Regarding the setting of the transmitter’s measurement range

It is a matter of measurement capability and range; manufacturers use a term called \"range ratio\" for this. The maximum measurement capability, or range, of an instrument is 200. Generally, the instrument’s range is chosen so that it falls within the range of 1/3 to 2/3 of this maximum value, corresponding to the pressure levels under normal operating conditions. If the maximum pressure in your operational environment is 30, then we would choose an instrument with a measurement range of 0–50, and so on. However, if the measurement range of the pressure transmitter produced by the instrument manufacturer that you have chosen is 0-200, you can change it to any value within 200. The purpose of reducing the measurement range is to minimize measurement errors, as these errors are directly proportional to the accuracy class and range of the pressure transmitter; therefore, in applications where high precision is required, the range is adjusted. If the requirements aren’t high, it’s fine to use a uniform measurement range of 0-200.

The range set is determined based on the process requirements. The range setting of a pressure transmitter is also related to the maximum range of the sensor itself; for example, if the maximum range of the sensor is 0~100 KPA, then the set range cannot exceed 0~100 KPA

It affects accuracy, and when it’s time to replace the instrument, knowing the range of measurement values is more precise than knowing the instrument’s range.

So when buying a gauge, it’s still necessary to base the decision on one’s needs – buy one with a range that matches what’s required, right? Only when, for example, a certain gauge breaks and there are no identical spare parts available, can a gauge with a larger range be used as a substitute; in such cases, is it necessary to adjust the gauge from a large range to a small range?

You’re putting the cause before the effect here; it’s the process design that determines that we need instruments with ranges of 0-50, 0-150, and 0-100； Then the instrument designers found that a range of 0-200 would meet the requirements, so they selected an instrument with a measurement range of 0-200 and configured it on-site to meet the process requirements. You need to keep in mind that the 4-20mA output range you set is not just a range for output; for the process in question, it represents the normal operating range. By expanding this range on your own, you might cause the process to consider certain values that are outside this normal range as normal. It’s not just a matter of that little bit of precision.

Don’t understand? Do you mean that the size should be chosen based on the required processing capacity? He wants 0-100, can’t I set the 0-200 range table so that 0-100 corresponds to 4-20mA?

Sure, just use a value within the range of 0-200

For the instrument itself, it basically does not affect accuracy; the instrument ensures accuracy across the entire range. But for process and process control, if a certain value is required to be kept below 50 – exceeding this value can cause equipment failures or result in defective products – then the 4-20 milliampere range of the instrument must be set between 0 and 50. The instrument outputs current values in the range of 4 to 20 milliamps, with 4 milliamps corresponding to a pressure of 0, and 20 milliamps corresponding to a pressure of 50, or it could represent the maximum level, such as a full tank or 100%. If only up to 50 is relevant and the range is set to 200, then the current value corresponding to a pressure of 50 will not be 20 milliamps, and the maximum level will not be indicated either. To show 50 as the maximum level, new logical relationships need to be added to the system; so why not simply set the instrument range to 0-50? It’s simpler and more convenient