Desen Instrument introduces the selection of various transmitters for you!

Sensors and transmitters play a crucial role in the fields of instruments, meters, and industrial automation. Unlike sensors, transmitters not only convert non electricity into measurable electricity, 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

The integrated temperature transmitter generally consists of a temperature measuring probe (thermocouple or thermistor 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, forming an integrated transmitter. Integrated temperature transmitters are generally divided into two types: thermistor and thermocouple.

The thermal resistance temperature transmitter is composed of a reference unit, R/V conversion unit, linear circuit, reverse protection, current limiting protection, V/I conversion unit, etc. After the thermal resistance signal is converted and amplified, the non-linear relationship between temperature and resistance is compensated by a linear circuit. After the V/I conversion circuit, a constant current signal of 4-20mA is output, which is linearly related to the measured temperature.

Thermocouple temperature transmitters generally consist of circuit units such as a reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, decoupling processing, reverse connection protection, and current limiting protection. It amplifies the thermoelectric potential generated by the thermocouple through cold junction compensation, 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. 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 breaks or is poorly connected, 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, wire saving, large output signal, strong anti-interference ability, good linearity, simple display instrument, solid module seismic 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; Can be used in conjunction with microcomputer systems or other conventional instruments. Users can also request to make explosion-proof or fireproof measuring instruments.

　　2、 Pressure transmitter

A pressure transmitter, also known as a differential transmitter, is mainly composed of pressure measuring elements, sensors, module circuits, display heads, watch cases, and process connectors. It can convert the received pressure signals of gases, liquids, etc. into standard current and voltage signals to supply secondary instruments such as indicator alarms, recorders, regulators, etc. for measurement, indication, and process adjustment.

The measurement principle diagram of the pressure transmitter is shown in Figure 3. The measurement principle is that the process pressure and reference pressure are respectively applied to the two ends of the integrated silicon pressure sensitive element, and the differential pressure causes the silicon wafer to deform (with a displacement of only μ m level), so that the fully dynamic Wheatstone bridge made of semiconductor technology on the silicon wafer can output a mV level 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 both high. During operation, the pressure transmitter converts the measured physical quantity into a mV level voltage signal and sends it to a differential amplifier with high amplification and the ability to offset temperature drift. The amplified signal is converted into a corresponding current signal through voltage current conversion, and then subjected to nonlinear correction to generate a standard current voltage signal that corresponds linearly to the input pressure.

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

　　3、 Liquid level transmitter

　　1. Floating ball liquid level transmitterIt consists of a magnetic float, measuring conduit, signal unit, electronic unit, junction box, and installation components. Generally, the specific gravity of a magnetic float is less than 0.5, and it can float above the liquid surface and move up and down along the measuring tube. The conduit is equipped with measuring elements that can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under external magnetic action, and convert the electronic unit into 4-20mA or other standard signal output. This transmitter is a modular circuit with advantages such as acid resistance, moisture resistance, shock resistance, and corrosion resistance. The circuit contains a constant current feedback circuit and an internal protection circuit, which can ensure that the maximum output current does not exceed 28mA. Therefore, it can reliably protect the power supply and prevent damage to the secondary instrument.

　　2. Float type liquid level transmitterIt is to replace the magnetic float with a float, which is designed based on Archimedes' principle of buoyancy. Float type liquid level transmitter uses tiny metal film strain sensing technology to measure the liquid level, boundary or density of liquids. It can perform routine setting operations through on-site buttons during operation.

　　3. Static pressure or liquid level transmitterWorking based on the measurement principle of 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 amplification 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, mainly used for remote continuous measurement and indication of liquid level or solid material level in conductive and non-conductive media.

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

　　5、 Ultrasonic transmitter

Ultrasonic transmitters are divided into two types: general ultrasonic transmitters (without headers) and integrated ultrasonic transmitters, with integrated ultrasonic transmitters being more commonly used.

The integrated ultrasonic transducer consists of a header (such as an LCD display) and a probe. This transmitter, which directly outputs 4-20mA signals, is assembled with miniaturized sensitive components (probes) and electronic circuits, making it smaller in size, lighter in weight, and cheaper in price. Ultrasonic transmitters can be used for liquid level. Measurement of material level and flow measurement of channels, open channels, etc., and can be used to measure distance.

　　6、 Antimony electrode acidity transmitter

Antimony electrode acidity transmitter is an industrial online analytical instrument that integrates pH detection, automatic cleaning, and electrical signal conversion. It is a pH measurement system composed of antimony electrode and reference electrode. In the tested acidic solution, due to the formation of an antimony trioxide oxide layer on the surface of the antimony electrode, a potential difference will be formed between the metal antimony surface and antimony trioxide. The magnitude of this potential difference depends on the concentration of antimony trioxide, which corresponds to the moderate concentration of hydrogen ions in the measured acidic solution. If the appropriate values of antimony, antimony trioxide, and aqueous solution are all taken as 1, their electrode potentials can be calculated using the Nernst formula.

The solid module circuit in the antimony electrode acidity transmitter consists of two main parts. For the sake of safety on site, the power supply uses AC 24V to power the secondary instruments. This power supply should not only provide driving power for the cleaning motor, but also be converted into corresponding DC voltage through a current conversion unit for use in the transmission circuit. The second part is the measurement transmitter circuit, which amplifies the reference signal and pH acidity signal from the sensor and sends them to the slope adjustment and positioning adjustment circuit to reduce the internal resistance of the signal and make it adjustable. The amplified pH signal is superimposed with the temperature compensated signal and then subtracted into the conversion circuit. Finally, a 4-20mA constant current signal corresponding to the pH value is output to the secondary instrument for display and control of the pH.

　　7、 Acid, alkali, salt concentration transmitter

Acid, alkali, and salt concentration transmitters determine concentration by measuring the conductivity of the solution. It can continuously detect the concentration and content of acids, bases, and salts in aqueous solutions during industrial processes online. This type of transmitter is mainly used in industrial production processes such as boiler feedwater treatment, chemical solution preparation, and environmental protection.

The working principle of acid, alkali, and salt concentration transmitters is that within a certain range, the concentration of acid-base solutions is proportional to their conductivity. Therefore, as long as the conductivity of the solution is measured, the level of acid-base concentration can be determined. When the tested solution flows into a dedicated conductivity cell, if electrode polarization and distributed capacitance are ignored, it can be equivalent to a pure resistance. When a constant voltage alternating current flows through, the output current is linearly related to the conductivity, which is proportional to the concentration of acid and alkali in the solution. Therefore, as long as the solution current is measured, the concentrations of acid, alkali, and salt can be calculated. The acid, alkali, and salt concentration transmitter mainly consists of a conductivity cell, an electronic module, a display head, and a housing. The electronic module circuit consists of excitation power supply, conductivity pool, conductivity amplifier, phase sensitive rectifier, demodulator, temperature compensation, overload protection, and current conversion units.

　　8、 Conductivity transmitter

It is a process instrument (integrated transmitter) that indirectly measures ion concentration by measuring the conductivity value of a solution, and can continuously detect the conductivity of aqueous solutions in industrial processes online.

Due to the fact that electrolyte solutions are good conductors of electricity, just like metal conductors, there must be a resistance effect when current flows through the electrolyte solution, which conforms to Ohm's law. But the resistance temperature characteristics of liquids are opposite to those of metal conductors, with negative temperature characteristics. To distinguish from metal conductors, the conductivity of electrolyte solutions is represented by conductivity (reciprocal of resistance) or conductivity (reciprocal of resistivity). When two mutually insulated electrodes form a conductivity cell, if the test solution is placed in the middle and a constant voltage alternating current is applied, a current circuit is formed. If the voltage and electrode size are fixed, there is a certain functional relationship between the circuit current and conductivity. In this way, by measuring the current flowing through the test solution, the conductivity of the test solution can be determined. The structure and circuit of the conductivity transmitter are the same as those of acid, alkali, and salt concentration transmitters.

　　9、 Intelligent transmitter

Intelligent transmitters are composed of sensors and microprocessors (microcomputers). It fully utilizes the computing and storage capabilities of microprocessors to process sensor data, including conditioning of measurement signals (such as filtering, amplification, A/D conversion, etc.), data display, automatic correction, and automatic compensation.

The microprocessor is the core of an intelligent transmitter. It can not only calculate, store, and process measurement data, but also adjust sensors through feedback loops to achieve optimal data collection. Due to its various software and hardware functions, microprocessors can accomplish tasks that traditional transmitters find difficult to accomplish. So intelligent transmitters reduce the manufacturing difficulty of sensors and greatly improve their performance. In addition, intelligent transmitters also have the following characteristics:

1. It has the ability to automatically compensate for nonlinearity, temperature drift, and time drift of sensors through software.

2. It can self diagnose and perform self inspection on the sensor after power on to check whether all parts of the sensor are normal and make judgments.

3. Data processing is convenient and accurate, and can be automatically processed according to internal programs, such as statistical processing, removal of abnormal values, etc.

4. It has bidirectional communication function. Microprocessors can not only receive and process sensor data, but also provide feedback to sensors to adjust and control the measurement process.

5. It can store and remember information, including sensor feature data, configuration information, and compensation characteristics.

6. It has a digital interface output function, which can easily connect the output digital signal to a computer or fieldbus.