Bimetallic Thermometer Calibration Tool
Fill in a three-point calibration record for a bimetallic dial thermometer. Calculates error rising and falling, hysteresis, span, and each as a percentage of span.
Bimetallic Thermometer Calibration Tool
Fill in the readings from a three-point calibration. The errors, the hysteresis, and the percentages of span are worked out for you.
Zero point
Mid-scale point
Full-scale point
Calibration record
| Point | Standard (°C) | Rising (°C) | Falling (°C) | Error rising (°C) | Error falling (°C) | Hysteresis (°C) |
|---|---|---|---|---|---|---|
| Zero point | 0.00 | 0.40 | 0.60 | +0.40 | +0.60 | 0.20 |
| Mid-scale point | 50.00 | 50.80 | 51.40 | +0.80 | +1.40 | 0.60 |
| Full-scale point | 100.00 | 99.50 | 99.50 | -0.50 | -0.50 | 0.00 |
As a percentage of span
Largest error is +1.40% of span. Largest hysteresis is 0.60% of span.
Error = reading − standard. Hysteresis = |reading rising − reading falling|. Percentage of span = value ÷ span × 100.
Documentation
This tool fills in a three-point calibration record for a bimetallic thermometer. Enter the reference temperature and the two readings taken at each point, and it works out the error, the hysteresis, and both as a percentage of span.
What a bimetallic thermometer is
A bimetallic thermometer measures temperature with a strip made of two metals bonded together. The metals expand by different amounts when heated, so the strip bends. The strip is usually wound into a coil that turns a pointer on a dial.
Because the coil is a mechanical part, it does not return to exactly the same position when the temperature falls as when it rises. That difference is called hysteresis, and calibration measures it.
Why three points
Calibration checks the instrument across its working range. Three points are the usual minimum:
- the zero point, at the low end of the range
- the mid-scale point, near the middle
- the full-scale point, at the top of the range
At each point the temperature is first approached from below, then from above, so both directions are recorded.
Formulas
error rising = reading while rising − standard
error falling = reading while falling − standard
hysteresis = |reading while rising − reading while falling|
span = standard at full scale − standard at zero
percentage of span = value ÷ span × 100
The standard value is the temperature shown by the reference instrument, which is trusted to be correct. A positive error means the thermometer reads high; a negative error means it reads low.
Example
A 0 to 100 °C thermometer is checked at three points.
| Point | Standard | Rising | Falling | Error rising | Error falling | Hysteresis |
|---|---|---|---|---|---|---|
| Zero | 0.00 | 0.40 | 0.60 | +0.40 | +0.60 | 0.20 |
| Mid | 50.00 | 50.80 | 51.40 | +0.80 | +1.40 | 0.60 |
| Full | 100.00 | 99.50 | 99.50 | −0.50 | −0.50 | 0.00 |
The span is 100 − 0 = 100 °C. The largest error is +1.40 °C, which is 1.40% of span. The largest hysteresis is 0.60 °C, or 0.60% of span.
Reading the result
Instrument accuracy is normally quoted as a percentage of span, so the percentages are what to compare against the manufacturer's accuracy class. A thermometer marked class 1 is expected to stay within 1% of span.
Large hysteresis with small errors usually points to friction or a tired coil rather than a shifted scale. An error that is about the same size at all three points suggests the pointer is simply offset and can often be reset at the zero adjustment screw.
Frequently asked questions
Why must the zero point not be below 0 °C?
In this record the zero point is the bottom of the working range being certified. The tool keeps it at 0 °C or above so the span is measured from the low end upward.
What causes hysteresis in a bimetallic thermometer?
Friction in the pointer mechanism and elastic lag in the bimetal coil. The coil does not relax along exactly the same path it followed while heating.
Should the error be measured rising or falling?
Both. Recording both directions is what makes the hysteresis visible. The worst of the two is the figure to judge the instrument by.
What is span?
The difference between the standard values at the full-scale point and the zero point. It is the width of the range being calibrated.
Can this record be used for other dial thermometers?
Yes. The same three-point method and the same formulas apply to any mechanical dial thermometer, including gas-filled and liquid-filled types.