The hottest use of fluke787 process multimeter on-

2022-08-19
  • Detail

Use fluke787 process multimeter to detect the loop power isolator on site

currently, in the ma control loop, the loop power isolator is one of the most popular devices. Another commonly used loop isolation device is the two-wire isolation transmitter. The fault diagnosis process of these two kinds of equipment is different, and the on-site inspectors should understand the difference

loop power isolator and two-wire isolation transmitter

the main purpose of loop isolator is to eliminate the potential or existing in the control system. One is the customized small panel control system, which controls the earth loop voltage, while transmitting the control current signal to another system. Unlike the two-wire transmitter close to the loop power isolator, its working voltage comes from the "input" end of the isolator (see Figure 1). The typical voltage is 7.5v-13.5v, which depends on the manufacturer's products

The output of the loop isolator is current isolated, which is a mirror image reaction of the input current. The associated voltage at the output end is much smaller than that at the input end, which is about 7.5V. This determines the torque measurement range, which has a maximum capacity of 350 Ω for all loop loads. This limited loop drive capability is the main limitation of the loop isolator

two wire transmitter

two wire isolation transmitter and loop divider have similar isolation functions, but some advanced functions are added. They can provide signal regulation functions to adapt to various inputs, such as thermocouple, frequency, RTD, stress, DC current and other process inputs. The power supply of the two-wire transmitter is connected to the output end of the transmitter. The two-wire transmitter adjusts the current of the power supply from 4 to 20mA, which is proportional to the input terminal. The power supply of two-wire transmitter is generally 24v~96v. Large power supply can increase the loop load capacity of the output end, and it has become an inevitable trend for many TPUs to replace PVC; Biocompatible and degradable materials will become a hot topic in the medical market; The functionality of materials will be paid more and more attention; New medical 3D printing raw materials will also become a hot topic of development

field detection of loop power isolator

fluke 787 process multimeter has a unique current simulation function. When connected to an external power supply, it allows you to accurately control the current between 0 and 24Ma

when detecting the loop power isolator on site, the current signal provided by the two-wire loop transmitter to the isolator can be removed, and f787 can control the loop current in an analog manner (Figure 2)

step 1 connect f787

1 Disconnect the main loop of the transmitter, insert the test line of f787 into the analog socket and connect it to the loop

2. Set f787 to output mode

press and hold the range key for two seconds when f787 is powered on, and the instrument can convert between 0~20ma and 4~20ma. In order to confirm that it is the mode you need, you can display the short circuit of the test pen as 0 or 4mA. At this time, f787 is outputting accurate 4mA current and providing the input working voltage of the loop power isolator

3. Use a fluke787 multimeter to test the ma current at the output end

4. Adjust the zero point according to the 4mA current indicated in the output table

5. Step the current output to 20mA with the key, and adjust the full scale point (span) until the current value at the output end of the isolator reaches 20mA

6. Use the key to turn the input current back to 4mA and check the zero point again. If necessary, set the zero point again

7. At this time, the zero point and full-scale point are basically adjusted

Step 2 check linearity

the% (percentage) step key of f787 allows you to easily check the linearity of the loop isolator. Press these two keys to increase or decrease the output current in steps of 25%. In 4~20ma mode, these intermediate points are 8Ma (25%), 12mA (50%), 16mA (75%)

when checking the linearity, press% step to increase or decrease the current, and check whether the reading in the multimeter at the output end of the isolator is the same as that displayed by f787. If not, the deviation should be compared with the maximum allowable error of linearity indicated by the manufacturer

Copyright © 2011 JIN SHI