The Dwyer A3300-250PA is a Series A3000 Photohelic® Differential Pressure Switch/Gage — a 3-in-1 instrument that combines a precision indicating gage with two independent setpoint switches (one low-limit, one high-limit) in a single 4-inch dial-face housing. This model is calibrated for a 125-0-125 Pa range on a zero-center bi-directional scale — ideal for applications where pressure can swing positive or negative around a reference (such as building static pressure relative to atmosphere, or ductwork balanced near zero differential).
The Photohelic builds on Dwyer's time-proven Magnehelic® gage movement with patented optical limit switches: as the indicator needle passes either setpoint, a helix-driven light shield interrupts an infrared beam between an LED and a phototransistor, triggering a DPDT relay (Form C, 10A @ 28 VDC, 10A @ 120/240 VAC). Deadband between switch make and break is less than 1% of full scale — just one pointer width — for chatter-free operation. Setpoints are adjusted with knobs on the gage face and are visible at all times. Gage reading is unaffected by switch operation, even during loss of electrical power. Maximum operating pressure: 25 psig (1.72 bar) - Standard.
Building static pressure control (positive or negative relative to outdoors), differential pressure across components where pressure can swing either direction (cleanrooms, isolation rooms, ductwork balanced near zero), and any application needing bi-directional pressure sensing with high- and low-limit alarms. Range: +/-125 Pa.
Key Features
Replaces three separate instruments — a pressure gage and two pressure switches — with one panel-mount unit. Reduces inventory, panel cutout count, wiring time, and total installed cost. Both setpoints are independently adjustable with knobs on the gage face and visible at all times alongside the actual pressure reading.
A helix-driven light shield interrupts an infrared beam between an LED and phototransistor as the indicator needle passes each setpoint — no mechanical contact with the gage movement, no wear on the switching mechanism. Reliable, long service life, and precise switching accuracy down to 1% of full scale deadband (one pointer width).
Two Form C relays (DPDT), one per setpoint, rated 10 A @ 28 VDC and 10 A @ 120/240 VAC. Standard circuit (HH) latches on pressure increase to either setpoint; optional circuit styles (LL, HL, LH) and manual reset configurations available for specialized control schemes including high/low limit fan or damper control between setpoints.
The Magnehelic®-based gage movement is mechanical — pressure reading remains visible and accurate even if electrical power to the switch circuitry is lost. On power or pressure loss, both relays de-energize to their normal "zero pressure" state, providing a predictable fail-safe behavior for alarm and control circuits.
Factory calibrated for +/-2% FS of full scale at 70 deg F (21.1 deg C). Repeatability +/-1% FS. Zero adjustment screw on the front cover for field zeroing without tools. NIST traceable calibration certificate available as -NIST option.
CE, CSA, and UL listed for use in industrial control panels. Plastic enclosure with polycarbonate connection board (self-extinguishing) and glass-epoxy printed circuit boards. 1/8 in female NPT pressure connections. 3/4 in conduit connection for electrical wiring. Standard 4-13/16 in panel hole for mounting; surface and remote-relay mounting also available.
Technical Specifications
| Specification | Value |
|---|---|
| SKU | A3300-250PA |
| Series | Dwyer Series A3000 Photohelic |
| Product Type | 3-in-1 Differential Pressure Switch/Gage |
| Pressure Range | 125-0-125 Pa |
| Scale Type | Zero-Center +/-125 Pa (bi-directional) |
| Service | Air and non-combustible, compatible gases |
| Accuracy | +/-2% FS at 70 deg F (21.1 deg C) |
| Repeatability | +/-1% of full scale |
| Deadband | <1% FS (one pointer width) |
| Pressure Limit | 25 psig (1.72 bar) - Standard |
| Negative Pressure Limit (Standard) | -20 in Hg (-0.677 bar) |
| Temperature Limits | 20 to 120 deg F (-6.67 to 48.9 deg C); low-temp -20 deg F option (-LT) |
| Process Connections | 1/8 in female NPT (high and low pressure ports) |
| Switch Type | Two Form C relays (DPDT), one per setpoint |
| Electrical Rating | 10 A @ 28 VDC, 10 A @ 120 VAC, 10 A @ 240 VAC |
| Electrical Connections | Screw terminals; use 167 deg F (75 deg C) copper conductors only |
| Conduit Connection | 3/4 in |
| Power Requirements | 120 VAC, 50/60 Hz standard; 240 VAC and 24 VAC optional |
| Setpoint Adjustment | Adjustable knobs on gage face |
| Mounting Orientation | Diaphragm in vertical position (consult factory for other orientations); -0 and -00 models must be mounted scale-vertical |
| Panel Mounting Hole | 4-13/16 in (122.24 mm) diameter |
| Behind-Panel Clearance Required | 4-3/8 in (111.13 mm) for electrical enclosure removal |
| Dial Face Diameter | 4 in (101.6 mm) |
| Overall Dimensions | 5 in (127 mm) OD x 8-1/4 in (209.55 mm) deep |
| Weight | 4 lb (1.81 kg) |
| Wetted Materials | Consult factory |
| Compliance / Listings | CE, CSA, UL |
| Calibration | Factory calibrated; NIST traceable certificate available (-NIST option) |
| Country of Origin | USA |
What’s In the Box
- Dwyer A3300-250PA Photohelic Differential Pressure Switch/Gage (4 in dial face)
- Mounting ring and snap ring for panel mounting
- Four 6-32 x 1-1/4 in mounting screws
- Adapters for rubber or soft plastic tubing
- Installation and operating instructions (also available below as PDF)
Related Products
Documentation
Frequently Asked Questions
The A3300-250PA is a Series A3000 Photohelic® Differential Pressure Switch/Gage — a 3-in-1 instrument calibrated for the 125-0-125 Pa range. It functions simultaneously as (1) an indicating gage that shows current pressure on a 4 in dial face, (2) a low-limit pressure switch (DPDT relay triggers when pressure passes the user-adjustable low setpoint), and (3) a high-limit pressure switch (DPDT relay triggers at the high setpoint). Both setpoints are independently adjusted with knobs on the gage face and visible at all times.
Zero-center scales are used when the measured pressure can swing either direction around a reference point — for example, building static pressure relative to outdoors (slightly positive during pressurization, slightly negative during exhaust imbalance), or ductwork balanced near zero differential. The A3300-250PA reads +/-125 Pa with zero at the center of the scale. Set the low-limit setpoint on the negative side and the high-limit on the positive side to alarm on both excursions with one instrument.
No mechanical contact with the gage movement. The Photohelic uses optical limit switches (Dwyer's patented design): an infrared LED beams continuously across a phototransistor inside the gage. The needle's helix-driven shaft carries a small light shield. When the needle reaches a setpoint, the shield interrupts the beam, the phototransistor registers the change, and a DPDT slave relay actuates. No wear on the gage movement, no contact bounce, no chatter, and switching deadband stays under 1% of full scale.
This model has a 25 psig (1.72 bar) - Standard maximum operating pressure. Negative-pressure handling: -20 in Hg (-0.677 bar). Standard rating (most A3000 models). If pressure exceeds the rating, the gage may "jump" from full scale back to zero until the overpressure clears — users should be aware of this possible false-zero indication after an over-pressure event.
Panel mounting is standard: cut a 4-13/16 in (122.24 mm) diameter hole in the panel, insert the unit from the front, slip the supplied mounting ring and snap ring over the back, and tighten the four 6-32 mounting screws. Allow 4-3/8 in (111.13 mm) of clearance behind the panel for electrical enclosure removal during wiring or service. Electrical wiring: 3/4 in conduit connection into the rear enclosure; screw terminals inside. Standard wiring is 120 VAC for the control transformer (Section E on terminal board), with the DPDT load relays wired to Sections A (high setpoint) and D (low setpoint). Use 167 deg F (75 deg C) rated copper conductors only. Surface mounting and remote-mounted relay (-RMR) options are also available.
Two 1/8 in female NPT ports on the gage: high pressure (HP) and low pressure (LP). Use 1/4 in OD metal or instrument-grade plastic tubing with 1/8 in NPT adapters at the gage. Adapters for rubber or soft plastic tubing are included. If only measuring differential pressure between two sources, connect each source directly to HP and LP. If measuring gauge pressure (positive only), connect the source to HP and leave LP open to atmosphere (use a Dwyer A-331 Filter Vent Plug to keep dust out of the open port). Important: zero the gage with both ports open to atmosphere before plumbing the pressure source.
Both relays de-energize and return to their normal 'zero pressure' state on loss of electrical power. On loss of pressure, the same de-energize behavior occurs for standard 0-to-FS scales. Exception for zero-center models: because the relays are always energized when the indicator needle is to the right of their setpoint, the relay action on pressure loss depends on where the setpoint is positioned relative to zero (since either setpoint could sit on the left of zero on a center-zero scale). Plan the failure mode into your control logic accordingly. The gage's pressure reading itself remains valid even during power loss — the indicator is purely mechanical.
+/-2% FS of full scale at 70 deg F (21.1 deg C); repeatability +/-1% FS. Standard A3000 accuracy class. For applications requiring documented calibration traceability, order with the -NIST option for a NIST-traceable calibration certificate. Re-calibration is typically not field-serviceable; return units to Dwyer for re-certification.
