Instrument % Span and % Error

Sensor is typically characterized and defined by its sensitivity, linearity and range with responsiveness for any given application…

In instrument calibrations, the span adjustment is meant to position the sensor together with its amplifier gain to have a flat most response in the most linear range of the sensor.

% Span is the sensor’s flat most response range in its characteristics curve, as compared to the amplifier’s linear most gain…adjusting for the amplifier’s maximum linear gain!

% Error is the combined errors… sensor’s error + amplification error, adjusting it for the minimum!

The overall response of the amplifier is being adjusted to have enough range and span with the least minimum error, while proportionately amplifying and linearly matching the sensor’s flat most response! …

% span is related with resolutions of instrument, whereas % error is related with accuracy or precision of that instrument.

How do we know which one to use?

It depends on field and types of measurand- flow, level, temperature, pressure, pH, torque, voltage, power or else.

  • %span will be good for direct readings or indicative types of instrument. Like- PMMC instruments, MI instruments, gauges etc.
    • Since they will be read by humans, more span or more resolution will improve the parallax error.
  • %error will be good for the case of sensors and tranducers. Preferably where you have incorporated DPTs or controllers [PID or PLC].
    • Best example is in 4–20 mA or 3–15 psi calibration.
    • They are equally important for all consideration. Error influence everything of instrumentation both- calibration and designing.

PMMC- Permanent Magnet moving coil. MI- Moving Iron. DPT- Differential Pressure transmitters. PID- Proportional Integral Differentiation. PLC- Programmable logic controller.

Credits: Wicida Sarmi, Abhishek

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