Selection of various types of transmitters

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Selection of various transmitters Sensors and transmitters play a crucial role in the fields of instruments, meters, and industrial automation. Unlike sensors, transmitters not only convert non-electrical quantities into measurable electrical quantities but also generally have a amplification function. This article briefly introduces the features of various types of transmitters to assist users in making their choices. I. Integrated temperature transmitter An integrated temperature transmitter generally consists of a temperature sensing probe (thermocouple or thermistor sensor) and a two-wire solid-state electronic unit. The temperature sensing probe is directly installed in the junction box in the form of a solid module, thereby creating an integrated transmitter. Integrated temperature transmitters are generally divided into two types: thermistor and thermocouple types. A thermistor temperature transmitter consists of a reference unit, an R/V conversion unit, a linear circuit, reverse connection protection, current limiting protection, a V/I conversion unit, and so on. After the signal from the temperature-sensing thermistor is amplified, a linear circuit is used to compensate for the non-linear relationship between temperature and resistance; following this, a V/I conversion circuit outputs a constant current signal of 4–20 mA that has a linear relationship with the measured temperature. A thermocouple temperature transmitter generally consists of circuit units such as a reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, open-circuit protection, reverse connection protection, and current limiting protection. It amplifies the thermoelectric potential generated by the thermocouple after cold-junction compensation, then uses a linear circuit to eliminate the nonlinear errors between the thermoelectric potential and temperature, and finally amplifies it to produce a 4–20mA current output signal. To prevent accidents caused by loss of temperature control due to broken wires in the thermocouple during measurement, the transmitter is also equipped with a power-off protection circuit. When the thermocouple wire breaks or the connection is poor, the transmitter outputs a maximum value (28mA) to cause the instrument to cut off power. Integrated temperature transmitters offer advantages such as a simple structure, reduced need for wires, high output signal strength, strong resistance to interference, good linearity, simplicity in terms of display instruments, shock and moisture resistance due to their solid module design, reverse connection protection and current limiting protection, as well as reliable operation. The output of the integrated temperature transmitter is a unified 4–20mA signal ； It can be used in conjunction with microcomputer systems or other conventional instruments. Measurement instruments can also be made explosion-proof or fire-resistant at the user’s request. II. Pressure Transmitter A pressure transmitter, also known as a differential transmitter, is primarily composed of a pressure-sensing element, module circuitry, a display gauge, a housing, and process connection components. It can convert pressure signals of gases, liquids, etc., received as input, into standard current and voltage signals for use by secondary instruments such as indicator alarms, recorders, and regulators for measurement, indication, and process control. The measurement principle diagram of the pressure transmitter is shown in Figure 3. Its measurement principle is as follows: process pressure and reference pressure act on opposite ends of an integrated silicon pressure-sensitive element; the pressure difference causes the silicon chip to deform (by a very small amount, only in the μm range), which enables the fully dynamic Wheatstone bridge fabricated on the silicon chip, using semiconductor technology, to generate a voltage signal in the mV range that is proportional to the pressure, driven by an external current source. Due to the excellent strength of silicon materials, both the linearity of the output signal and the distortion metrics are very high. During operation, the pressure transmitter converts the physical quantity being measured into a voltage signal in the mV range, which is then sent to a differential amplifier with a high gain that can cancel out temperature drift. The amplified signal is converted into a corresponding current signal through voltage-to-current conversion, followed by nonlinear correction, to ultimately produce a standard current-voltage signal that has a linear relationship with the input pressure. Pressure transmitters can be divided into ordinary pressure transmitters (0.001 MPa to 20 MPa) and micro-differential pressure transmitters (0 to 30 kPa), depending on their pressure measurement range. III. Level Transmitters 1. Float-type Level Transmitter A float-type level transmitter consists of a magnetic float, a measurement tube, a signal unit, an electronic unit, a junction box, and mounting components. Generally, the specific gravity of magnetic floating balls is less than 0.5, allowing them to float on the surface of the liquid and move up and down along the measurement conduit. The conduit is equipped with a measuring element that, under the influence of an external magnetic field, converts the liquid level signal being measured into a resistance signal proportional to the change in liquid level; it also converts the electronic signal into a 4–20mA signal or other standard signals for output. This transmitter is a modular circuit that offers advantages such as acid resistance, moisture resistance, shock resistance, and corrosion resistance. It contains a constant current feedback circuit as well as internal protection circuits, which ensure that the maximum output current does not exceed 28mA. This allows for reliable protection of the power supply and prevents damage to secondary instruments. 2. Float-type level transmitter: The float-type level transmitter replaces the magnetic float with a buoy; it is designed based on Archimedes’ principle of buoyancy. Float-type level transmitters utilize micro metal diaphragm strain sensing technology to measure the level, interface, or density of liquids. It can be used for routine setting operations via on-site buttons while it is in operation. 3. Hydrostatic or level transmitter: This transmitter operates on the principle of measuring hydrostatic pressure of liquids. 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, before being output in the form of a current of 4–20mA or 0–10mA. IV. Capacitive level transmitters: Capacitive level transmitters are suitable for use by industrial enterprises in measuring and controlling production processes. They are primarily used for the remote, continuous measurement and indication of liquid levels in conductive and non-conductive media, as well as of levels of granular solids. A capacitive level transmitter consists of a capacitive sensor and an electronic module circuit. It operates on a two-wire 4–20mA constant current output as its basic configuration; after conversion, it can also provide outputs via three-wire or four-wire connections, with the output signal taking the form of standard signals such as 1–5V, 0–5V, or 0–10mA. A capacitive sensor consists of an insulating electrode and a cylindrical metal container filled with a measuring medium. As the material level rises, since the dielectric constant of non-conductive materials is significantly lower than that of air, the capacitance changes as the height of the material changes. The modular circuit of the transmitter consists of components such as a reference source, pulse width modulation, conversion, constant current amplification, feedback, and current limiting. The advantages of using the pulse width modulation principle for measurement include a lower frequency, resistance to RF interference from surrounding components, good stability, good linearity, and no significant temperature drift. V. Ultrasonic transmitters Ultrasonic transmitters are divided into two types: conventional ultrasonic transmitters (without a gauge) and integrated ultrasonic transmitters, with the latter being more commonly used. An integrated ultrasonic transducer converter consists of a display unit (such as an LCD monitor) and a probe. This type of transmitter, which outputs a 4–20mA signal directly, combines a compact sensor element (the probe) with electronic circuits, thereby resulting in a smaller size, lower weight, and lower cost. Ultrasonic transmitters can be used for level measurement. It can be used for measuring level, as well as flow rates in canals and open channels, and it can also be employed to measure distances. VI. Antimony electrode pH transmitter The antimony electrode pH transmitter is an industrial online analysis instrument that combines pH detection, automatic cleaning, and electrical signal conversion; it is a pH measurement system composed of an antimony electrode and a reference electrode. In the acidic solution under test, an antimony trioxide oxide layer forms on the surface of the antimony electrode, which creates a potential difference between the surface of the metallic antimony and the antimony trioxide. The magnitude of this potential difference depends on the concentration of antimony dioxide trioxide, which corresponds to the concentration of hydrogen ions in the acidic solution being measured. If the activities of antimony, antimony trioxide, and aqueous solution are all set to 1, their electrode potentials can be calculated using the Nernst equation. The solid module circuit in the antimony electrode acidity transmitter consists of two main parts. For safety during on-site operation, the power supply section uses 24V AC to power the secondary instruments. In addition to providing the driving power for the cleaning motor, this power supply must also be converted into the appropriate DC voltage through a current conversion unit for use by the transmission circuit. The second part is the measurement transmitter circuit, which amplifies the reference signal from the sensor as well as the PH acidity signal, and then sends them to the slope adjustment and positioning adjustment circuits, thereby reducing the impedance of the signals and allowing for adjustments to be made. The amplified PH signal is combined with the temperature-compensated signal, then fed into a differential conversion circuit; finally, a 4–20mA constant current signal corresponding to the PH value is output to the secondary instrument in order to enable display and control of the PH value. VII. Acid, Alkali, and Salt Concentration Transmitters Acid, alkali, and salt concentration transmitters determine concentration by measuring the electrical conductivity of the solution. It can continuously detect online the concentration of acids, bases, and salts in aqueous solutions within industrial processes. These transmitters are primarily used in industrial processes such as boiler feedwater treatment, preparation of chemical solutions, and environmental protection. The working principle of acid, base, and salt concentration transmitters is that, within a certain range, the concentration of acid or base solutions is proportional to their electrical conductivity. Therefore, by measuring the value of the solution’s conductivity, it is possible to determine the level of acid or base concentration. When the solution under test flows into a dedicated conductivity cell, if electrode polarization and distributed capacitance are ignored, it can be treated as an equivalent pure resistor. When a constant-voltage alternating current flows through it, the output current is linearly related to the conductivity, which in turn is proportional to the concentration of acids and bases in the solution. Therefore, by measuring the solution current, the concentrations of acids, bases, and salts can be calculated. Acid, alkali, and salt concentration transmitters are mainly composed of a conductivity cell, an electronic module, a display gauge, and a housing. The electronic module circuit consists of units such as an excitation power supply, a conductivity cell, a conductivity amplifier, a phase-sensitive rectifier, a demodulator, temperature compensation, overload protection, and current conversion. VIII. Conductivity Transmitter: It is a process instrument (integrated transmitter) that indirectly measures ion concentration by detecting the conductivity of a solution, enabling continuous online measurement of the conductivity of aqueous solutions in industrial processes. Since an electrolyte solution is a good conductor of electricity just like a metal conductor, resistance must be present when electric current flows through it, and Ohm’s law applies. However, the resistance-temperature behavior of liquids is opposite to that of metal conductors, exhibiting a negative temperature coefficient. To distinguish them from metal conductors, the conductivity of an electrolyte solution is expressed in terms of conductance (the reciprocal of resistance) or conductivity (the reciprocal of resistivity). When two mutually insulated electrodes form a conductivity cell, and a solution to be tested is placed between them with a constant-voltage alternating current applied, an electrical circuit is created. If the voltage magnitude and electrode size are fixed, there is a certain functional relationship between the loop current and conductivity. In this way, by measuring the current flowing through the solution to be tested, its conductivity can be determined. The structure and circuit of the conductivity transmitter are the same as those of the acid, alkali, and salt concentration transmitters. IX. Intelligent Transmitters An intelligent transmitter is composed of a sensor and a microprocessor (microcomputer). It makes full use of the computing and storage capabilities of microprocessors to process sensor data, including the conditioning of measurement signals (such as filtering, amplification, A/D conversion, etc.), data display, automatic calibration, and automatic compensation. The microprocessor is the core of intelligent transmitters. It can not only calculate, store, and process measurement data, but also adjust the sensors through a feedback loop to ensure that the collected data is of the highest quality. Due to its various software and hardware functions, the microprocessor is capable of performing tasks that are difficult for traditional transmitters to accomplish. Therefore, intelligent transmitters reduce the manufacturing difficulty of sensors and significantly improve their performance. In addition, intelligent transmitters also have the following features: 1. They possess automatic compensation capabilities, allowing for automated correction of sensor nonlinearities, temperature drift, and time drift through software. 2. It supports self-diagnosis: upon power-up, the sensor can be self-checked to determine whether all its components are functioning properly. 3. Data processing is convenient and accurate; it can automatically handle data according to internal procedures, such as performing statistical analyses or removing abnormal values. 4. It has bidirectional communication capabilities. Microprocessors can not only receive and process sensor data but also send information back to the sensors, thereby adjusting and controlling the measurement process. 5. It can store and retain information, capable of storing characteristic data of sensors, configuration information, and compensation parameters, etc. 6. It features a digital interface output function, allowing the output digital signals to be easily connected to computers or fieldbuses, etc.