What does the range ratio of a gauge refer to?

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The range ratio is the ratio of the maximum measurement range to the minimum measurement range. A larger range ratio provides more room for adjustment; it enables the measurement range of the transmitter to be changed more easily when process conditions change, without the need to replace the instrument. It also helps reduce the number of instruments in stock, facilitating management and preventing capital accumulation. Therefore, the range ratio of a transmitter is a very important technical parameter. However, when choosing instruments, it’s not the case that the larger the range ratio, the better the performance of the instrument; there is also the concept of operating range to consider. For example, for a pressure transmitter with a measurement range of 0–250 kPa, when the actual pressure being measured is 60 kPa, the range we should choose should be 0–100 kPa; this range is what is known as the operating range. If the range ratio of this pressure transmitter is 10:1, it means that while maintaining the accuracy of this pressure transmitter (for example, 0.055%), the minimum operating range we can choose is 0–25 kPa. If the operating range is lower than this (for example, 0–20 kPa), the accuracy of the instrument will decrease and will not reach 0.055%. On the other hand, if the operating range is between 0–25 kPa and 0–250 kPa (for example, 0–50 kPa), the accuracy of the instrument can still be maintained. Therefore, the operating range and the maximum range should not differ too much, otherwise the accuracy of the instrument will decrease. In fact, for instruments with a range ratio of 10:1, in our field of control instrumentation, when indicating the instrument’s range, we often take 0.1 times the full range as the minimum value of the measurement range, and the full range itself as the maximum value (for example, the measurement range is usually expressed as 2.5~25 kPa, 10~100 kPa, rather than 0~25 kPa or 0~100 kPa). Although the measurement ranges for instruments in this field are often expressed as 2.5–25 kPa, the scales on the instruments in use actually start at 0 kPa rather than 2.5 kPa. Measurement range, upper and lower limits, and scale: Every measuring instrument has a measurement range, which is the range of the variable being measured that the instrument can detect with the specified accuracy. The minimum and maximum values of the measurement range are referred to as the lower measurement limit and the upper measurement limit, respectively; these are simply called the limit and the upper limit. The range of an instrument indicates the size of its measurement range, and it is equal to the algebraic difference between the upper and lower limits, that is, Range = Upper measurement limit – Lower measurement limit. The use of the lower and upper limits allows for a complete description of the instrument’s measurement range, as well as the determination of its range. If the lower limit of a temperature measuring instrument is -50°C and its upper limit is 150°C, then its measurement range can be expressed as -50°C to 150°C, with a range of 200°C. It is clear that by knowing the measurement range of an instrument, one can determine its upper and lower limits as well as its range; conversely, if only the range of the instrument is given, it is not possible to determine its upper and lower limits or its full measurement range. The range ratio of a transmitter refers to the ratio of the maximum value to the minimum value that can be measured while meeting the accuracy requirements. Generally, the larger the range ratio, the lower the measurement accuracy. In traditional concepts, the quality of a transmitter is assessed mainly based on its measurement accuracy, range ratio, and temperature/static pressure influence factors – is that correct? What are the most important factors affecting a transmitter’s performance? Experts believe that relying solely on the transmitter’s accuracy, range, and temperature/static pressure influence factors does not fully reflect its overall performance, as these values are isolated from one another and none of them can indicate the transmitter’s comprehensive capabilities. For modern transmitters, long-term stability and reliability should be given more importance; this is also what users should care most about. Therefore, when the measurement accuracy is similar, long-term stability and reliability are the most important criteria for evaluation. Generally, the larger the range ratio, the lower the measurement accuracy. However, in recent years, with microcontrollers being integrated into transmitters, fully intelligent transmitters have begun to gradually overcome such technical limitations. Smart circuits can provide various types of compensation while amplifying signals, thereby reducing the impact of interference signals. Nevertheless, in general, it is still advisable to avoid excessive range shifts on-site; try to determine the range before placing an order, so as to select the best diaphragm box. In terms of linearity, through multi-point correction over the full range, the intelligent transmitter can perform 17 corrections on-site. However, too many revisions not only increase the workload on site but also affect long-term stability. Stability and reliability depend on the transmitter’s time drift and temperature drift. Smart products with advanced technology and manufacturing processes generally achieve a full-scale accuracy drift of ±0.1% within one year. The effect of temperature, within the range of -40 to 85°C, results in a total full-scale drift of ±0.1% for both the zero point and the full scale. Stability and reliability are particularly important for end-users, as they help reduce the need for on-site maintenance and minimize the risk of failures.