Introducing NJK Precision Airflow Measuring Stations

This new and unique airflow measuring station can accurately measure airflows at very low flow rates and can operate in physically restrictive applications. Inspired by proven aviation and automotive technologies, NJK Precision has developed a new airflow measuring station that is entirely different from the methods that have been used in the HVAC industry for a generation.

NJK Precision has developed an airflow measuring station technology that includes 21 patented claims. The result is an airflow measuring station that is not subject to the same inaccuracy, placement restrictions, low flow limitations, pressure drop issues, and size restrictions that are inherent in other airflow measurement methods. This airflow measuring station provides a unique, robust, and patented method of measuring airflow, even in the most challenging HVAC applications.

The NJK Precision Airflow Measuring Station has an airfoil profile that installs in the perimeter of a duct or an outdoor air opening. The Flow Frame has a patented Dual-Chambered Airfoil Design. There is a precision-formed linear slot opening on both the front and backside of the airfoil chambers running the length of the Flow Frame Like an airplane wing, the momentum of air flowing over this anodized extruded aluminum frame creates a pressure differential. On an airplane wing, this is called “lift.” And just like it happens on a wing, the heavier or denser the air, the greater amount of lift and the greater mass of air movement. This is the principal at work in the NJK Precision Airflow Measuring Station.

This dual-chamber airfoil Flow Frame has three functions. First off, the chamber induces a differential pressure that is proportional to the mass of air flowing through it. Secondly, it averages flow samples from the perimeter of the duct. Lastly, the Flow Frame dynamically normalizes the turbulent flows inherent in a duct. These functions improve measurement accuracy. The chambers are connected through the Flow Measuring Module. The lift or momentary pressure change created by air passing through the Flow Frame induces air to flow through the frame and the Flow Measuring Module.

Manufacturers typically recommend installing an air measuring station as far away as possible from an elbow, branch, transition, or fan. The more equivalent duct diameters away from these air distribution components, the better the measurement. This is because the air velocity profile becomes more uniform the further it travels down a straight section of the duct. NJK Precision achieves this uniform velocity profile when air enters the Flow Measuring Module through a half-inch diameter communication hole.

NJK Appplication
NJK Application

The air then flows 7.5 equivalent duct diameters before reaching the advanced thermal dispersion technology thin film resistor, which measures the mass flow of the air. The Flow Measuring Module simulates a desirable duct run. This is the same sensor technology used to measure the mass flow rate of air entering a fuel-injected combustion engine. Each Airflow Measuring Station from NJK Precision is calibrated and scaled for accuracy from the minimum flow through the maximum flow on a primary NIST-certified airflow test stand. The result is a Flow Measuring Module reading within a 2.0% uncertainty and at a 95% confidence interval. This means high accuracy and repeatability of airflow readings at all temperatures and humidity conditions.

More Accurate Measurements

The taming of the turbulent airflow results in more accurate measurements, which means the NJK Precision Airflow Measuring Station can be placed in more physically restrictive situations. While some products already accurately measure airflow at less than the ASHRAE Standard 111 recommendation for equivalent duct diameters away from an elbow, branch, transition, or fan, the NJK Precision Airflow Measuring Station sets a new, smaller equivalent duct diameter standard for station placement.

For example, instead of requiring that an air measuring station be located three equivalent duct diameters downstream from a 90° elbow, the NJK Precision Airflow Measuring Station can be placed just one equivalent duct diameter downstream of the elbow. (For details, consult the NJK Precision Application Guide).

Increased Installation Options

Placing a NJK Precision Airflow Measuring Station in tight locations increases the number of applications to apply an airflow measuring station. The station is suitable for new construction and existing buildings. The NJK Precision Airflow Measuring Station’s design also allows for some of the lowest flow measurements on the market. The station can measure airflow amounts with velocities as low as 50 feet per minute (FPM). This provides greater precision and can be used for applications like ventilation, building pressurization control, and critical applications by a building automation system.

The Flow Frame of the NJK Precision Airflow Measuring Station comes in two sizes to minimize the cross-sectional area inside the duct. For rectangular air stations where both the height and width are less than 18”, the smaller Flow Frame profile (only 2.75” wide) is standard. For rectangular air stations where either the height or width is 18’ or greater, the larger Flow Frame profile (only 5.5” wide) is standard. The airfoil shape and proportionally smaller cross-section have a very low-pressure drop and create very little fan pressure loss. There are no sampling tubes traversing the cross-section of a duct or outside air opening.

Airflow Measuring Station Components and Assembly

The NJK Precision Airflow Measuring Stations are built to order. Measuring stations are shipped unassembled to reduce shipping costs and to prevent damage. Field assembly is fast and simple. The Flow Measuring Module is already factory mounted onto a Flow Frame section. The sections are connected together with slip fit, field adjustable corner pieces that are tightened by a screw. Because field-provided dimensions are not always exact, each frame is built 0.5” smaller than the requested duct dimension.

Signal Processing Unit w-Display
Signal Processing Unit with Display
Flow Frame and Slip Fit Corner Pieces
Flow Frame and Slip Fit Corner Pieces

In the event of an incorrect duct dimension, the small profile frame is adjustable +0.5”, and the larger profile frame is adjustable +1.25”. This adjustability allows secure and proper fit inside the duct or outdoor air opening. Round air sensing stations have mounting bracket adjustments so that a single station can fit into a range of duct diameters. In applications where you are placing a measuring station at the opening of an outdoor air hood or measuring flow from an open plenum, NJK Precision offers perimeter edge extension pieces.

These 1” wide pieces go on the outside edge of the air opening. This simple addition makes accurate airflow measures in a rain hood, plenum, or other unducted installations a reality. The Flow Measuring Module can be mounted inside or outside the duct. Measuring stations come with a Signal Processing Unit that includes a display. The display readout from the Signal Processing Unit is in cubic feet per minute at standard air conditions (SCFM).

Maintenance is also simple. The Flow Frame needs to be clear of debris that can affect sensor operation, which can occur in an unfiltered outdoor air application. While particulates like Cottonwood will have little effect, items like leaves, straw, and litter can plug the Flow Frame and affect operation.

Rain hood and unfiltered outside air installations may require periodic cleaning. Every year the Flow Measuring Module should be removed, and the sampling port must be cleaned with a small amount of air, water, or electric contact cleaner. The NJK Precision Flow Measuring Module can be cleaned by removing the four mounting screws that attach the module to the Sensor Flow Frame and flushing the sampling port with either a small amount of air or electrical contact cleaner. Air pressures in excess of 110 PSIG may damage the Sensing Probe. It is recommended that the rubber sealing gaskets be replaced whenever the Flow Measure Module is removed and replaced. Recommended cleaning interval is two years for a standard application.

Multiple Sensor Applications

For multiple sensor installations, NJK Precision offers an economical option not available from competitors. Instead of requiring a dedicated display unit for each airflow measuring station, NJK Precision offers a Multi Sensor Hub and a Multiple Sensor Signal Processing Unit (SPU-M). This combination saves on hardware and installation costs. Up to eight stations can be connected to a multiple sensor SPU (SPU-M) through the NJK Precision Multi Sensor Hub. From one location, you can simultaneously monitor four airflow values at a time or view all eight air station measurements individually.

Multiple Sensor Signal Processing Unit (SPU-M)

The Signal Processing Unit  is powered by 24 volts either A.C. or D.C. The Multiple Sensor Signal Processing Unit provides four 1-10 VDC output values to the building automation system. Airflows can be summed together for a totalization of airflows.


Multiple (SPU-M) Requirements

  • Multiple sensors (up to eight) can be monitored from a single SPU-M
  • Three sensors per channel or group can be added together on each channel
  • Overall cable length is limited to 300’ per SPU-M and Multi Hub
  • Sensors added to each channel will be viewed at BAS as one flow signal only
  • Each sensor in the channel or group can be viewed and modified at the SPU-M
  • The Multi Hub and Flow Measuring Modeules are all powered from the SPU-M


Multiple (SPU-M) Advantages

  • Multiple (like) sensors can be added to create one flow signal to the Building automation system
  • Unlike sensors (outside, return, supply, and exhaust air) can be viewed from a single SPU-M
  • Equipment cost savings
  • Installation cost savings