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Although TD devices are subjected to condensation from coils or carryover from exterior louvers, the impact of liquid-water immersion on TD performance is temporary, with normal performance resuming as the sensor surface returns to ambient relative-humidity levels.

In rare instances, insulating materials bind to tiny thermistor sensing elements, compromising thermal-transfer characteristics. In such cases, occasional light cleaning of sensors is necessary. Typically, however, common dust and airborne particulate found in outside air and conditioned-air ventilating systems is not capable of accumulating and producing an insulating effect great enough to materially impact the thermal-transfer characteristics of a sensor.

PITOT-STATIC TUBES and VELOCITY-PRESSURE ARRAYS

The laboratory Pitot-static tube is a "primary" instrument, which means it has physical properties that have been scientifically proven to provide a predictable level of measurement performance, albeit with known application limitations. The Pitot array is not a primary instrument. The Pitot-static tube and the Pitot array share only the use of the velocity-pressure relationship in the determination of velocity and, thereafter, the calculation of air volume.

In the hands of a skilled testing-and-balancing technician at a suitable location, a Pitot-static-tube traverse is a highly trusted method of obtaining field-test information. It is important to the initial testing and balancing of many systems and the basis of many laboratory test standards (e.g., ANSI/AMCA Standard 210-07/ANSI/ASHRAE 51-07, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating, and ASHRAE Standard 41.2-1987, Standard Methods for Laboratory Airflow Measurement), but susceptible to error because of normal inconsistencies in human application.

There are at least six manufacturers of velocity-pressure-based duct-averaging products and at least 12 manufacturers of variable-air-volume- (VAV-) box pickups, fan-inlet Piezo rings, flow-balancing hoods, and calibration and research devices based on the same principles in North America.

As applied to commercial ventilation systems, velocity-pressure-based measurement technology has been around since the 1960s.

A pneumatic output no longer is a preferred interface for building controls, as differential pressure must be converted to an electronic signal and transmitted to host controls or an intermediate device programmed to make the analog output linear to flow. Most Pitot-array manufacturers offer pneumatic-to-electronic (P/E) transducers and other electronic control devices to enable functionality with direct-digital-control systems.

Velocity-pressure arrays, Pitot arrays, and self-averaging arrays are names for the same product species. They are bifurcated devices that separately equalize total and static pressure within a length of partitioned tubing through small sampling holes positioned at a cross-sectional plane in ductwork. A single differential pressure is measured between two compartments or equalizing manifolds and assumed to be the "average." This analog result then is output electronically to a controller or display. It is made linear by an integrated transmitter, a separate intermediate device ("square-root extractor"), or other method of calculation.

Testing-adjusting-and-balancing (TAB) contractors measure cross-sectional average velocity in a duct by recording individual readings at specific locations on a plane perpendicular to airflow direction. Each reading is determined using the following equation:

V = 4,005 x (¦¤P)^0.5

where:

¦¤P is expressed in inches of water

This generalized relationship uses a constant (4,005) that assumes standard conditions of altitude and temperature. The equation does not account for changes in air density attributed to variations in air temperature and barometric pressure (altitude). The readings simply are added and divided by the number of measurements; a final average airflow rate then is determined.

Taking a high number of readings throughout a duct can help compensate for changes in velocity profile. However, it increases the time required to determine all of the data needed for averaging (ISO 3966:2008, Measurement of Fluid Flow in Closed Conduits -- Velocity Area Method Using Pitot Static Tubes).

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