Glossary · Sensors and measurement
Span
Also known as: Measuring span, Range span
German: Messspanne
In instrumentation, the span is the algebraic difference between the upper and lower range values of an instrument's set measuring range; for a transmitter set to −10 to 90 °C, the span is 100 K.
- Sensors
In one sentence
The span is the difference between the upper and lower range values of a set measuring range, the basis for percent-of-span errors.
Example
A level transmitter set from 0.5 to 4.5 m has a span of 4 m, so an accuracy of 0.1 % of span means ±4 mm.
How it applies
- Engineering: Many error figures are given in percent of span. Reducing the span (turndown) makes the absolute error in engineering units smaller only if the error figure is referred to the set span, not to the sensor's full range.
- Commissioning: Zero and span adjustment sets the lower and upper range values. In analog loops, 4 mA and 20 mA correspond to these values.
- Calibration: A two-point check at zero and span confirms the setting; intermediate points reveal nonlinearity.
- Documentation: Record lower range value, upper range value and span for each transmitter. Scaling in the controller must use the same values.
Span vs. measuring range
The Measuring range is the interval itself, for example −10 to 90 °C. The span is its width, 100 K. The German Messspanne and Messbereich are used in the same way.