Rangeable describes a type of measurement instrument that has an output signal which can be adjusted by the operator to a different scaling.
A rangeable instrument can be rescaled by adjusting the zero and span setting to a measurement between zero and the 100% full scale range of the instrument.
The different methods used to re-range a measurement instrument output include externally accessible variable potentiometers, push button controls or digital communications.
Some rangeable instruments have a limit on how much the scale can be reduced, which is typically expressed as a ratio of the maximum full scale. For example a 10 bar range pressure transmitter which can be reduced by up to 80%, would be described as having a 5 to 1 turndown ratio.
Often the accuracy performance will be compromised when a range is re-scaled to a reduced span, particularly with analogue based devices.
Some digital rangeable devices will offer a fixed percent of full scale accuracy to a certain level of turndown, beyond which the percent full scale accuracy will typically increase pro-rata.
- Rangeable DP Sensors - Rangeable differential pressure sensors with output signals that can be adjusted by the user to elevate or suppress the zero position and turn down the full range span.
- Rangeable Pressure Transmitters - Rangeable pressure transmitter which can be down-ranged by the operator to scale the 4 to 20mA output signal to a different pressure range.
Turn down accuracy
For a rangeable transmitter of lets say 100 bar g full scale, and a 100:1 turn down with an accuracy of 0.1%FS, the error would be +/-100 mbar over the range of 0 to 100 bar g. What would be the accuracy if it was re-ranged to 60 bar. Would it be +/-100mbar or +/-60mbar?
It will depend on the make and type, since some will improve the accuracy at lower ranges by using a % of reading method. The worse case error should be within 100 mbar at any point or range. You should be able to find some statement on the product data sheet explaining how the accuracy is affected by turndown.