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.
Sensor products with manually or digitally re-scalable output signals allowing users to standardise on one product configuration and adapt it for use in a number of different applications.
- 41X High Precision Low Range Pressure Transmitter
- XMD Process Plant DP Cell Differential Pressure Transmitter
- DS210 Combined Low Pressure Switch, Indicator and Sensor
- 33X High Accuracy Digital Output Pressure Sensor
- DS200P Sanitary Low Range Pressure Gauge, Switch and Sensor
- 36XW Digital Submersible Hydrostatic Level Sensor
- LEO3 Current or Digital Output Pressure Gauge
- PD39X High Differential Pressure Transmitter
- PD33X High Accuracy Differential Pressure Transmitter
- DS200 Combined Pressure Switch, Gauge and Sensor
- DS201P High Range Flush Pressure Gauge, Switch and Sensor
- PD33X Digital Differential Pressure Sensor
- XMPi Process Plant Gauge and Absolute Pressure Transmitter
- DMP331i High Accuracy Pressure Transmitter
- Xacti Rangeable Flush Pressure Transmitter with LCD Readout
- DMP333i Rangeable High Pressure Precision Transducer
- 35X Flush Diaphragm Digital Output Pressure Sensor
- PD39X Twin Absolute Digital DP Sensor
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.