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Switches · Interactive Instrumentation Guide
Differential Pressure Switch
Raise and lower the process to cross your set and reset points. Watch the SPDT contacts and remember what happens inside the deadband.
Custom process schematic · not to scale · blue H connection / orange L connection, where applicable.
Animation range and application settings
Equation and live calculation
Deadband = |Set Point − Reset Point|; rising: Reset < Set; falling: Reset > Set
Live substitution appears when the demonstration loads.
PV = process value; LRV/URV = lower/upper range values; I = current in mA. Pressure calculations use kPa internally. Selected units apply to inputs, limits, diagrams, and readouts; changing units converts the same physical values without changing the current. Tank calculations use meters internally; z is positive above the bottom tap. ρ = 1000 × specific gravity in kg/m³; g = 9.80665 m/s². H in tank equations means measured height, not the H port. Entered LRV/URV define the current scaling; calculated empty/full tank DP values are reference checks. Set and Reset Points define switch operation; deadband is their absolute difference. Temperature reference equations use °C internally. Mercury pressure units use conventional NIST conversion factors. Vacuum entries are negative gauge pressure; the absolute-pressure readout remains nonnegative. Models assume ideal instantaneous response. Displayed current is limited to the nominal 4–20 mA range: I_display = clamp(I_ideal, 4, 20), where clamp holds a value at its lower or upper limit. This teaching limit does not reproduce manufacturer-specific saturation or alarm currents.
Where it is used
Automatic-reset switches used for alarms or process-state indication. The contact sketch shows an ideal SPDT changeover.
What the demonstration shows
A rising-action switch resets below its trip point. A falling-action switch resets above it. Inside the deadband, the previous state is retained.
Common mistake to avoid
NO and NC describe contacts in the normal unactuated condition, not a universal safe state. Real switch deadbands may be fixed and model-dependent.
Model assumptions
Ideal automatic-reset SPDT switch. Adjustable deadband is provided for teaching; actual hardware may have fixed deadband. The animation range automatically expands to include valid Set and Reset entries. Changing action initializes the switch to normal, then applies the entered process condition.
Technical references
Ashcroft — Deadband and hysteresisNIST — SI unit conversion factors (conventional mercury units)Educational model. Consult the instrument manual for installation, configuration, limits, and approved test procedures. No connection to live plant equipment is made.