Instrument system accuracy
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One temperature is converted to 4-20mA using a temperature sensor, and this signal is sent to the PLC’s current module. The temperature resolution is 0.5 degrees, while the precision of the PLC’s current module is 0.25 degrees. The temperature range is from 0 to 1000 degrees. Signals are supplied from on-site thermocouples. What degree of error is considered acceptable for the PLC? How is system error calculated? Is it additive? Are there any relevant standards? Thank you
I took a course in college called Error Theory, published by Hefei University of Technology; however, it has not been in print for many years now. I remember there being a method for calculating systematic errors in that book. Systematic errors are additive. I don’t understand what you mean by “qualified”; isn’t it sufficient to meet the error requirements set by the client? Generally speaking, temperature does not require extremely high precision; a difference of a few degrees is not a problem.
I’m not sure what degree of PLC error is considered acceptable, but from the information I’ve seen, there are two viewpoints: one is that the accuracy of a system can only be determined through overall calibration of the system; in the case of digital transmission, only the accuracy of the field instruments needs to be taken into account, while for analog signals, the accuracy of the field instruments, the system itself, and the transmission process all need to be considered. Another perspective is that it can be obtained through calculation, and there are generally two methods. For a conservative estimate, one could use the approach of squaring first, then adding the values, and finally taking the square root; for an optimistic estimate, the largest error among all the components of the system can be taken as the error of the entire system. Since I’m also struggling with this issue now, I hope everyone can discuss it together.
The system accuracy is obtained by squaring the accuracy of the thermocouple, the temperature sensor, the PLC module, and the display device respectively, adding those values together, and then taking the square root.
What was said on the 4th floor is correct; the system accuracy is calculated using the root mean square value. If the accuracy of the measuring elements, the transduction unit, and the indicator is all 0.5%, then the system accuracy will be 0.86%
I usually encounter the issue that thermocouple transmitters don’t have good linearization, so in the PLC settings, it’s necessary to match them with the curve of the thermocouple. As for the accuracy of errors, I still support the view from the 4th floor. However, I have encountered exceptions where the accuracy of the thermal resistor was poor, and the accuracy of the transmitter module was also poor; yet when the two were connected together, the accuracy of the entire temperature measuring instrument improved.
I think your case is an exception; for example, if the RTD value is high while the temperature variation is low, this results in the better accuracy you mentioned, but it’s a very one-sided view.
Reply to 7# zzzi: I already explained it as “special case” when I answered; this gentleman doesn’t need to comment any further – what I said is already one-sided enough. Are you right?
The square root of the squared accuracy of all instruments in the system would be the system accuracy, right?
Agree with the view from the fourth floor; the calculation error is within plus or minus 5 degrees