The SRD991 is Schneider Electric's intelligent digital valve positioner - a microprocessor controller that turns a 4-20 mA signal into precise pneumatic action on linear and rotary actuators using fast piezo-valve technology.

SRD991 or SRD960 - which do you specify?

The SRD991 is the cost-effective digital positioner for everyday control valves: HART 7, push-button calibration, and valve-health diagnostics at a fraction of the universal SRD960's cost. The alternative - the SRD960 - only pays off where SIL 3, PST, and premium signatures are mandated.

How precise is the positioning?

Piezo technology holds linearity around ±0.5% (±0.2% on high-accuracy builds) with ±0.2% repeatability, on 1.4-7 bar supply, for linear strokes of 10-100 mm and rotary angles of 90-120°. The alternative - an analog positioner - cannot self-report its own repeatability drift.

What does the diagnostics actually catch?

It logs valve signature, leakage, and air-supply pressure trends, so a packing that is starting to bind shows up in a handheld check rather than a process upset. The alternative - a positioner with no comms - hides that wear until the valve sticks.