What are the measuring range and the turndown ratio?
The measuring range is 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 measurement range to the minimum measurement range of an instrument. A large turndown ratio provides more room for adjustment. When the process conditions change, it is convenient to change the measurement range of the transmitter without replacing the instrument. It can also reduce the number of spare instruments in stock, which is beneficial for management and prevents 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 measurement range of 0-250 kPa, when the actual pressure we measure is 60 kPa, the measuring range we should select is 0-100 kPa, and this measuring range is the operating range. If the turndown ratio of this pressure transmitter is 10:1, it means that, while ensuring the accuracy of this pressure transmitter, the minimum operating range we can select is 0-25 kPa. If the operating range is smaller than this value, the accuracy of the instrument will decrease; while if the operating range is between 0-25 kPa and 0-250 kPa, the accuracy of the instrument can be ensured. So, the operating range and the maximum measuring range should not differ too much, otherwise the accuracy of the instrument will decline. For cold water meters, the turndown ratio is the ratio of the rated flow (Q3) to the minimum flow (Q1), and it can also be represented by the letter “R” followed by a number.
Post time: Apr-13-2025