Short Answer:

Hysteresis error in measurements refers to the difference in the readings of an instrument when the measured quantity is increasing versus when it is decreasing. Even though the value of the input may be the same, the output reading changes depending on the direction of change.

This error mainly occurs due to the material’s internal friction, magnetic effects, or mechanical lag in the instrument. It is commonly found in devices like pressure gauges, magnetic sensors, and mechanical systems where past input affects current output. Hysteresis reduces the accuracy and consistency of measurements.

Detailed Explanation:

Hysteresis Error in Measurements

In electrical and mechanical measuring instruments, accuracy and consistency are crucial for correct results. Sometimes, the same value of input gives different output readings depending on whether the input is increasing or decreasing. This difference is known as hysteresis error. It is a common source of inaccuracy in many measuring systems.

Meaning of Hysteresis Error

Hysteresis error is defined as the maximum difference in output value when a measured quantity is approached from two different directions—once by increasing the input and once by decreasing it. Even when the input returns to the same value, the output may not return to the same point. This shows that the system “remembers” its previous state, which leads to measurement error.

For example, in a pressure sensor, increasing the pressure to a certain point may give a different reading than decreasing the pressure back to the same level. The difference in these two readings is due to hysteresis.

Causes of Hysteresis Error

Material Properties
Materials like rubber, spring steel, or magnetic substances have memory effects. When they are stretched, compressed, or magnetized, they do not return to their exact original state, causing a lag.
Internal Friction
In mechanical parts, friction resists smooth movement, especially when the direction changes. This leads to different positions or outputs for the same input level.
Magnetic Hysteresis
In electrical instruments with magnetic cores, such as transformers or magnetic sensors, the core retains some magnetism even after the field is removed. This retained magnetism leads to different readings when magnetic field increases or decreases.
Temperature Effects
Heating and cooling may affect materials differently. Expansion or contraction can cause mechanical shifts, contributing to hysteresis.

Examples in Electrical Instruments

Magnetic Sensors: Magnetic hysteresis in core materials causes different voltage readings for increasing and decreasing magnetic fields.
Pressure Gauges: Elastic elements like diaphragms may not return fully to original shape, causing reading differences.
Thermostats and Switches: They often use hysteresis intentionally to avoid frequent on-off switching, but in measurements, this causes small delays or reading differences.

Impact on Measurement Accuracy

Hysteresis error affects the repeatability and reliability of readings. It becomes a serious problem in instruments that need very high precision. In such systems, hysteresis needs to be minimized or compensated.

In control systems, hysteresis can lead to incorrect decisions or delayed responses.
In calibration, hysteresis must be considered to ensure standard values are properly matched.
In sensor design, materials with low hysteresis are preferred.

Reducing Hysteresis Error

Use Better Materials: Select materials with low internal friction or magnetic memory.
Improve Mechanical Design: Reduce loose parts and friction-causing joints.
Temperature Control: Maintain stable operating temperature to avoid expansion/contraction issues.
Regular Calibration: Identify and adjust for any hysteresis in system performance.

Conclusion

Hysteresis error in measurements occurs when an instrument gives different readings for the same input based on whether the value is increasing or decreasing. It is caused by internal friction, material memory, magnetic effects, or mechanical lag. This error affects measurement accuracy and is especially important in precise instruments and control systems. By selecting proper materials, improving design, and applying calibration, hysteresis error can be reduced for more consistent and reliable results.