What are the measuring range and the turndown ratio?

The measuring range refers to the value of the difference between the upper and lower limits of the nominal range of an instrument. If the lower limit of measurement of an instrument is zero, then the maximum value of the physical quantity that can be measured is equal to its measuring range. For example, the measuring range of an ammeter is the maximum value of the current that can be measured.
The turndown ratio is the ratio of the maximum measuring range to the minimum measuring range of an instrument. A large turndown ratio provides more room for adjustment. When the process conditions change, it is convenient to change the measuring range of the transmitter without the need to replace the instrument. It can also reduce the number of backup instruments in stock, which is beneficial for management and helps prevent the backlog of funds. Therefore, the turndown ratio of a transmitter is a very important technical indicator.
When selecting an instrument, it is not the case that the larger the turndown ratio, the better the performance of the instrument. There is also the concept of the operating range. For example, for a pressure transmitter with a measuring range of 0~250 kPa, when the actual pressure we measure is 60 kPa, the selected measuring range should be 0~100 kPa, and this range is the operating range. If the turndown ratio of this pressure transmitter is 10:1, it means that, while ensuring the accuracy of the pressure transmitter, the minimum operating range we can choose is 0~25 kPa. If the operating range is smaller than this value, the accuracy of the instrument will decrease. However, if the operating range is between 0~25 kPa and 0~250 kPa, the accuracy of the instrument can be guaranteed. So, the operating range and the maximum measuring range should not differ too much; otherwise, the accuracy of the instrument will decline.
Regarding cold water meters, the turndown ratio is the ratio of the commonly used flow rate Q3 to the minimum flow rate Q1, and it can also be represented by the letter R followed by a number.