Electronic Digital Hygrometers

5.3.1 Electronic Digital Hygrometers

• wet bulb temperature, press the “wet bulb

These multi-purpose instruments determine

button” a second time

relative humidity, air temperature, and dew-

• ambient temperature, press the “wet bulb”

point temperature and are convenient and

button a third time

The display indicates when dew point and wet bulb temperatures are selected.

5-8 Environmental Testing

Press the “Hold” button once to freeze the tor-based inaccuracy examples are: the displayed readings and stop taking measure-

reading was taken in direct sunlight, the ments. To continue taking readings, press

instrument was left in place too long, or the “Hold” button again. Some instruments

removed before it stabilized. have minimum, maximum, storage, save,

Equipment-based erroneous readings are print and recall abilities for easier record-

very likely due to calibration or equipment keeping.

malfunction and, therefore, must be These instruments, from any manufacturer,

replaced.

need to meet all coating thickness standards for quality and use in accordance with

5.3.2 Sling Psychrometer

ANSI/NCSL Z540-6 (National Calibration The sling psychrometer (sometimes called Standard).

a whirling hygrometer) is the type of psy- chrometer ( Figure 5.10 ) most often used in

Hygrometers come from the manufacturer coating inspection, especially in hazardous already calibrated; however, this calibra- environments. It measures the ambient air tion requires certification by an indepen- temperature (dry-bulb and wet-bulb temper- dent lab as well as verification in the field. atures) as close to the worksite as is practica-

The accuracy and precision of electronic ble. These temperatures are used to calculate digital hygrometers inspectors use must be

the dew point and relative humidity. near the top of its scale (i.e., close to 100%

It is the inspector’s responsibility to have all RH). This is the critical point at which con- equipment information and manufacturers’ tractors and inspectors make decisions on instructions prior to arrival on the jobsite. whether to work or not. Most manufactur-

ers’ guidelines state the degree of accuracy The psychrometer consists of two identical in both Celsius and Fahrenheit, as well as

thermometers, using either mercury or red the range and resolution for each reading

spirit (alcohol). One thermometer bulb is (i.e., temperature, relative humidity, dew

covered with a sock saturated in distilled point, and wet bulb).

water. The covered thermometer is called the wet bulb, the other the dry bulb.

The repeatability of results depends on indi- vidual instrument manufacturers, so consult the correct manufacturers’ technical data sheet.

Routinely question readings when highs and lows are outside known parameters. Check local weather for the work site each morning to get a good idea of expected ambient con- ditions. Use this as the benchmark for the day.

Common hygrometer errors are either opera- tor-based and/or equipment-based. Opera-

Environmental Testing 5-9

the psychrometer ventilated by whirling (sling psychrometer). The methods described are applicable within an ambient temperature range of 32–122°F (0–50ºC), wet bulb temperatures not lower than 33.8°F (1ºC) and restricted to ambient pressures not differing from standard atmospheric pres- sure by more than 3%.

How do you “field-calibrate” the sling psy- chrometer? It is very simple: (1) remove the two thermometers, (2) place them in a cup of

Figure 5.10 Sling Psychrometer

room temperature water, and (3) remove the thermometers from the cup in 3 to 4 min-

The dry-bulb thermometer measures the air utes. The two thermometers should have temperature as it is; the wet-bulb thermome- identical readings. If the readings are not ter measures a lower temperature, because identical, replace the thermometers. A sling of latent heat loss from water evaporating psychrometer must have two thermometers from the wet sock. The faster the water that read the same in a like temperature envi- evaporates, the more cooling occurs, result-

ronment.

ing in lower humidity and dew point temper- ature.

Sling psychrometer operating parameters follow in this section. Crucial factors are:

To use the sling psychrometer:

• The accuracy and precision of the sling • Saturate the sock with clean water. psychrometer is ± 2% when used properly.

• Whirl the instrument rapidly for about 40 The repeatability is unlimited with proper care. seconds.

• Read the wet-bulb temperature.

Repeat the process (spin and read without additional wetting) until the temperature sta- bilizes. When the wet-bulb temperature remains constant, record the result. Read and record the dry-bulb temperature after the sta- bilized wet-bulb reading is recorded.

The standard for sling psychrometers is ASTM E337-02 (2007), Method B Standard Test Method for Measuring Humidity with a Psychrometer (the measurement of Wet- and

Figure 5.11 Sling Psychrometer Split Bulb

Dry-Bulb temperatures). This test method Routinely question readings when the highs details how to determine humidity of atmo-

and lows are outside known parameters. In spheric air by means of wet and dry bulb

other words, know the local weather forecast temperature readings and incorporates use of

5-10 Environmental Testing

before arriving on site. This provides a Coatings are not usually applied benchmark temperature range for the day.

at low temperatures.