Instrumentation is an instrument or equipment used to detect, measure, observe and calculate various physical quantities, material compositions, physical property parameters, etc.
1. Instrument size. That is, the size of the instrument, which is a very basic problem. The digital display meter should be installed on the cabinet, so it is necessary to consider the overall coordination, too large may not be able to fit, too small to see the displayed numbers clearly, in addition, the large instrument is generally more expandable, the same function price may be expensive, and the small size of the instrument may be less functional expandable.
2. Display the number of digits. Generally speaking, the higher the number of digits, the more accurate the measurement, and the more expensive the price, mainly the following: two digits (99, special); three (999, very few); three and a half digits (1999, ordinary digital display tables dominate); four-digit (9999, smart digital display meter dominates); Four and a half (19999); Fourth, three-quarters (3999); Five digits and more than five digits (common in counters, accumulators and high-end instruments), users can choose a digital display meter with several digits according to the measurement accuracy requirements.
3. Input signal. It refers to the measurement signal directly input to the instrument, some industrial signals are directly connected to the instrument measurement, and some signals are connected to the instrument after conversion, and the nature of the measurement signal must be clarified, otherwise the purchased instrument cannot be used, or even damage the instrument and the original equipment. To clarify the signal type: current or voltage, AC or easy-to-sell industrial DC, pulse signal or linear signal, etc., it is also necessary to clarify the size of the signal. The name of the meter is not the same concept as the input signal, to give a few examples: the input signal is a 0-75mVdc ammeter (the name is an ammeter, but the input signal is a voltage signal, because the current passes through the shunt to obtain the voltage signal); The input signal is a tachometer from 0 to 10Vdc (the name is a tachometer, but the input signal is a voltmeter, because the frequency converter converts the speed signal into a voltage signal).
4. Working power supply. All digital meters need to work power, and the working power supply of digital display meters mainly is: 220Vac; 110/220Vac; 85-265VAC/DC switching power supply, 24Vdc (generally customized), 5Vdc (small panel meter).
5. Instrument function. The instrument function is generally modular, optional, the price of the instrument will also vary with different functions, the digital display meter mainly has the following optional functions: alarm function and the number of alarm outputs (i.e., relay action output), the size and power of the feed power output and output voltage, the type of transmission output and transmission output (4-20mA or 0-10V, etc.), the communication output and communication mode and protocol (RS485 or RS232, Modbus or other protocols), For the adjustment control instrument, there are more optional functions, specifically refer to the manufacturer's selection spectrum to select a standardized model, and communicate with the manufacturer and confirm that it is correct before ordering.
Several important parameters should be paid attention to: measurement accuracy (the smaller the value, the more accurate it is), the response speed (the smaller the value, the faster the response), the working environment, temperature coefficient (the smaller the value, the less affected by temperature), and overload capacity.
