Transmitter selection

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1. What type of pressure is the transmitter intended to measure? First, determine the maximum pressure value that needs to be measured in the system. Generally, it is necessary to choose a transmitter with a pressure range that is about 1.5 times greater than this maximum value. This is mainly because in many systems, especially those involved in pressure measurement and processing, there are peak values as well as continuous irregular fluctuations. Such sudden peak values can damage pressure sensors, while continuously high pressure levels or those slightly above the transmitter’s rated maximum can shorten the sensor’s lifespan; however, this approach leads to a decrease in accuracy. Therefore, a buffer can be used to reduce pressure spikes, but this will decrease the sensor’s response speed. Therefore, when selecting a transmitter, the pressure range, accuracy, and stability must be taken into full consideration. 2. What kind of pressure medium: We need to consider the medium that the pressure transmitter is designed to measure. Viscous liquids and mud can clog the pressure connections, while solvents or corrosive substances may damage the materials in the transmitter that come into direct contact with these media. These factors will determine whether to choose a direct isolation film and materials in direct contact with the dielectric. The material used for the part of pressure transmitters that comes into contact with the medium is usually 316 stainless steel. If your medium does not corrode 316 stainless steel, then virtually all pressure transmitters are suitable for measuring the pressure of that medium. If your medium does corrode 316 stainless steel, then chemical sealing is necessary; this not only allows for the measurement of the medium’s pressure but also prevents the medium from coming into contact with the parts of the pressure transmitter that are in contact with it, thereby protecting the transmitter and extending its lifespan. 3. What level of precision is required for the transmitter? Factors that determine precision include non-linearity, hysteresis, non-repeatability, temperature effects, zero-point offset, and the influence of temperature. But due mainly to non-linearity, hysteresis, and non-repeatability, the higher the precision, the higher the price. Every electronic measuring instrument has accuracy errors, but the accuracy levels specified vary from one standard to another. For example, in countries like China and the United States, the specified accuracy refers to the part of the sensor’s range where its linearity is best, that is, within 10% to 90% of the measurement range. In contrast, the European standard defines accuracy for the parts of the range where linearity is poorest, namely between 0% to 10% and 90% to 100% of the measurement range. If the accuracy specified by European standards is 1%, then it is 0.5% according to Chinese standards. 4. Temperature range of the transmitter: Typically, a transmitter is calibrated for two temperature ranges – the range for normal operation and the range within which temperature can be compensated for. The normal operating temperature range refers to the temperature range within which the transmitter can operate without being damaged; outside this range, it may fail to meet its specified performance criteria. The temperature compensation range is a typical range that is smaller than the operating temperature range. Operating within this range, the transmitter will definitely meet its specified performance criteria. Temperature variations affect its output in two ways: one is zero drift, and the other is an impact on the full-scale output. For example: +/-X%/℃ of full scale, +/-X%/℃ of the reading; +/-X% of full scale when outside the temperature range, and +/-X% of the reading within the temperature compensation range. The absence of these parameters leads to uncertainty in use. The change in the transmitter output is caused by pressure changes or by temperature changes. The effect of temperature is the most complex part when understanding how to use transmitters. 5. What kind of output signal is required: mV, V, mA, or frequency; digital output. The choice of output depends on various factors, including the distance between the transmitter and the system controller or display, as well as the presence of \"noise\" or other electronic interference signals. Whether an amplifier is needed, the location of the amplifier, etc. For OEM devices where the distance between many transmitters and controllers is short, transmitters with mA output represent the most economical and effective solution; if it is necessary to amplify the output signal, it is best to use transmitters with built-in amplification. For long-distance transmission or in the presence of strong electronic interference signals, it is best to use mA-level output or frequency output. In environments with high RFI or EMI levels, in addition to choosing mA or frequency output, special protection or filters also need to be considered. (Due to various acquisition requirements, there are many different types of output signals for pressure transmitters available on the market. The most common ones include 4...20mA, 0...20mA, 0...10V, 0...5V, etc. However, 4...20mA and 0...10V are the most frequently used. Among the output signals mentioned above, only 4...20mA is a two-wire system; when we talk about the number of wires in an output system, we do not include the ground or shield wire, so all the other options represent three-wire systems.) 6. What excitation voltage to choose: The type of output signal determines which excitation voltage should be used. Many amplification transmitters have built-in voltage regulation circuits, which allows for a wide range of supply voltages. Some transmitters are configured with a fixed quantity of output, and they require a stable operating voltage; therefore, the available operating voltage determines whether to use sensors equipped with regulators. When selecting a transmitter, it is necessary to take into account both the operating voltage and the system cost. 7. Whether interchangeable transmitters are required: Determine whether the required transmitter can be used in multiple systems. Generally speaking, this is very important. Especially for OEM products. Once the product reaches the customer, the cost incurred by the customer for calibration is quite high. If the product has good interchangeability, then even changing the transmitter used will not affect the performance of the entire system. 8. Stability is required after the transmitter has operated for an extended period of time: Most transmitters experience \"drift\" after operating for too long, so it is essential to understand their stability level before purchasing them. This preliminary assessment can help avoid various problems that may arise during use. 9. Transmitter enclosure: The enclosure of the transmitter – and in particular its frame – is something that is often overlooked; however, its shortcomings will become apparent over time as the transmitter is used. When selecting a transmitter, it is essential to take into account the future operating environment of the transmitter, such as the humidity levels, how the transmitter will be installed, and whether there will be any severe impacts or vibrations. 10. What kind of connection should be used between the transmitter and other electronic devices? Is a short-distance connection necessary? If a long-distance connection is required, is a connector needed? 11. Other: After determining some of the parameters mentioned above, it is also necessary to identify the process connection interface of your pressure transmitter as well as its supply voltage; in special applications, explosion protection and protection ratings must also be taken into consideration.