Outside Air Measuring with Air Monitor’s VOLU-flo/OAM
The measurement of Outside Air presents a variety of challenges that cannot be overcome utilizing the standard Pitot and thermal technologies.
Among these challenges are:
- The total absence of any upstream/downstream straight duct run to obtain accurate and/or repeatable measurement.
- The potential for outside air temperature variances of 150+ ºF in certain climatic zones and installed locations ranging from sea level to 5,000+ feet elevation make a temperature and altitude air density compensating system necessary.
- Airborne particulate and condensing moisture transported by outside air will impair the functionality and accuracy of both thermal and standard Pitot based technologies.
- Varying directional wind loads and gusts cause fluctuating airflow measurements.
- Airflow velocities often down to 150 FPM are below the recommended operating range of standard Pitot technologies.
The VOLU-flo/OAM has been specifically designed to measure the airflow rate within ±5% of actual airflow in all of these challenging conditions by measuring the pressure drop across a fixed inlet resistance device, such as a louver, layer of expanded metal, etc.
The VOLU-flo/OAM consists of an Outside Reference Sensor and an Inlet Airflow Sensor, both specially designed to be unaffected by dirt or moisture, plus a temperature sensor and a microprocessor-based Monitor/Controller which converts the measured pressure drop and temperature to actual airflow utilizing proprietary algorithms, density compensation, and signal conditioning.
Components and Features:
Standard and Expanded Ranges. With its 4:1 range of measurement, the Standard VOLU-flo/OAM is ideally suited for minimum outside air applications, while the Expanded VOLU-flo/OAM’s 16:1 range of measurement will cover the full range of outside air, from the minimum set-point to design maximum.
Microprocessor Based Functionality. All functions of monitor configuration, output and display scaling, calibration, and controller tuning are performed by the VOLU-flo/OAM’s onboard microprocessor. All input to the microprocessor is via four pushbuttons and menu selections.
Air Density Correction. The VOLU-flo/OAM corrects measured inlet airflow for ambient temperature variances by means of an integral 100 ohm platinum RTD temperature sensor and corrects for atmospheric pressure by entering site altitude into the microprocessor during initial system start-up.
Continuous Display of Process. A standard 2×20 liquid crystal display (LCD) is used during the configuration and calibration of the VOLU-flo/OAM, and provides a means to display two measured processes (volume, velocity, temperature) during normal operation.
BAS Interface. While the VOLU-flo/OAM is capable of operating as a stand-alone monitor/controller, a wide variety of inputs and outputs were designed into the Monitor/Controller Module to facilitate coordinated operation with a facility building automation system (BAS). Interfaces with the BAS include:
- Digital inputs for fan system start status, switching to economizer setpoint, or to use the BAS provided adjustable operating setpoint.
- Digital outputs for high and low alarms, triggered when the measured airflow falls outside of a user selectable deadband around the control setpoint.
- Dual analog output; a 4-20mADC output corresponding to measured inlet airflow, and a damper controller output field configurable for 0-5VDC, 0-10VDC, or 4-20mADC via jumpers.
- An analog input, field selectable for 0-5VDC, 0-10VDC, or 420mADC via jumper, that is used as the controller setpoint.
Enclosures. The VOLU-flo/OAM is offered in four different enclosure packages to fit any application and installation:
- NEMA 1 rated, painted carbon steel enclosure designed to be located indoors or inside the air handler.
- NEMA 4 rated, painted carbon steel enclosure designed for outdoor locations where the ambient temperature is consistently at 32°F and above.
- NEMA 4 rated and insulated enclosure which includes a thermostatically controlled enclosure heater. For outdoor locations where the ambient temperatures may go as low as –40°F.
- OEM mounting arrangement that permits the Monitor/Controller Module to be mounted into an existing OEM enclosure.
The VOLU-flo/OAM Station has been designed for ease of installation and commissioning by Factory mounting its sensors
onto a layer of expanded metal of known fixed resistance welded into a galvanized sheet metal casing. In selected applications, the known fixed resistance of the VOLU-flo/OAM Station allows the overall System to be pre-calibrated at the Factory for plugn-play operation.