Hysterisis Definition

[learner08]

Hi!

Would like to get a definition for hysterisis. Looking it up gives 2 definition.

1. lagging behind of flow upon movement of valve
2. difference in measurement for the same amount of flow measured when flow is increasing against when flow is decreasing.

Does the context in which it is used defines the meaning? Example (1) when it is used for valves and (2) when it is used for flowmeters?

Also for situation (2) how come a flowmeter can have repeatability of 2% when the hysterisis is 15%? Doesn't make sense to me.....

Can anybody pls help? Thanks in advance.

 

[insult2injury]

Both definitions are really equivalent. The first just says that you can have two different measured flow rates at the same valve opening % depending on whether it is opening or closing. This is exactly what your second definition says.

I2I

 

[25362]

As explained by insult2injury hysteresis and repeatability can coexist with hyst. % > repeat. %.

Repeatability, meaning in fact non-repeatability, expressed usually as % of span, is the maximum difference between readings when the same input is applied consecutively, approaching from the same direction.

Reproducibility, on the other hand, expresses the closeness of agreement of repeated measurements of an output for the same input under equal operating conditions over a given time period, but approaching from both directions, thus including deadband, hysteresis, drift and repeatability.

 

[Ashereng]

Good answers!

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Ashereng]

Just to add a bit.

Hysteresis is the effect of different result depending on preceding conditions.

For example, agar melts at 85 °C and solidifies at 40 °C.

So, if you have liquid agar at 100 °C, and start to cool it, it is still a liquid at 50°C, and will remain so until 40 °C when it will solidity. Once you have solid agar, and start to heat it up, it is solid at 50 °C, and will remain so until 85 °C when it will melt.




"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[automatic2]

adjusting a pressure switch yesterday that would turn on at 1000 psi, yet turn off at 900. It did so very precisely. Hysteresis at work.

 

[davefitz]

hysterisis as applied to valves can also have other causes and effects.

Relative to the time delay between movement of the flow of process fluid thru the valve vs the transmission of the control signal, time delays that contribute to increased hysterisis include control signal transmission( neglible in modern DCS), action of actuator fluidic control valves ( ie, pnuematic actuator air control valve, hydraulic actuator control valves), inertia of control fluid, inertia ( time constant) of accumulator used by control fluid, frictional pressure drop in control fluid transmission line, inertia of actuator piston, inertia of valve plug+ stem , stick-slip friction in actuator and also between plug and cage.

If the sum of above time delays are negligible compared to the governing time constant of the controlled process, then hysterisis is not an issue.