Rangeable

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.