Measuring instruments

To assess comfort conditions in an air conditioned room the following instruments may be used: mercury-in-glass thermometer, sling or aspirated psychrometer, Kata thermometer, stop watch, globe thermometer, and hot wire anemometer.

Dry-bulb temperature is measured by a mercury-in-glass thermometer, properly shielded from radiation, wiped dry beforehand and given time to settle down to a steady-state reading. The accuracy from a recent calibration should be known and the scale should be apt for the range of values to the measured.

The sling psychrometer comprises dry — and wet-bulb thermometers and has a reservoir which should be filled with clean, fresh water at a temperature about equal to the wet-bulb reading anticipated. The wick on the wet-bulb should be clean and free of scale or other solid deposits. Avoid handling the wick as this can cause contamination. Both thermometer stems and the bulb of the dry-bulb instrument must be wiped free of moisture. When the wick is completely wetted by capillary action from the reservoir the sling should be whirled rapidly for about 10 seconds to eliminate radiation errors (see section 2.17). As soon as whirling is stopped the wet-bulb reading is taken and then the dry-bulb immediately afterwards. The wet-bulb must be read first because the reduction of the evaporation rate and the onset of radiation error when whirling ceases causes the indicated temperature to rise rapidly. A similar rise in the dry-bulb occurs but this is less rapid. This procedure is repeated several times, ensuring each time that the wick is properly wetted, until three sets of successive readings show close agreement. Failure to ensure the wick is properly wetted, whirling the sling too slowly, or being late in reading the wet-bulb, will give high and unrealistic wet-bulb temperatures that imply the humidity is much greater than is really the case. Accurate measurements of wet-bulb temperature are quite difficult to obtain. An aspirated version uses a chromium-plated cylinder, having a clockwork or battery-driven fan at the top which induces air at an adequate velocity over the thermometer and gives stable readings free from radiation errors.

The Kata thermometer is not in very common use today because of the lengthy, elaborate procedure involved (requiring simultaneous readings of the dry-bulb and a significant time taken for the fluid within the Kata to fall between two markings on the stem as it cools). A thermos bottle of hot water is also needed to raise the temperature of the Kata above that corresponding to the upper markings on the stem, in the first place. Instead of using the Kata thermometer to measure very small air velocities, the hot-wire anemometer is adopted today. This instrument is basically a Wheatstone bridge, one resistance element of which is the anemometer velocity sensor. When an electric current flows through the sensor element its temperature is raised to a value at which its rate of heat loss to the environment is stable, being a function of ambient air velocity and dry-bulb. Since electrical resistance depends on the temperature of a conductor an indication can be obtained of air velocity. The instrument is easy to use down to a velocity of 0.05 m s_1 although there may be

Problems with local transient air movement. Temperature as well as air velocity may be measured simultaneously and both can be recorded.

The globe thermometer is an ordinary mercury-in-glass thermometer inserted in a blackened copper sphere so as to have its bulb at the centre. It is affected by dry-bulb temperature, air velocity and mean radiant temperature but in still air reads the mean radiant temperature exactly. A simultaneous dry-bulb reading must be taken and about 20 minutes allowed for the globe to settle down. Mean radiant temperature, Tm in kelvin, is then determined from the following equation

7* x 10~9 = Tg4 x lO“9 + 1.442(Tg — Ta) Vv (4.14)

Where Tg is the globe reading in kelvin and T. A the ambient dry-bulb also in kelvin.

Posted in Air Conditioning Engineering

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