversion circuit, a 4-20mA constant current signal with a linear relationship with the measured temperature is output.

Thermocouple temperature transmitters are generally composed of reference source, cold end compensation, amplifier unit, linearization processing, V/I conversion, broken couple processing, reverse connection protection, current limiting protection and other circuit units. It amplifies the thermoelectric potential generated by the thermocouple through cold end compensation, and then eliminates the nonlinear error between the thermoelectric potential and temperature through a linear circuit, and finally amplifies and converts it into a 4-20mA current output signal. In order to prevent accidents caused by temperature control failure due to thermocouple wire breakage during thermocouple measurement, the transmitter is also equipped with a power-off protection circuit. When the thermocouple wire is broken or the connection is poor, the transmitter will output the maximum value (28mA) to cut off the power supply of the instrument.

The integrated temperature transmitter has the advantages of simple structure, saving lead wires, large output signal, strong anti-interference ability, good linearity, simple display instrument, solid module anti-vibration and moisture-proof, reverse connection protection and current limiting protection, and reliable operation.

The output of the integrated temperature transmitter is a unified 4-20mA signal, which can be used in conjunction with a microcomputer system or other conventional instruments. It can also be made into an explosion-proof or fire-proof measuring instrument as required by the user.

2. Pressure transmitter

Pressure transmitter is also called differential transmitter, which is mainly composed of pressure measuring element sensor, module circuit, display head, case and process connection parts. It can convert the received pressure signal of gas, liquid, etc. into standard current and voltage signal to supply secondary instruments such as indicator alarm, recorder, regulator for measurement, indication and process regulation.

The measurement principle diagram of the pressure transmitter is shown in Figure 3. The measurement principle is: the process pressure and the reference pressure act on the two ends of the integrated silicon pressure sensitive element respectively, and the differential pressure causes the silicon chip to deform (the displacement is very small, only μm level), so that the full dynamic Wheatstone bridge made of semiconductor technology on the silicon chip outputs a mV voltage signal proportional to the pressure under the drive of an external current source. Due to the excellent strength of silicon materials, the linearity and variation index of the output signal are very high. When working, the pressure transmitter converts the measured physical quantity into a mV voltage signal and sends it to a differential amplifier with a high amplification factor that can offset the temperature drift. The amplified signal is converted into a corresponding current signal through voltage-current conversion, and then undergoes nonlinear correction to finally generate a standard current and voltage signal that is linearly corresponding to the input pressure.

Pressure transmitters can be divided into two types according to the pressure measuring range: general pressure transmitters (0.001MPa~20MP3) and micro differential pressure transmitters (0~30kPa).

3. Liquid level transmitter

1. Float type liquid level transmitter

The float type liquid level transmitter consists of a magnetic float, a measuring tube, a signal unit, an electronic unit, a junction box and mounting parts.

The specific gravity of the general magnetic float is less than 0.5, and it can float on the liquid surface and move up and down along the measuring tube. The measuring element is installed in the tube, which can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under the action of external magnetism, and convert the electronic unit into 4-20mA or other standard signal output. The transmitter is a modular circuit with the advantages of acid resistance, moisture resistance, shock resistance, and corrosion resistance. The circuit contains a constant current feedback circuit and an internal protection circuit, which can make the maximum output current not exceed 28mA, so it can reliably protect the power supply and prevent the secondary instrument from being damaged.

2. Float type liquid level transmitter

The float type liquid level transmitter is a float that replaces the magnetic float ball. It is designed based on the Archimedes buoyancy principle. The float type liquid level transmitter uses tiny metal film strain sensing technology to measure the liquid level, interface or density. It can perform regular setting operations by pressing buttons on site during operation.

3. Static pressure or liquid level transmitter

The transmitter works on the principle of measuring liquid static pressure. It generally uses a silicon pressure sensor to convert the measured pressure into an electrical signal, which is then amplified by an amplifier circuit and compensated by a compensation circuit, and finally output in the form of 4-20mA or 0-10mA current.

4. Capacitive level transmitter

Capacitive level transmitters are suitable for industrial enterprises to measure and control the production process during the production process. They are mainly used for long-distance continuous measurement and indication of liquid level of conductive and non-conductive media or powdery and granular solid material level.

The capacitive level transmitter is composed of a capacitive sensor and an electronic module circuit. It is based on a two-wire 4-20mA constant current output. After conversion, it can be output in a three-wire or four-wire mode, and the output signal is formed into a standard signal such as 1-5V, 0-5V, 0-10mA. The capacitive sensor consists of an insulated electrode and a cylindrical metal container containing a measuring medium. When the material level rises, the capacitance changes with the change of the material height because the dielectric constant of the non-conductive material is significantly smaller than the dielectric constant of the air. The module circuit of the transmitter is composed of reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting units. The advantages of using the pulse width modulation principle for measurement are low frequency, no RF interference to the surrounding elements, good stability, good linearity, no obvious temperature drift, etc.

5. Ultrasonic transmitter

Ultrasonic transmitters are divided into two categories: general ultrasonic transmitters (without meter heads) and integrated ultrasonic transmitters. Integrated ultrasonic transmitters are more commonly used.

The integrated ultrasonic transmitter consists of two parts: a meter head (such as an LCD display) and a probe. This transmitter that directly outputs 4-20mA signals is a miniaturized sensitive element (probe) and an electronic circuit assembled together, making it smaller, lighter, and cheaper. Ultrasonic transmitters can be used for liquid level measurement, open channel and open channel flow measurement, and can be used to measure distance.

6. Antimony electrode acidity transmitter

The antimony electrode acidity transmitter is an industrial online analyzer that integrates pH detection, automatic cleaning, and electrical signal conversion. It is a pH value measurement system composed of an antimony electrode and a reference electrode. In the measured acidic solution, an antimony trioxide oxide layer is generated on the surface of the antimony electrode, so a potential difference is formed between the metal antimony surface and the antimony trioxide. The magnitude of this potential difference depends on the concentration of antimony trioxide, which corresponds to the degree of hydrogen ions in the measured acidic solution. If the degrees of antimony, antimony trioxide, and aqueous solution are all regarded as 1, the electrode potential can be calculated using the Nernst formula.

The solid module circuit in the antimony electrode acidity transmitter consists of two parts. For the safety of on-site operation, the power supply part uses AC 24V to power the secondary instrument. In addition to providing driving power for the cleaning motor, this power supply should also be converted into a corresponding DC voltage through a current conversion unit for use in the transmitter circuit. The second part is the measurement transmitter circuit, which amplifies the reference signal and the pH acidity signal from the sensor and sends them to the slope adjustment and positioning adjustment circuit to reduce and adjust the signal internal resistance. The amplified pH signal is compared with the temperature compensated

The signals are superimposed and then sent to the conversion circuit, and finally a 4-20mA constant current signal corresponding to the pH value is output to the secondary instrument to display and control the pH value.