How instruments measure

A frequency-modulated radar beam reflects off the surface; the frequency offset between emitted and returned signal gives distance.

Two process pressures deflect a sensing diaphragm between capacitor plates; the capacitance change is proportional to the pressure difference.

Oscillating tubes twist in proportion to mass flow; phase shift between sensors gives direct mass flow, density and temperature.

Ion-selective electrodes and conductivity cells convert chemical activity into potential or current.

Faraday's law: a conductive liquid moving through a magnetic field induces a voltage proportional to velocity.

Low-power radar pulses travel down a probe; the reflection at the liquid surface sets the level, and a second echo can show the interface.

A plug moves along its stem axis against a seat, throttling flow with a characterised trim.

Light emission, absorption or scattering is measured; luminescence quenching gives oxygen, scattered light gives turbidity.

A silicon or ceramic diaphragm deforms under pressure and a bridge of strain-sensitive resistors converts that deflection into a millivolt signal.

A positioner compares the 4–20 mA setpoint with stem position and drives the actuator until they match.

The resistance of a platinum element rises predictably with temperature, typically 100 Ω at 0 °C for Pt100.

A segment or ball rotates in the body to modulate flow.

Two dissimilar metals joined at a junction generate a voltage related to the temperature difference to the cold junction.

Ultrasonic pulses travel faster with the flow than against it; the time difference gives velocity.

A sound pulse is timed from the transducer to the surface and back.

A piezo-driven tuning fork vibrates at resonance; contact with product damps the frequency and trips the switch.

A bluff body sheds vortices at a frequency proportional to velocity; sensors count the vortices.