Ozone analyzer failure
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The ozone analyzer communicates properly with the host computer, but a current of 32–35 mA is measured in the 24V power supply circuit of the instrument. As a result, the incorrect analog current signal received by the host computer causes the displayed value to be negative. When the instrument is powered separately using a manual controller, the output value is 4 mA. Since there is no ozone present on site, it can be assumed that there is no problem with the instrument itself. The wiring runs from the instrument to the field relay box and then to the terminal block in the PLC cabinet; however, without diagrams, it is not possible to determine the terminal numbers or to understand where this current value is coming from. The instrument’s terminal connections are: 485+, 485–, 24V power+, S (probably signal–), and G (ground). On site, 24V+ and S are connected together and linked to the manual controller, resulting in irregular current values of 32–35 mA. Theoretically, it should be 4mA; however, the upper-level system failed to detect 4mA and only detected 32mA, which is why the maximum value is displayed. I seek advice from experts
What model of PLC is it? Is a separate 24V power supply connected to the instrument, while the PLC module is powered via a two-wire system? That’s why the current level is abnormal.
Thank you so much; to be honest, I’m no expert in PLCs. It handles both signaling and power supply; it’s not like the DCS card keys that are powered directly. I’ll check the relevant drawings and discuss them with you later. I’m heading off for now, thanks
Has the original poster’s problem been resolved? Has it been determined what the issue is? Based on the description, it seems to not be a fault with the ozone analyzer itself. We encountered a similar situation as well, but it wasn’t ozone – it was testing equipment for another project; we are in contact with the PLC team to get their help in troubleshooting it.
Use 24+ V GNd three-wire power supply for the instrument; G should be the common terminal. The currents measured are s and G.
Suspecting a fault with the ozone detector, I bought two more domestic units, but they still did not provide a 4-20MA output signal. The circuit was checked and found to be fine; other types of instruments were tried (the temperature and humidity sensors also output signals in the 4-20ma format, and the display on the host computer shows values consistent with those from the instrument terminals). Professionals familiar with PLCs were also consulted, and they said they had not encountered similar problems either. I’m now preparing to rewire it and connect it to my DCS system. Because ozone does not use a position control loop. It is just a record of the disinfection data for the clean area. In fact, the problem still hasn’t been solved in the end
The instrument is a three-wire system, with terminals for 24V+, S, and G. Before there were any problems, only the 24V+ and S terminals were connected. Because it needs to be calibrated every year; after it’s taken away for calibration and brought back, there are no readings anymore. Using a handheld device, the analog signal at the PLC terminal was measured to be less than 4 milliamps; however, when another type of 2-wire instrument was used, the master unit displayed normal readings. Problems with the wiring or the PLC can be basically ruled out, because if there were issues with the wiring or the PLC, I wouldn’t get any readings regardless of which instrument I used.
Are there any photos from the scene? Are the table headers visible there?