The ACI A/MLP2-D50-W-U-A-A-0P is part of ACI's MLP2 Series Miniature Low Differential Pressure Transmitter family, configured for a 0 to 0.5 inch WC unidirectional range with a 4-20 mA output (2-wire loop powered (or 3-wire VAC powered)) and +/-0.5% FSO accuracy. Designed specifically for OEM and high-density panel mounting applications, the MLP2 features an integral 35 mm DIN rail mounting foot (U.S. Patent No. 7,416,421) with both rear and side-mount options, vertically oriented wiring and pressure connections, a de-pluggable 3-position screw terminal block for fast field installation, and a pushbutton zero function on the front cover for in-place offset calibration. Sensing is via a high-accuracy piezoresistive silicon element that reduces warmup shift and minimizes the effect of package stress for superior long-term stability. This unit is factory-calibrated with NIST-traceable equipment but ships without a printed certificate; if your project requires documented calibration, see the 3-Point or 5-Point NIST variants in the same family. Made in USA by ACI in Middleton, Wisconsin.
This range is best suited for low-pressure HVAC applications: building static pressure, room-to-room pressure differentials, filter loading on residential and light commercial systems, fan coil monitoring, and most cleanroom envelope pressure measurements. Applications listed by ACI: building and duct static pressure, filter monitoring, air flow measurement, process control, roof-top units, air handlers, clean rooms, isolation rooms, and data centers.
Key Features
The MLP2 is purpose-built for OEM and panel-density applications: integral 35 mm DIN rail mounting foot (U.S. Patent No. 7,416,421) with both rear-mount (integrated) and side-mount (included adapter, much thinner profile) options, vertically oriented wiring and pressure connections, and a de-pluggable 3-position screw terminal block that accepts 14-24 AWG wire. The compact 4.21" x 2.085" x 1.354" footprint and 0.17 lb weight pack significant capability into minimum panel space.
A micro-machined piezoresistive silicon element senses differential pressure with a linear analog output proportional to the specified range. The silicon technology reduces warmup shift compared to older diaphragm-based designs and minimizes the effect of package stress on the sensing element - dramatically improving long-term stability versus competing low-pressure sensor technologies.
4-20 mA current output, available as 2-wire loop-powered (16-36 VDC at 250 ohm load, 21-36 VDC at 500 ohm load) or 3-wire VAC-powered (24 VAC). Output is limited to 20.5 mA maximum. Current output is preferred for long cable runs (over ~150 ft) or in electrically noisy environments near VFDs.
Accuracy specification includes hysteresis, linearity, and repeatability at 71 deg F (21.5 deg C) typical. This SKU is the standard +/-0.5% accuracy variant. For higher-precision applications requiring +/-0.25% FSO, see the matching variants with accuracy code 'B' in the same family (note: +/-0.25% is not available on the smallest 0.10 inch WC range or 25 Pa range due to sensor physics). Thermal effects: +/-0.056% FSO per deg F relative to 77 deg F (25 deg C).
A front-cover pushbutton ZERO performs in-place offset calibration without opening the cover. Procedure: ensure no pressure at either HI/LO fitting (or short them together with a tubing loop), hold the ZERO button for 5 seconds, then release. Recommended after the 15-minute warmup period or after re-positioning the unit. Periodic re-zeroing eliminates the effect of long-term drift, which is primarily an offset shift rather than a span shift.
The MLP2 uses a digital pot for Span calibration (factory-set, not user-adjustable - units requiring re-calibration must be returned to ACI). Long-term drift is primarily a function of stress relaxation over time in the silicon package, which shifts the offset (not the span). Periodic re-zeroing via the pushbutton eliminates this drift component without service interruption.
Proof pressure of 270 inch WC (67.2 kPa) and burst pressure of 415 inch WC (103.3 kPa) - far above the maximum operating range of this SKU. Tolerates transient overpressure events from duct dampers slamming closed, fan start-up surges, and HVAC commissioning test pressures without sensor damage or calibration drift. Note: media must be non-corrosive, non-ionic gases (dry air or inert gases) - liquids and corrosive vapors will damage the silicon sensing element.
Two brass push-on pressure fittings (HI and LO) accept 0.25" (6.35 mm) O.D. x 0.170" (4.318 mm) I.D. flexible poly tubing - standard HVAC pressure-sensing tubing. ACI recommends a maximum tubing length of 75 feet (23 meters); longer runs introduce signal lag and reduce response time. Pair with ACI's pitot tube accessories (A/PT, A/SPT, A/VPT) or pickup ports (A/SP-PUP, A/R-PUP) for sealed-tap applications.
The 3-position de-pluggable screw terminal block accepts 14 AWG (1.628 mm²) to 24 AWG (0.5105 mm²) wire with 4.43-5.31 in-lb torque. The de-pluggable design lets technicians pre-wire the panel offline, then plug the terminal block into the unit - dramatically reducing in-panel labor on OEM builds. Pin assignments: VIN / GND / OUT. ACI recommends shielded cable with the shield grounded only at the controller end (single-ended grounding prevents ground loops).
Technical Specifications
| Specification | Detail |
|---|---|
| Sensor Style | Active differential pressure transmitter for low-pressure air |
| Sensing Element | Piezoresistive silicon (micro-machined) |
| Measured Value | Differential pressure (HI - LO ports) |
| Pressure Range | 0 to 0.5 inch WC (Unidirectional) |
| Output Signal | 4-20 mA - 2-wire loop powered (or 3-wire VAC powered), output limited to 105% of full scale |
| Accuracy | +/-0.5% FSO (includes hysteresis, linearity, repeatability at 71 deg F) |
| Thermal Effects | +/-0.056% FSO per deg F (+/-0.10% FSO per deg C) relative to 77 deg F |
| Warm-Up Time | 15 minutes (recommended before zeroing) |
| Response Time (T95) | 4 seconds |
| Output Update Rate | 500 ms |
| Zero Function | Front-cover pushbutton ZERO (hold 5 sec to re-zero) |
| Proof Pressure | 270 inch WC (67.2 kPa) |
| Burst Pressure | 415 inch WC (103.3 kPa) |
| Power Supply | 16-36 VDC (250 ohm load) / 21-36 VDC (500 ohm load) / 24 VAC +/-10% 50/60 Hz |
| Supply Current | 23 mA minimum (0.83 VA power consumption) |
| Operating Temperature | 32 to 185 deg F (0 to 85 deg C) |
| Compensated Temperature Range | 32 to 122 deg F (0 to 50 deg C) |
| Storage Temperature | -40 to 176 deg F (-40 to 80 deg C) |
| Operating / Storage Humidity | 10-95% RH non-condensing |
| Media Compatibility | Non-corrosive, non-ionic gases (dry air, inert gases) |
| Pressure Fitting | Brass push-on, accepts 0.25" O.D. x 0.170" I.D. flexible poly tubing |
| Mounting | 35 mm DIN rail (rear or side mount) + #8 x 3/4" panel screws (U.S. Patent No. 7,416,421) |
| Wiring | 3-position de-pluggable screw terminal block, 14-24 AWG, 4.43-5.31 in-lb torque |
| Enclosure | Polycarbonate UL 94 V-0, operating -40 to 248 deg F (-40 to 120 deg C) |
| Dimensions | 4.21" x 2.085" x 1.354" (106.94 x 52.96 x 34.04 mm) |
| Weight | 0.17 lb (0.078 kg) |
| NIST Calibration | No NIST Certificate - Factory NIST-traceable calibration |
| Approvals | CE, RoHS2, WEEE, REACH, UKCA |
| Manufacturer | ACI (Automation Components, Inc.) - Middleton, WI USA |
| Manufacturer Part Number | A/MLP2-D50-W-U-A-A-0P |
| Warranty | 5-Year ACI Limited Warranty |
What's In the Box
- MLP2 transmitter with integral rear-mount 35 mm DIN rail foot
- Side-mount DIN rail adapter clip (for thinner-profile high-density panel mounting)
- De-pluggable 3-position screw terminal block (pre-installed)
- Two (2) #8 x 3/4" self-drilling panel-mount screws
- Installation & Operation Instructions (ACI document I0000791)
- Pressure-sense tubing (medical-grade clear PVC, 0.17" ID x 0.25" OD) - A/10' TUBE, A/20' TUBE, A/50' TUBE, A/100' TUBE
- Pitot tubes with foam gasket for air-flow measurement - PT 3", PT 5.2", PT 7.5", PT 9.7"
- Static pitot tubes (aluminum, flat tip) - SPT-3.5", SPT-8.0"
- Velocity pitot tubes (aluminum, pointed tip) - VPT, VPT-8"
- Pickup ports for room/wall pressure taps - A/SP-PUP, A/R-PUP, A/R2-PUP, A/O-PUP-H, A/O-PUP-V
- Power supply transformer and BAS controller wiring
- Shielded cable (ACI recommends 14-24 AWG twisted pair shielded)
Documentation
Frequently Asked Questions
What does the A/MLP2-D50-W-U-A-A-0P measure and output?
It measures differential air pressure in the 0 to 0.5 inch WC range (unidirectional) and outputs a linear 4-20 mA analog signal proportional to the pressure. Accuracy is +/-0.5% FSO including hysteresis, linearity, and repeatability. The MLP2 is intended for HVAC and OEM applications using dry air or other non-corrosive, non-ionic gases - not liquids, refrigerants, or corrosive vapors.
Should I use inches WC or Pascal units?
Inches of Water Column (WC) is the standard in North American HVAC; Pascals (Pa) is used internationally and in scientific or European-spec applications. The MLP2 is range-specific - this SKU is calibrated in inches WC. For Pascal calibration on smaller ranges, see the matching variants with unit code P (note: smaller Pa ranges like 50, 100, 300 Pa are Pascal-only because they're below the equivalent WC range boundary).
Should I use unidirectional or bidirectional mode?
Unidirectional (0 to +X) is correct for almost all HVAC applications where the HI port is always at higher pressure than the LO port - duct static measurements (HI tapped into duct, LO at atmosphere), filter loading (HI upstream, LO downstream), and room-to-corridor pressurization. For applications where pressure can swing both positive and negative across zero (reversible-flow systems, makeup-air with mode switching), see the bidirectional variants (direction code B) where 0 differential maps to mid-scale output.
When and how should I zero the device?
Zero on initial installation, after re-positioning the unit, and any time the unit reads a non-zero offset with no pressure applied. Procedure: (1) Wait at least 15 minutes after power-up (warm-up period). (2) Ensure no pressure at the HI or LO fittings - either disconnect both tubes from the process or connect a short tubing loop between the two brass fittings to neutralize external pressure. (3) Press and hold the ZERO button for 5 seconds, then release - the unit enters zero mode after the button is released. The Span calibration is digital and factory-set; units requiring Span recalibration must be returned to ACI.
How do I mount this on a DIN rail?
The MLP2 offers two DIN rail options: rear mount (integrated into the enclosure - just clip the unit onto the rail with the integral foot) or side mount (uses the included adapter clip for a thinner-profile installation in high-density panels). To install the side-mount clip: pry off the front cover with a flat-blade screwdriver on the upper edge or side slots, slide the clip into position, then snap the front cover back into place. Both options are 35 mm DIN rail compatible (U.S. Patent No. 7,416,421). For panel-screw mounting, use the two included #8 x 3/4" self-drilling screws through the top and bottom flanges.
Does this come with a NIST certificate?
No - this is the standard 0P (no NIST certificate) variant. The MLP2 is still factory-calibrated using NIST-traceable equipment, but no printed certificate is shipped. If your project requires a documented certificate of calibration, see the 3P (3-point at 10%/50%/90% FSO) or 5P (5-point at 10%/30%/50%/70%/90% FSO) variants of the same SKU.
What accessories do I need for installation?
Required: pressure-sense tubing (0.25" OD x 0.17" ID medical-grade PVC - ACI offers A/10' TUBE, A/20' TUBE, A/50' TUBE, A/100' TUBE), 14-24 AWG shielded cable for the wiring, and a 24 VAC or 12-36 VDC power supply. Application-dependent: pitot tubes for air-flow measurement (PT 3", PT 5.2", PT 7.5", PT 9.7" with foam gaskets; SPT-3.5" / SPT-8.0" static tips; VPT / VPT-8" velocity tips), pickup ports for wall/room pressure taps (A/SP-PUP stainless, A/R-PUP beige, A/R2-PUP white, or A/O-PUP-H / A/O-PUP-V outdoor weatherproof), and isolation/equalization valves if the install requires them. Keep tubing runs to a maximum of 75 feet (23 meters) to maintain response time.
Any tips for getting accurate readings on this ultra-low range?
Below 1 inch WC, several factors can swamp the signal: (1) Mount vertically with brass fittings pointing down - this prevents condensation from affecting the sensor (horizontal mounting can cause a small zero shift). (2) Keep tubing runs as short as possible - long runs introduce lag and can act as low-pass filters that distort transient readings. (3) Re-zero after the 15-minute warmup and after any temperature swing in the panel. (4) Avoid sharing the 24 VAC transformer with inductive loads (VFDs, relays, solenoids) - induced noise on the supply rail can show up as small offsets on the output signal at these low ranges.
How does the MLP2 compare to Belimo's 22ADP?
Both are active differential air pressure transmitters. MLP2 advantages: smaller form factor (designed for OEM/panel density), DIN rail mounting (rear or side), front-cover pushbutton zero, optional 3-point or 5-point NIST Certificate of Calibration, Made in USA, lower price point. Belimo 22ADP advantages: Modbus RTU communication (on -55x and -56x variants), 8 DIP-selectable sub-ranges on a single SKU (vs MLP2's fixed single range), volumetric flow output, optional LCD display, NEMA 4X/IP65 enclosure (MLP2 is indoor-only), UL 2043 plenum rating. Choose MLP2 for panel-density OEM applications with documented NIST certification; choose Belimo 22ADP for Modbus integration, multi-range stocking flexibility, or wash-down/outdoor installations.
What media can this sensor handle?
Dry air and inert non-conductive gases only: clean ambient air, nitrogen, argon, CO2 (dry). The MLP2 is NOT compatible with: liquids (any form), refrigerants, corrosive gases (chlorine, ammonia, hydrogen sulfide, acid fumes), conductive gases, condensing moisture, or any media containing particulates. Liquid contamination at the sensor diaphragm causes immediate damage and voids the warranty. For wet/wet differential pressure applications, see Senva PW30 or Belimo 22PDP/22WDP series in the Liquid Pressure subcategory.
- Series:
- MLP2
- Pressure Range:
- 0 to 0.5 inch WC
- Range Class:
- Low
- Direction:
- Unidirectional
- Output Signal:
- 4-20 mA
- Accuracy:
- +/-0.5% FSO
- Display:
- No
- NIST Certificate:
- None
- Mounting:
- DIN Rail / Panel
- Made in USA:
- Yes
