.square root .sqr   .sre

The square root extractor is an electronic (or pneumatic) device that takes the square root of the signal from the flow transmitter and outputs a corresponding linear flow signal.

Purpose of its use

The square root extractor is used to convert the measured differential pressure into flow rate. signal is found to vary as the square of the measured flow rate, Q, based on eqn (*) and the basic flow rate equation Q = A (flow area) x V (velocity)
That is, Q2 α ∆P, where Q α √∆P.
Hence, a square root extractor is required on the ∆P signal in order to measure Q.

Why it is not used in level/temperature transmitter.

Because the relationship between the variables are linear so no need of an square root extractor.