If you work with pressure transmitters, pressure gauges, control valves, or valve positioners, you may have encountered this situation during calibration:
You gradually increase the input to a certain value and record the instrument output. Then you approach exactly the same input value from the decreasing direction — but the instrument gives you a slightly different output.
Same input, different output.
This phenomenon is commonly known as hysteresis.
1. What Is Instrument Hysteresis?
In simple terms, hysteresis is the difference in instrument output at the same input value depending on whether that input is approached from the increasing or decreasing direction.
More technically:
Hysteresis is the maximum difference between the output readings obtained at the same input value when the input is approached from the increasing and decreasing directions under the same operating conditions.
In practical language:
The instrument gives one reading on the way up and another reading on the way down.
2. A Practical Example
Suppose we are calibrating a pressure transmitter with a measuring range of:
0–2 MPa
During the increasing-pressure test:
- Input: 1.0 MPa
- Output: 12.00 mA
Then we increase the pressure further and gradually decrease it back to 1.0 MPa.
During the decreasing-pressure test:
- Input: 1.0 MPa
- Output: 12.10 mA
The hysteresis at this point is:
Hysteresis = |12.10 − 12.00| = 0.10 mA
For a standard 4–20 mA transmitter, the output span is:
20 − 4 = 16 mA
Therefore:
Hysteresis = (0.10 / 16) × 100% = 0.625% FS
However, a proper hysteresis test should not be based on only one calibration point.
For example:
| Input | Increasing Output | Decreasing Output | Difference |
|---|---|---|---|
| 0% | 4.00 mA | 4.00 mA | 0.00 mA |
| 25% | 8.00 mA | 8.04 mA | 0.04 mA |
| 50% | 12.00 mA | 12.10 mA | 0.10 mA |
| 75% | 16.00 mA | 16.05 mA | 0.05 mA |
| 100% | 20.00 mA | 20.00 mA | 0.00 mA |
In this example, the maximum difference is 0.10 mA, so the maximum hysteresis is:
0.625% of output span
3. What Causes Hysteresis?
Hysteresis can come from several sources, depending on the instrument design.
1. Mechanical Friction
Mechanical components such as valve stems, packing, shafts, gears, and linkages generate friction.
When the direction of movement changes, the direction of the friction force also changes. This can cause the instrument to respond differently during increasing and decreasing input.
This is especially common in control valves and mechanical instruments.
2. Elastic Hysteresis
Diaphragms, bellows, springs, and other elastic sensing elements do not always follow exactly the same path during loading and unloading.
As a result, the output during increasing pressure may differ slightly from the output during decreasing pressure.
3. Mechanical Backlash
Gears, linkages, shafts, and other transmission mechanisms may have small mechanical clearances.
When the direction reverses, this clearance must first be taken up before the mechanism begins moving in the opposite direction.
Think of it like a mechanical transmission system with a small amount of free movement when the direction changes.
4. Magnetic Hysteresis
Some electromagnetic sensing components can exhibit magnetic hysteresis.
Their response may depend partly on the previous magnetic state, resulting in slightly different outputs for increasing and decreasing inputs.
4. How Should Hysteresis Be Tested?
A proper calibration should include both upscale and downscale measurements.
For example:
0% → 25% → 50% → 75% → 100%
and then:
100% → 75% → 50% → 25% → 0%
Compare the upscale and downscale output readings at each corresponding input point.
The largest difference found between the two sets of readings represents the maximum hysteresis observed during the test.
For accurate results, avoid overshooting the calibration point and then correcting backward, as this can affect the direction-dependent measurement.
5. How Do You Know Whether the Hysteresis Is Acceptable?
Do not automatically assume that every instrument must meet the same hysteresis limit.
The acceptable value should be determined according to the:
- Manufacturer’s technical specifications
- Instrument accuracy requirements
- Applicable calibration or verification procedure
- Relevant industry standards
For some instruments, the allowable hysteresis may be related to the specified basic error limit, while others have a separate hysteresis or deadband specification.
Therefore, an instrument should not be considered acceptable simply because its zero and span are correct.
Its performance during both increasing and decreasing input should also be checked.
A Special Note About Control Valves
For control valves and valve positioners, differences between increasing and decreasing travel can involve more than hysteresis alone.
Possible causes include:
- Valve stem friction
- Packing friction
- Mechanical backlash
- Deadband
- Actuator problems
- Positioner performance
- Linkage problems
Therefore, if a control valve shows a significant difference between its opening and closing characteristics, do not immediately assume that the problem is simply “hysteresis.”
The entire valve-positioner-actuator assembly should be evaluated.
Final Takeaway
Hysteresis can be summarized in one sentence:
Same input, different output depending on the direction of approach.
When calibrating an instrument, don’t just check whether it reaches the correct reading during the increasing cycle.
Always check the return cycle as well.
A device may have a correct zero point and span but still show excessive hysteresis — and that can lead to poor measurement accuracy or control performance in real industrial applications.