Dwyer RSM-1-B : Room Status Monitor, +/-0.05 in w.c., 120 VAC Supply, Selectable 4-20 mA / 0-5 V / 0-10 V Output, Audible + Visual + Relay Alarm, NEMA 1 / IP20

The RSM-1-B is a wall-mounted Room Status Monitor that continuously measures the static pressure difference between a critical environment (such as an isolation room) and a reference space (typically the adjacent hallway or anteroom) across a ±0.05 in w.c. range. When the room pressure breaches the configured upper or lower limit and the alarm delay expires, the RSM triggers three coordinated alarms: an audible buzzer (up to 85 dB), visual indicators (red LED plus red RGB backlight), and an SPST relay output for remote signaling (BAS, fire panel, or remote annunciator).

Accuracy is specified as a percentage of full scale. A ±0.05 in w.c. sensor delivers ±0.00025 in w.c. resolution at full scale — more than enough to discriminate the typical 0.01–0.03 in w.c. working differentials used in BSL labs and cleanrooms. A wider-range RSM (say ±1 in w.c.) used at the same point would have 20× coarser effective resolution at the typical operating pressure. Match the range to your spec's working differential, not to worst-case transients (the ±15 in w.c. pressure tolerance handles spikes regardless of selected range).

Yes — the -B variant accepts 85–265 VAC at 50/60 Hz, which is a true universal mains supply. It works on 100 V (Japan), 110/120 V (North America), 208/230/240 V (Europe / industrial), and everything in between. No transformer or switching required. If your site has 24 VAC Class-2 control wiring already available and you'd prefer to use that instead of mains, order the RSM-1-A variant.

Many low-pressure differential sensors work by allowing a small amount of air to flow between the two pressure ports across a sensing element — the flow rate is proportional to the pressure differential. That's fine for HVAC duct measurement, but it's a problem for critical environments because it creates a small flow path between the protected and reference spaces (a cross-contamination risk), and the sensor drifts over time as dust and particulates foul the flow path. The RSM uses a dead-ended capacitive element: the sensing diaphragm flexes minutely under differential pressure with no flow between sides. No cross-contamination, no fouling drift, no annual cleaning required.

The output is field-selectable via the touchscreen menu among three options: 4–20 mA (2-wire current loop, max 510 Ω load), 0–5 VDC, or 0–10 VDC (both 3-wire). No external excitation supply is needed for any of them — the output is powered from the RSM. Pick 4–20 mA for long cable runs to a remote BAS controller (noise-immune); pick 0–10 VDC for the most common BAS analog input scaling; pick 0–5 VDC if your controller expects the 5V half-scale convention. Same hardware, no part-number change — selection is purely a menu setting.

The RSM has two operating modes selectable via the touchscreen: Occupied (normal operation — alarms triggered when pressure breaches limits) and No Isolation (alarms suppressed — the room is intentionally not maintaining isolation, such as during cleaning or maintenance). Mode switching requires either the Operator-level password (limited to mode-switch only) or the Supervisor-level password (full menu access). Two levels of password protection prevent untrained staff from inadvertently changing alarm setpoints, output type, or calibration.

The RSM has a low-voltage (5 VDC) door switch input that accepts a normally-open SPST or SPDT contact wired to the door jamb. When the door is open, the RGB backlight turns yellow and the monitoring screen shows door-open status — serving as a pre-alarm visual cue (the pressure differential will often collapse momentarily during door operation, which would otherwise trigger a false alarm). The door switch is optional and the input is disabled by default; enable it in the Alarm Setup screen if a door contact is wired in. Door switch hardware is not included with the RSM.

The RSM-1-B does not include BACnet communication. To get BACnet MS/TP, order the corresponding RSM-1-D variant instead — same hardware, same pressure range, same analog outputs, plus an isolated RS-485 BACnet MS/TP interface for integration into a building automation system. The BACnet option allows the supervisory system to monitor pressure and alarm status across all RSM devices in the facility from a single workstation, push configuration changes, and log historical data. BACnet variants cannot be added to a non-BACnet RSM in the field — the BACnet hardware is internal.