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? 

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.

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