Issues regarding the selection of the measurement range for pressure transmitters
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This post was last edited by The project’s progress on 2023-11-6 at 11:37. Generally, we determine the measurement range required for the instrument by taking twice the operating value specified in the process requirements; when selecting an instrument, we check the measurement range indicated in its specifications, as long as the range we choose falls within that specified range. I would like to ask what the minimum and maximum ranges of the sample indicate, and whether the operating values required by the process are related to this Should the operating value be within this range? For example, the measurement range of the EJA210AM diaphragm gauge is 1–100 KPa, while the process value is 700 Pa. If the measurement range is set to 0–1.6 KPa, can this gauge be used? 2. When adjusting the pressure transmitter, is the range of 0-1.6 corresponding to 4-20mA? 3. There is another question: since the gauge’s diaphragm senses pressure, does the diaphragm move only when the pressure exceeds a certain level? If so, does the gauge have such a limit? 4. If the measurement range in the sample is, for example, 1–100 kPa, does that mean the measurement range ratio of the instrument is 100:1? 5. Once the operating range of the instrument has been selected, how is the minimum scale determined?
For EJA, the minimum measurement value within the 1–100 KPa range is 1 KPa, while the maximum range is 100 KPa; thus the range ratio is 1:100. Values below 1 KPa can be measured and displayed, but accuracy is not guaranteed. You can adjust the range arbitrarily within the 0–100 KPa range, and the output in this case will always be in the 4–20 mA range
When selecting the range for a pressure transmitter, there are several key points to keep in mind: 1. The minimum and maximum ranges of the instrument: The minimum and maximum ranges of the instrument (what you refer to as the measurement range) are actually determined by the manufacturer based on the physical and technical conditions involved in the design and manufacturing of that instrument. Within this range, the instrument’s measurements should be relatively accurate and stable. If it exceeds this range, the instrument may be unable to take accurate measurements due to design and technical limitations, or the measurement results may be unstable. 2. Operating values required by the process: The operating values required by the process should be within the range you have selected. In other words, the range you select should cover the entire operating range required by the process. For example, you specified that the measurement range for the EJA210AM diaphragm gauge is 1–100 KPa, but the actual process value is 700 Pa; therefore, a measurement range of 0–1.6 KPa was chosen. This is completely possible, as the process measurement value falls within the selected range, and that selected range is within the instrument’s measurement range. 3. For the adjustment of pressure transmitters, it is generally necessary to establish a correspondence between the measurement range (such as 0–1.6 KPa) and the output signal (such as 4–20 mA). This is a linear relationship: the higher the pressure, the greater the output current signal. As for your last question, the diaphragm moves in response to the pressure it senses, but it’s not necessary for that pressure to be above a certain level for movement to occur. As long as there is a pressure difference, the diaphragm box will move. Furthermore, both the design and materials of the diaphragm box affect this displacement; therefore, manufacturers typically establish a minimum measurement range to ensure the accuracy and stability of the instrument. Therefore, in general, instruments have a minimum measurement range, which is what you refer to as the “limit”. .
For example, an output of 4–20 mA corresponds to a measurement range of 0–100 kPa, with a maximum operating pressure of 400 kPa. How to choose the measurement range? With a maximum operating pressure of 400 kPa, pressure transmitters with ranges from 0.01 to 1 bar and above can be selected. Set the measurement range to 100 kPa; if 0.01... 1 bar is selected, the maximum measurement range is 100 kPa. Range ratio r = 100/100 = 1, accuracy ≤0.065%, error = 100*0.065/100 kPa = 0.065 kPa ； If 0.16... 16 bar is selected, the maximum measurement range is 1600 kPa. The range ratio is r = 1600/100 = 16; the precision is ≤ (0.004*16 + 0.045)% = 0.109%, and the error is 1600*0.109/100 kPa = 1.744 kPa. The difference between the two is 1.744 - 0.065 = 1.679 kPa. Therefore, the measurement range of the pressure transmitter is set to 0.01 ... 1 bar.