A balometer, also known as a capture hood, allows you to quickly and easily measure the air flow at grilles and diffusers. These are the supply and exhaust points of HVAC systems. Unlike situations where you could use an anemometer or manometer to measure air flow, such as within a duct traverse, a balometer is designed to fit over the grille or diffuser, seal around it, and read an average velocity across several measurement points in this cross-section of known dimensions. Previously, such measurements had to be made in a complicated fashion, making use of a deflecting vane anemometer with a diffuser probe, and having to apply the probe manufacture’s “Ak” factors. Eliminating these steps has resulted in the balometer becoming a time- and therefore cost-saving tool in HVAC troubleshooting today.
Balometers are available in analog/mechanical and digital types. Analog balometers measure via the manifold, through a user-adjustable and mechanical range selector and then into a deflecting vane anemometer giving the cubic feet per minute (CFM) flow readout. The advantage here is that neither batteries nor power supply are needed, while the device can be used on ceiling, sidewall or floor outlets. Digital balometers have an auto-range feature that hot wire/thermo-anemometer sensing method, or a pressure sensor, instead of a mechanical range selector. Pressure sensor models automatically ‘zero’ during normal operation. The meter is typically a manometer. Digital balometers as expected provide digital readouts, typically in numeric or gauge format as desired. In both analog and digital types, models are available capable of measuring low air flow.
Analog balometers are comprised of six subassemblies:
- Air collector
- Instrument base
- Range selector
- Low flow adapter.
The air collector is a cloth hood sized to a specific diffuser at one end. This end is edged with foam to create the air seal. At the other end, the hood is a standard size, a bit larger than one square foot. This standard size allows for air flow measurement.
The instrument base is what the standard size of the air collector is attached to at one end. The other end is larger, creating a flared shape. This acts either as a stable bottom, or as the input end for exhaust/return measurements. The base houses the anemometer, range selector and manifold.
The manifold allows for standardized measurements of air flow. Since the cross-sectional area is known, the velocity of air flowing through gridded holes patterned into the manifold is proportional to its volume.
The range selector is a control which switches the direction of air flow measurement, as well as the specific range of air flow to be measured.
The anemometer displays the air flow measurement figure.
The low flow adapter is a screen which reduces the area through which air flows. Since this increases the air’s velocity, it makes measuring low air flow easier.
Digital balometers are comprised of similar subassemblies, except the anemometer is replaced by a manometer which also incorporates the range selector.
Balometers, especially after the user’s first time setting one up, are easy to use and have few safety considerations. However, if you are measuring air flow at ceiling diffusers—especially if on a ladder—ensure that you can safely lift the balometer and hold it to take the reading. Do not attempt to measure the flow of any gas except air: dangerous, corrosive or explosive gases should not be measured with a balometer.
In terms of maintenance, it is critical that the nylon hoods do not become torn. If they do, the balometer will not be able to provide accurate measurements. In the case of analog balometers, it is possible that a static electric charge will impede the anemometer from holding a zero setting or make it give incorrect readings. If so, take a cloth slightly moistened with anti-static solution and wipe down the anemometer’s case and widow. Check the manifold often to make sure sensing holes remain free of clogging particles. The manifold may be cleaned without immersing it in water. If any cracks appear in the manifold, it must be replaced and cannot be repaired. In this situation, do not use the balometer because it will provide incorrect measurements.
You are welcome to publish this article for free of charge on your website, newsletter or e-zine provided:
- You do not alter the article in any way,
- You include the entire article, including the About the author section,
- All hyperlinks remain intact,
- You agree to indemnify the author,
- You provide a courtesy copy of your publication to the author.
Jason Kanigan is a technical writer for Global Test Supply, a distributor of test and measurement equipment.