Concepts and distinctions between primary instruments, secondary instruments, local instruments, and remote transmission instruments
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Primary instrument: A instrument that is directly installed on the process pipelines or equipment, or near the measurement point but in contact with the medium being measured; it is used to measure and display process parameters or to send parameter signals to secondary instruments. A primary instrument is one of the components (elements) of an automatic detection device. It is equipped with sensing elements to detect changes in the parameters of the medium being measured. It may have a scale to indicate the reading; or it may not have a scale and thus not indicate any reading. Secondary instrument: An instrument that receives electrical or pneumatic signals sent from transmitters, converters, sensors (including thermocouples and thermal resistors), etc., and indicates the values of the process parameters being monitored. One of the components (elements) of the automatic detection device. Used to indicate, record, or accumulate the measurement results from primary instruments. There are generally three types of standard signals received by secondary instruments: ① Pneumatic signal: 20–100 KPa (0.02–0.1 MPa); ② Type II electric unit instrument signal: 0–10 mADC. ③Type III electric unit meter signal: 4~20mADC. There are also some devices that do not use standard signals; the primary instrument generates an electrical signal, and the secondary instrument displays the value directly, such as remote pressure gauges. Secondary instruments are usually installed on the dashboard. Based on the installation location, they can be divided into panel-mounted instruments and rack-mounted instruments. In the production process, measuring instruments are often classified qualitatively based on the number of times of energy conversion: those that convert energy once are called primary instruments, while those that convert it twice are called secondary instruments. Taking temperature measurement with thermocouples as an example, the thermocouple itself converts thermal energy into electrical energy, which is why it is called a primary instrument. If the electrical energy is then converted into mechanical energy that causes a pointer to move, using a potentiometer (or millivoltmeter), this second conversion of energy results in what is known as a secondary instrument. Those that undergo more than two conversions are usually referred to as secondary instruments; for example, in flow measurement using orifice plates, the orifice plate itself is a primary instrument, while the differential pressure transmitter has no specific designation. Secondary instruments, on the other hand, are used to indicate, record, or accumulate the measurement results from primary instruments. Local instruments: As the name implies, local instruments are those that are installed in close proximity to chemical processing equipment within the process area; typical examples include pressure gauges and glass tube level gauges. Remote transmitters: These are instruments installed at the site that can transmit the signals from there to the control room in electrical or pneumatic form, such as integrated pressure transmitters and similar devices. Many traditional field instruments now also possess remote transmission capabilities. For example, magnetic flap level gauges can function as standalone field indicator devices, similar to glass tube level gauges; however, by adding a transmission module, they can serve both as field indicators and for remote transmission. And if a display module is added to the pressure transmitter, it can also be used for on-site indication. But it is rare for so many functions to be used together in this way. Therefore, in situations where both on-site indication and remote control are required, the main function of on-site indication is to facilitate local operation in case of automatic control failure; it can also be said to serve to increase the safety factor. Meters that correspond to the above situations need to be listed separately for explanation.
Primary instruments are instruments that come into direct contact with the medium being measured and measure its parameters. They are typically installed on process pipelines or equipment, and can be either display-type or non-display-type; examples include thermocouples that detect temperature changes. Secondary instruments receive the signals sent by primary instruments and carry out tasks such as display, recording, or calculation. Common signal types include pneumatic signals and electrical unit signals; these instruments are usually installed on control room dashboards. In-situ instruments are those installed at the production site to provide direct readings on-site, such as pressure gauges and glass tube level gauges. Remote transmitters are capable of transmitting measurement signals over long distances to the control room; they usually have transduction functions, such as integrated pressure transmitters. Such devices can support both on-site reading and remote control. To distinguish these instruments, one mainly looks at their installation location, whether they are in direct contact with the medium, and the method of signal transmission. .
There is a question: for an integrated temperature and pressure transmitter, how many instruments should it be counted as?
The terms \"primary instruments\" and \"secondary instruments\" were commonly used in the past, but they are no longer used this way today. The reason is as mentioned in the question on floor 3: the structure of instruments has changed significantly, and the distinction between primary and secondary instruments has become blurred.
I’ve learned from it; in a previous project, the various installation teams had disputes over the definition of primary instruments and secondary instruments. :Q