ACI’s New LP2 Differential Pressure Sensors

Recently, the ACI Low Pressure Transmitter Series has undergone a few product improvements that are designed to improve not only the overall functionality of the unit, but to also reduce its installation time. These changes were due to many of the newer building requirements specified by the LEED and Green Building codes. Among the improvements are the improved accuracy from +/- 1.1% FSO to +/- 0.85% FSO, field selectable input pressure and analog output ranges, Overpressure Indication, and optional LCD display. The increased accuracy of this sensor will allow for better measurement and control of the static pressure within the ductwork in your buildings but will still be one of the best values in the HVAC marketplace.

Today, energy consumption in buildings can account for up to 50% of the total operating expenses for most commercial type buildings with laboratories, consuming 5 to 10 times more energy per square foot than most office buildings. Since many of today’s fan system efficiencies have already been optimized, the proper design and monitoring of your building system’s static pressure is critical to being able to reduce the overall operating costs. Using the A/LP2 sensors in locations where you think the static pressure should be the lowest will allow you to properly set your duct static pressure set point to supply the proper amount of air to all of your VAV boxes, or pressure outlets, into your controlled environment. Keep in mind that the static pressure will not always be the lowest in those locations, so the use of multiple A/LP2 Series pressure sensors may be required to properly control the air flow in your air handling system. This will allow you to control your duct static pressure set points based on the lowest static pressure measured at key points throughout your entire air handling system. Also, by properly sizing and sealing your ducts, you will be able to increase the overall energy savings from your air handling system. Note that the more you can reduce your static pressure, the easier it is for your fan to move the proper amount of air through your ductwork.

The three most common applications in which the A/LP2 Series are to measure duct static pressure, velocity or total pressure, and building static pressure depending on your design requirements.

The A/LP2 Series low differential pressure transmitters can be used to monitor your system’s duct static pressure from 1 inch to a maximum of 20 inches of water column when used with a static pitot or pickup tube. The selection of these devices should be based upon the desired duct static pressure range to meet your overall system design. For best accuracy and sensor resolution, the full scale output of the transmitter should be field adjusted using the integral dip switches within each of the units. The increased accuracy of the A/LP2 will allow for better control of the duct static pressure especially at low static pressures of 1 to 2 inches of water column. As explained above, by lowering the static pressure inside of the duct, you will also be able to reduce the speed at which your fan may be operating thereby reducing some of the noise created from the air being moved through your supply grills. The LCD display should be used as an installation tool to easily monitor the duct static pressure without having to use an analog current or volt meter, as well as, to determine if the duct static pressure is more than 10% over the selected full scale pressure range by displaying “OVP” on the LCD Display.

When using the A/LP2 Series Pressure Transmitter to measure the velocity or total pressure, a pitot or pickup tube should be mounted so that the tip of the tube is facing into the air flow from the fan. The pitot tube should be mounted a minimum distance of 8 1/2 duct diameters downstream from the fan due to the turbulence created by the fan. The length of the pitot tube should be long enough to reach the center of the duct, since this is the location where the highest velocity pressure will occur. The tubing should then be connected to the high port on the A/LP2 Series and a number of readings should be taken before using them to mathematically calculate the overall velocity pressure. This method can be used in place of hot wire anemometers for low flow applications or more expensive variable area flow meters.

When using the A/LP2 Series transmitter to maintain a positive building static pressure, the low pressure port should be connected to the area that you are going to use as your reference pressure. The high pressure port should then be connected to a pickup port in the room or area that needs to be controlled. The pressure will then be controlled using the differential pressure between the two different environments. Application examples for controlling building static pressure include maintaining a slight positive pressure in one room over another to prevent the spread of bacteria or germs in a hospital isolation room, or to keep dust particles and other airborne contaminants from entering into a clean room. When using the A/LP2 Series pressure transmitter to control the overall building static pressure, a comparison between the indoor and outdoor pressure will be used to maintain a slightly positive control of the indoor pressure to prevent your doors from slamming shut in a school, office or any other type of commercial building. This positive pressure can also be used to minimize the amount of cold air from entering the building every time a door is opened and closed during the winter months in northern climates.

As previously explained, pressure sensor accuracy is a key element in maintaining system optimization. It can also be said that accuracy and sensor resolution are highly integrated, so it is not recommended to rescale a device that uses a pressure element designed for higher ranges and then only using a fraction of the element’s full scale. ACI recommends researching your application before deciding on an appropriate range for your pressure sensor.

Regardless of the application, ACI has engineered a product that has been cost optimized and is designed to provide a high quality, long-term solution for your low pressure sensing needs. In addition, every unit is manufactured to the strictest standards at ACI’s manufacturing facility in Middleton, Wisconsin. We hope you take advantage of our accuracy ratings