The Dwyer Digihelic AT-DH3 packs three instruments into one ATEX/IECEx-certified enclosure: a backlit 4-digit LCD differential pressure gauge, two independently configurable SPDT relay switches, and a 4–20 mA process transmitter. For an engineer designing a hazardous-area air or gas monitoring panel, this means one process connection, one set of wiring, and one device to stock instead of three. The flame-proof aluminum housing is rated IP66 and carries ATEX II 2G Ex db IIC and IECEx certification for gas and dust atmospheres.

How do the relay setpoints and transmitter output work together?

Both SPDT relays have adjustable setpoints and deadbands configured through the front-panel keypad menu — no tools, no opening the enclosure in the hazardous area. Each relay can operate in high, low, or high/low alarm mode with auto or manual reset and an adjustable alarm delay. The 4–20 mA output is independently scalable and can output a linear signal proportional to differential pressure, or a square-root signal when the controller is configured for velocity or volumetric flow measurement. This lets one AT-DH3 serve as both the safety interlock (via relays to a PLC or shutdown system) and the continuous monitoring channel (via 4–20 mA to the DCS).

What ranges and measurement modes are available?

Nine unidirectional ranges span from 0.25 to 100 inches w.c., with six bidirectional ranges from ±0.25 to ±10 in w.c. Accuracy varies by range — ±0.5% of full scale on ranges 10 in w.c. and above, ±1% on 0.5 to 5 in w.c., and ±1.5% on the most sensitive 0.25 in w.c. span. The controller can display and transmit in pressure units (in w.c., mm w.c., Pa, kPa, mbar, psi), velocity (FPM, m/s), or volumetric flow (CFM, m³/h) with user-entered K-factor and duct area. A selectable digital damping filter smooths readings in pulsating flow applications.