Thermo-anemometers, also known as “hot wires,” operate on the principle of heat transfer. A sensor is heated above room temperature by passing current through an electrical resistance and the energy is converted to heat. Through convective heat transfer, the heat is transferred to air passing over the heated element. The electronics provide more power to the heated wire to maintain the initial temperature at zero flow, and this increased power curve signal is converted to a velocity reading on the display.

Thermal anemometers accurately measure a wide range of velocities, anywhere from 10 to 10,000 fpm (.05 to 51 m/s). They have small probe diameters which allow them to be placed in tight areas, such as measuring air velocity in between circuit boards in a PC or in small ducts. Thermo-anemometers have air temperature limitations. Most units have a maximum temperature of approximately 180 to 200°F (82 to 93°C), which generally limits their use to ambient conditions.

Thermo-anemometers are commonly used to measure velocity in paint spray booths, laminar flow hoods, air flow system balancing and duct traverses, pharmaceutical manufacturing areas, clean-room biological safety cabinets and fume hood testing, room air currents, and draft measurements.
 

Basic Thermo-Anemometers  These small, compact models are useful for general air velocity measurements. Velocity ranges are typically less than 4000 fpm (20 m/s) and temperature measurement capability along with a telescopic probe may be available on some models. Most models offer the following:  Low cost instruments with good accuracy.  Available with a digital LCD or analog meter readout.  Available as intrinsically safe models.  Available in English or metric models.	AVM410
Thermo-Anemometers with Mathematical Calculations  These models are commonly used to perform duct traverses and face velocity measurements of fume hoods and HEPA filters due to their ability to take multiple measurements and to average the readings. Optional portable printers can be used with these instruments to print test data in real time. They usually provide the following:  Wide velocity and temperature range.  Velocity to volume flow conversion based on duct dimensions.  Telescopic or telescopic articulated probes.  User-adjustable dampening or time response settings.  English or metric readout capability.	AVM430
Comprehensive Thermo-Anemometers  These models provide the same accurate velocity and temperature measurements as the other models. Additional functions include humidity measurement in %RH and conversions to wet bulb and dewpoint temperatures. A data logging function allows the user to program the number of samples to be taken and the time interval between samples. This can be a very useful feature when tracking trends over a period of time. These meters offer the following:  Segregate data groups in separate test IDs.  Recall stored data on instrument display.   Download stored information to a PC or portable printer.  Offer telescopic or telescopic articulated probes.  Perform density correction.  Have backlit LCD display.	AVM440