Airflow-measurement-device selection is critical to the performance of today's state-of-the-art HVAC control systems, as accuracy and repeatability can vary dramatically between instruments. Many of the requirements and limitations of one measurement technology, however, often are thought to apply to all.

This article discusses functional and performance differences between the two most popular technologies for permanently duct-mounted measurement systems in commercial buildings: velocity-pressure devices (Pitot arrays, probes, Piezo rings, and other delta-P methods) and thermal-dispersion (TD) devices (microprocessor-based instruments using some form of thermistor sensor). It excludes vortex shedding and resistance-temperature-device-type instruments, which generally are applied in contaminated and/or high-temperature industrial environments.

THERMAL DISPERSION

Introduced in 1985, advanced TD airflow-measurement technology is used in a wide range of office, laboratory, health-care, and educational facilities. Some manufacturers produce TD instruments with a combination of electronic components, while others provide independent sensing elements. One manufacturer produces TD instruments factory-calibrated to National Institute of Standards and Technology- (NIST-) traceable velocity reference standards. When designed properly and applied with sensing elements of sufficient density, a TD instrument can minimize, if not overcome, placement limitations and measurement uncertainties inherent in the use of velocity-pressure instruments.

Pre-1993 TD arrays were influenced significantly by duct-turbulence and placement conditions. Because the technology determines airflow by relating the rate of heat transfer from a warm body to an air stream, airflow-measurement stations placed close to duct disturbances often registered “false high” readings. More heat was removed from sensors in duct locations with excessive eddies and turbulence than sensors exposed to steady-state factory wind-tunnel calibration and, thus, higher readings.

During the early 1990s, enhancements to one design placed the heated sensor in the turbulent wake created by the sharp leading edge of the sensor-probe assembly. This “preconditioning” effect essentially made airflow across the sensor more “turbulent” than the worst-case duct-disturbance effect, allowing the condition to be created consistently. During the calibration process, a known flow rate was associated with the actual conditions encountered by the sensor. As a result, TD sensors were influenced far less by duct disturbances. This has been proved in full-scale laboratory testing.¹ With advanced TD instruments, 0.75 to 1.5 simple equivalent duct diameter [(width + height) ÷ 2] often is sufficient for accurate measurement. Contrast this with the three-duct-diameter minimum industry standard.

During the spring of 2000, more than 350 advanced TD devices were installed at NIST's Advanced Measurement Laboratory in Gaithersburg, Md. The devices have performed without the need for field calibration and are said to be functioning well, without a single reported sensor failure.

TD technology should not be confused or compared with thermal anemometers, hot-wire devices, or other forms of analog electronic velocity measurement, as it often — and incorrectly — is.

Hand-held thermal (single-point) instruments generally use unshielded thermistors, which makes them highly sensitive to the direction and vectors of airflow. Typically, they are analog devices and, thus, have a tendency to drift from zero. Also, they usually require regular recalibration and “zeroing” and tend to perform satisfactorily only within a narrow temperature band and at favorable locations.

At least one permanently mounted TD product has a published operating range of -20°F to +160°F and a design limiting the impact of rotational misalignment. Tests confirm the device's immunity to improper installation (rotation of airflow angle) (Figure 1). The test was conducted by monitoring the output signal while the device was rotated perpendicular to airflow.

There are some operating conditions to which TD instruments are not immune. TD performance is dependent upon not only the transfer of energy (heat) from the sensing element to the measured air stream, but the precise determination of air-stream temperature at the point of measurement. Conditions that could affect thermal transfer (insulating materials, liquid water) also could impact the ability of an instrument to function as designed. The TD manufacturer's choice of sensing elements ultimately impacts:

• Instrument cost.

• Instrument mean time between failures.

• Instrument sensitivity to environmental changes.

• Instrument stability over time and changing temperatures.

• The instrument's ability to perform without mechanical failure amid continuous cycling between heating and cooling.

• The validity of the manufacturer's factory calibration process.

• The lack of/need for interchangeability of instrument components or subsystems.

• Limitations on product placement.

Each of these factors must be considered prior to instrument selection.

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