What is a primary meter and what is a secondary meter?

Instruments that perform on-site measurements and transmission are generally referred to as primary instruments, while those located in the control room are called secondary instruments.

Do the measuring and transmitting instruments merely be responsible for taking measurements and outputting analog signals, with the secondary instrument then receiving those signals and converting them into numerical values?

The instruments located at the site are called secondary instruments, while those located in the control room are also known as secondary instruments. Primary instruments include detection devices (transmitters, sensors); actuators are also referred to as primary instruments. Secondary instruments include those for control, indication, recording, calculation, alarm generation, etc. Nowadays, most of these functions are carried out by DCS and PLC systems, and secondary instruments are mostly virtual instruments (i.e., internal modules).

Thermocouples and pressure gauges are called primary instruments, while digital displays fall under secondary instruments

A pressure gauge should not be called a primary instrument; it should be referred to as an on-site indicating instrument. (Personal opinion)

Do we need to discuss it both once and twice?

A primary instrument is one that directly measures the meson being tested, while an instrument that displays the results after conversion is a secondary instrument.

Primary instruments include detection devices (transmitters, sensors); actuators are also referred to as primary instruments. Secondary instruments include those for control, indication, recording, calculation, alarm generation, etc. Nowadays, most of these functions are carried out by DCS and PLC systems, and secondary instruments are mostly virtual instruments (i.e., internal modules).

This post was last edited by yunrun on 2018-3-6 21:38. Terms such as measurement points, primary components, primary valves, primary elements, primary calibration, secondary instruments, field instruments, secondary calibration, instrument components, and flowcharts with control points frequently appear in literature and drawings related to automated instrumentation; these are all common terms used in the field of instrumentation. A measurement point (primary point) refers to the point in a detection system or control system that is in direct contact with the process medium. Such as the pressure sampling points in pressure detection systems, and the installation points of thermocouples and thermal resistors in temperature detection systems. A point can be on the process pipeline or on the process equipment. A primary component (source component) generally refers to a instrument component installed at a primary point, such as a pressure tapping fitting in a pressure monitoring system or a thermometer boss in a temperature measurement system. A primary valve (pressure tapping valve) refers to a valve installed on the primary components, such as the valves in the pressure measurement system connected to the pressure tapping fittings, and the valves connected to the inlet and outlet pipes of the positive and negative pressure chambers of the orifice plate. Primary elements (sensors) refer to components that are installed on-site and in contact with the process medium, such as thermocouples and thermal resistors. Primary instruments are a type of field instrument; they are those that are installed on-site and in direct contact with the process medium, such as Bourdon tube pressure gauges, bimetallic thermometers, differential pressure transmitters, etc. One calibration (individual calibration) refers to the verification carried out before the installation of the instruments; in accordance with the requirements of the \"Code for Construction and Acceptance of Industrial Automation Instrumentation Projects\", in principle, each instrument must undergo one calibration. The focus of calibration is to check the indication error and drift of the instruments, as well as to adjust errors in the instrument’s proportional gain, integration time, differentiation time, control point deviation, and balance. An instrument that meets the requirements specified in the design or product manual can be installed only once, in order to ensure the quality of the secondary calibration. Secondary instruments at yunrun.com.cn/tech/list_58.html refer to the general term for various instruments whose indication values are not directly related to contact with the process medium. The input signal of secondary instruments is usually a standard signal converted by a transmitter. There are generally three types of standard signals received by secondary instruments: pneumatic signals, 0.02–0.1 MPa ； Type II electric unit combination instrument signal, 0-10mA ； Ⅲ Electric unit combination instrument signal, 4-20mA or 1-5V. Field instruments refer to the general term for instruments installed on-site, including all primary instruments as well as secondary instruments mounted at the site. Secondary calibration (secondary alignment, system calibration) refers to the inspection of the entire detection circuit or automatic control system after the instruments have been installed on-site, and once the piping and wiring in the control room are completed and verified; it represents a second comprehensive calibration before the instruments are put into formal use. Its verification method usually involves adding a binary signal during the testing phase, and then carefully checking whether each instrument in the system is operating within the allowable error range. If it exceeds the allowable error range and no cause can be found, all the instruments making up the system must be recalibrated. Instrument processing parts refer to the general term for all metal and plastic mechanically processed components used in instrument installation, and they play a special role in such installations. A flow chart with control points is a diagram that uses symbols from process monitoring and control system design to describe the automation aspects of a production process. It represents in detail the automation scheme and level, serves as a basis for control system design, and is used as a reference during construction, installation, and operational activities.

The meter used for on-site measurement and signal conversion is the primary meter, while the one used for display and alarm functions is the secondary meter