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Hi, I’m the Industrial Control Assistant. Today, I’m here to share some knowledge about transmitters. Pressure transmitters are being used more and more frequently, and many non-professionals are also starting to study the principles behind them. We continue to explore their intricacies. One of the most common problems is not knowing how to deal with transmitter failures or how to carry out repairs. So today, I’ll share some methods for dealing with such situations. The common faults of transmitters are roughly as follows: 1. The PV value of the pressure transmitter does not match the actual pressure. 2. The PV value of the pressure transmitter matches the actual pressure, but the 4-20mA output does not correspond to that of a standard gauge. 3. Both of the above parameters do not match. The following analyzes and addresses the common issues of pressure transmitters for each of these problems. When we use the HART375 handheld programmer to calibrate the diaphragm head, we are essentially correcting the aforementioned curve. We call them: SENSORTRIM! FULLTRIM and ZEROTRIM, etc. Usually, these operation menus are found under SENSORTRIM; whether to choose FULLTRIM or ZEROTRIM depends on the specific level of error. However, different transmitters have varying limits on the degree of curve correction, typically 3-5% of the full scale. A. Use ZEROTRIM when, after installation, the gauge is at zero point, or when the differential pressure gauge is in a balanced state but there is static pressure in the pipes; in such cases, the gauge’s output will exhibit a small deviation (0.25–0.31 Kpa), depending on the installation location and type of gauge. ZEROTRIM should be used to correct this issue. This operation only translates the calibration curve and does not affect the slope; therefore, it is a single-point verification for the D/A converter. If the user does not have a HART375 communicator, and the instrument itself has an external zero adjustment button, this correction can also be carried out by activating (holding down for more than 2 seconds) that external button. The difference is that this operation only adjusts the current zero PV value to 4mA; the pressure reading remains unchanged! It’s equivalent to performing a slight zero-point shift. B. Use FULLTRIM; this option involves two-point verification and requires a standard pressure source for pressurization, therefore it can only be carried out offline. This operation is a correction for the A/D inside the membrane head. To perform this operation, it is necessary to first carry out low-end adjustment (LowsensorTrim), and then high-end adjustment (UppersensorTrim). After performing FULLTRIM, the calibration curve of the instrument was fitted. After entering the diaphragm head calibration menu (see figure below), select 2, or simply press key 2 to proceed with the operation! The standard pressure for low-end models can be a value other than zero; for example, for gauge pressure meters, this pressure is usually the atmospheric pressure. (14.7Psia) Next, perform the upper limit calibration. The calibration tolerance generally does not exceed +/-5% of the upper range limit. When the overload limit is reached, an operation alarm prompt will appear on the instrument. It should also be noted that the instrument must have write protection disabled in order to perform this calibration. Under normal circumstances, after the aforementioned checks are completed, the PV value of the pressure transmitter should match the standard value; however, this does not guarantee the accuracy of the output. Therefore, it is generally necessary to perform analog output trimming (TrimAnalogOutput). C. 4-20mA output verification. See the aforementioned section: D/ATrim. D. In cases where the PV value is correct and the 4-20mA output current value matches that indicated by the standard gauge, but the PV value does not correspond to the 4-20mA value, and this occurs under zero migration conditions; for example, if the instrument’s range is 0-100 Kpa, at the zero point the reading on the handheld controller’s AO function is not 4 mA but rather 5.6 mA, and verification using a standard ammeter confirms that it is indeed 5.6 mA. This type of failure is an internal circuit fault that cannot be repaired. E. If the PV value is incorrect and cannot be corrected using FULLTRIM, it is usually due to damage to the membrane head; in such cases, characteristic analysis is required, and this type of repair should be carried out by the factory. F. For some faults, the instrument will display hardware error alerts, and users can troubleshoot the issue by referring to these alerts along with the relevant explanations in the instrument’s operation manual. Generally, users are unable to repair hardware failures. There are many other situations and various types of transmitters; we cannot list them all here. If fans have any related questions, we will continue to share information and keep updating the content – feel free to stay tuned.