Terminal Based Linearity Calculator
Separate linearity from zero and span error. The calculator draws a straight line through the 0 % and 100 % readings and finds how far the middle points deviate from it.
- Line through end points: I(p) = I₀ + (I₁₀₀ − I₀) × p
- Deviation = (measured − line) ÷ 16 mA × 100
Deviation from the terminal line
| Point | Measured | Terminal line | Deviation |
|---|---|---|---|
| 0 % | 4.010 | 4.010 | +0.000 % |
| 25 % | 8.030 | 8.013 | +0.109 % |
| 50 % | 12.045 | 12.015 | +0.188 % |
| 75 % | 16.040 | 16.018 | +0.141 % |
| 100 % | 20.020 | 20.020 | +0.000 % |
Terminal Based Linearity
Nonlinearity = max |Imeasured − I(p)| ÷ span
Zero and span errors can be trimmed out; nonlinearity cannot. Best fit straight line (independent) linearity, often quoted in datasheets, is typically about half the terminal based value.
Worked example
With 4.010 and 20.020 mA at the ends, the line predicts 12.015 mA at 50 %. The reading of 12.045 mA is 0.030 mA high: +0.19 % nonlinearity.
Frequently Asked Questions
What is terminal based linearity?
The maximum deviation of the calibration curve from a straight line drawn through its end points, as a percent of span.
How is it different from independent linearity?
Independent linearity uses the best fit line through all points, which gives a smaller number than the terminal method.
Can linearity be adjusted?
Not with zero and span. Smart transmitters may allow a multi point sensor trim; otherwise the sensor must be replaced.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article