How to measure the resistance value of a platinum resistor and the 4-20mA signal output by a Rosemount 3144P multimeter
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Dear experts: As the title suggests, the professional engineer is not available at the moment. The ambient temperature readings obtained show a value that is 5-10 degrees higher than the actual temperature. How should I carry out inspections? I just want to learn; please avoid criticizing me. Temperature change: 3144P, PT-100, 4-wire. In the diagram, the 4 wires of the platinum resistor are connected to terminals 1, 2, 3, and 4 respectively. 1. What are the functions of -, +, and T on the left side? Things like measurement… 2. How does one use a multimeter to measure the resistance value of a platinum resistor? Is it necessary to disconnect the 4 wires connected to the platinum resistor? The multimeter uses a white and a red lead – what are the specific wiring methods and procedures? 3. How to measure the 4-20mA signal generated by temperature variation using a multimeter? Should one of the power cables be disconnected, and then connected in series with the circuit using the multimeter? What are the specific wiring methods and procedures?
To measure a PT100 with a multimeter, it is necessary to disconnect the wires; the measurement can be taken directly on the gauge. In the four-wire system, two wires are connected to each of the two points of the platinum resistor. By measuring any two of these points, the resistance value can be determined, as long as it isn’t 0Ω. The temperature corresponding to a PT100 can be calculated using the formula (measurement value – 100) / 0.385. This gives an approximate value; for a more precise result, calculation software is required.
Photos cannot be uploaded. The sensor should be placed vertically, with the input port at the top and the platinum resistance thermometer sleeve at the bottom. Once the back cover is removed, the wiring is as follows: At the top, there are 3 terminals from left to right: T, +, and -. The – terminal has a wiring screw, while T and + resemble hex head bolts; neither of these terminals is connected to any wires. On the left side, there are terminals numbered 1, 2, 3, and 4 (these are connected to the platinum resistance thermometer; red wires for 1 and 2, white wires for 3 and 4). There is also a terminal numbered 5, which remains unused. Below terminal 5 is the ground terminal. Near the center of the back cover, there are 2 terminals: – and +, which are connected to the two power wires (these two terminals are not shown in many pictures found online)；
Positive and negative are the power supply terminals; the T terminal is used for testing and is generally not utilized. To measure the current, one power cable must be removed; then the two probes of the multimeter are used to connect the terminals to the cable that has been removed. To determine the resistance value, one red probe and one white probe should be used
Subtract 100 from the measured resistance value, then divide by 0.385 to obtain the temperature corresponding to that resistance value
1 and 2, as well as 3 and 4, are short-circuited to each other. When using a multimeter to measure, the resistance between 1 and 2, and between 3 and 4 should be 0. As for the value of the thermal resistor, you can use a multimeter to measure the resistance between 1 and 2, and between 3 and 4. Of course, to obtain accurate measurements, it is recommended to disconnect one of the wires connecting 1 to 2 or 3 to 4 before taking the readings. As for measuring the circuit current, many modules come equipped with a circuit testing function; for example, there are three terminals for connecting to the power supply: + and – for the power supply, and another terminal labeled LOOP or TEST. You can simply use a multimeter set to the current range to measure the current between the LOOP terminal and the negative terminal. If such a function is not available, then you will need to disconnect one wire and insert the multimeter’s current probe into the circuit