As a quantitative processing test tool, multimeters have improved a lot of test capabilities, and their testing scope and functions are gradually strengthened, especially in handheld tool instruments, which have exceeded the traditional range (measurement range), accuracy, resolution, stability And other basic indicators. Taking into account the characteristics of the field, the multimeter has developed many new products in terms of product reliability and test functions. The world's most famous multimeter manufacturer is Fluke. As China's largest producer of low-end multimeters, many manufacturers have also begun to upgrade the technical capabilities of multimeters. From simple manufacturing, they have gradually transformed into Chinese creation. The test function of multimeters is in their design. There are more and more blueprints. The most typical meaning is the application in industrial field. The 4-20mA% function for small current test has become the main function in many multimeters, such as OI859CFⅡ high-precision handheld multimeter.
1. About the working principle of 4～20mA
4～20mA is widely used in the field of sensing and measurement and control in industrial field. It mainly solves the long-term transmission problem of industrial field sensor signal and signal amplification. In the past, people used voltage in long-distance transmission, there are mainly the following four problems:
(1) These signal voltages are generally relatively small and are susceptible to load interference;
(2) The voltage signal will be affected by the impedance of the transmission line during transmission, resulting in noise interference;
(3) No matter how good the transmission line itself has wire resistance, it is easy to cause voltage division and make voltage signal attenuation
(4) Due to interference and environmental problems in industrial sites, it is also very difficult to provide stable small operating voltages.
In this way, in order to solve the problem of long-term voltage transmission, people find that using small current signal transmission is more useful, and it is not necessary to solve the problem of noise and interference. The small current signal forms a current loop in the closed-loop system to ensure the stability of the sensing signal of the measurement and control and the sensitive change of the sensing induction. In the 4-20mA current loop, 4mA is generally used to indicate the zero signal, and 20mA is used to indicate the full-scale signal. The 4-20mA loop current indicates that the signal is reliable and safe. The signal below 4mA or more than 20mA is an unsafe signal. There will be an alarm or an error in the system.
2. How does the multimeter measure 4～20mA?
The 4-20mA current loop is mainly used in signal sensing and transmission in industrial scenes. For example, long-term drive field drive devices (such as valves) are widely used. There are generally two types: (1) two-wire system, ( 2) Three-wire system. In the application, the signal of the sensor is converted into a 4-20mA signal through an appropriate circuit. At this time, small changes in the current will affect the transmission effect, and it is very important for the tracking and correction of the current signal.
Generally, a multimeter can only measure the size of the current, it is not easy to intuitively judge the proportional relationship of 4-20mA, and the resolution and stability of the multimeter are not enough when measuring small current signals. The new multimeter sets the %4～20mA function, and uses 4～20mA% (percentage) to indicate the deviation of the current. For example, when the current is 20mA, the% reading 100 means full scale, so it is easy to judge the problem of 4～20mA, and it is convenient for on-site adjustment And correction.