Some Common Errors and Causes

5.3.3 Some Common Errors and Causes

Operator-based errors include:

• Not ventilating the psychrometer long enough to reach equilibrium

• Not getting the wick wet enough • Letting the wick dry out too much • Holding the psychrometer too close to the

body • Taking too long to read the thermometers

Figure 5.12 Air Flow Psychrometer

• Touching the bulb ends with hands while

Psychrometric tables ( Figure 5.13 ) are

reading

used after dry-bulb and wet-bulb tempera-

• Not facing into the breeze

tures are measured. Psychrometric tables from the US Department of Commerce

All of these lead to a wet bulb readings that Weather Bureau are used to determine the are too warm; i.e., preventing inspectors relative humidity and dew-point temperature from doing the job properly.

of the air.

Equipment-based errors are easier to iden- The difference between dry-bulb tempera- tify with this instrument, i.e., it is obvious if ture and wet-bulb RH is calculated; it is the thermometers or the units are broken. called the wet-bulb depression. Both the Ensure that the sock does not get too dirty, dry-bulb temperature and wet-bulb depres- as it frequently does in blasting and painting sion are found on the vertical and horizontal operations. If it becomes too dirty, replace it, axes, respectively, of psychrometric tables. or inaccurate readings will result. The point where the two temperatures inter-

The powered air flow psychrometer ( Figure sect indicates the relative humidity or dew-

5.12 ) operates similarly to the sling psy- point temperature (depending on the particu- chrometer, but air is moved using a fan,

lar table).

rather than slinging the instrument. After Please note that both relative humidity and about 2 minutes, the temperature stabilizes. dew-point temperature may vary with baro- Monitor only the wet-bulb temperature; metric pressure. The differences are gener- when it remains stable, record both wet- and ally small, and although many tables dry-bulb temperatures. Be very careful when (calculated at different pressures) are pro- temperatures are below 32°F (0°C). Sling or vided in a typical book of tables, it is reason- fan-operated psychrometers are not reliable ably accurate to use tables based on a at those temperatures since the water barometric pressure equivalent to 30 inches freezes. When the temperature is this low, of mercury. For absolute accuracy, deter- determine humidity with a direct-readout mine the actual barometric pressure and use humidity instrument.

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the appropriate table to determine RH and dew point.

Figure 5.13 Psychrometric Tables Figure 5.15 Psychrometric Calculator

A psychrometric chart ( Figure 5.14 ) is a

5.4 Wind Speed

graph of the physical properties of moist air High wind speed can be a safety hazard. at a constant pressure (often equated to an When work is performed at a particular elevation relative to sea level). The chart height, winds of 40 mph (64 kph) or more graphically shows how various properties are considered dangerous. When work is relate to each other, and is thus a graphical performed on offshore platforms, safety equation. The thermo/physical properties boats and rescue craft cannot operate reli- found on most psychrometric charts are the ably with high wind speeds, so work close to same as the psychrometric tables (i.e., wet the sea or below deck level may be post- bulb, dry bulb, barometric pressure, etc.). poned until the wind speed drops.

Many countries provide weather bureau information or meteorological services. Applicators or inspectors can obtain both daily and historical data concerning weather conditions. Weather forecasts for the next day and for several days in advance may be obtained from the local weather bureau.

Weather bureau records may also be useful to check conditions when records of weather

conditions were not kept on site or when the Psychrometric calculators ( Figure 5.15 ) may

Figure 5.14 Psychrometric Chart

accuracy of recorded measurements is sus- also be used to determine relative humidity

pect.

and dew point temperature. Keep the manu- facturers’ instruction guide on the job and know how to use it.

Record results of measurements in a daily report using a standard format shown in Table 1 .

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Figure 5.16 Wind Speed Monitor

A wind speed monitor ( Figure 5.16 ) is an effective instrument to determine if condi- tions are appropriate for coating application. When using a wind speed monitor there are several items beneficial to know.

Instructions for the proper use of the instru- ment are in the manufacturers’ instruction guide, (the essential knowledge base for instruments). A critical part of an inspector’s responsibility is to ensure that the wind speed monitor instruction manual is at hand on the job. Always stand facing the wind with the digital dial facing the user. Hold the

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Table 1: Inspection Details — Ambient Conditions

Time Wet-bulb temperature Dry-bulb temperature RH% Dew point Steel temperature OK to work? Yes/No

instrument at arms length so the air flows

• The user is not holding the instrument

through without obstruction.

away from the body

Remember that manufacturers’ instruments • The wind is too high for the instrument

(will wear out the roller bearing)

should meet all coating thickness standards for quality and use in accordance with ANSI/NCSL Z540-6 (National Calibration Standards). Wind speed monitors come from the manufacturer already calibrated and should never need recalibration.

Operating parameters for the wind speed monitors include:

• The accuracy and precision of the instru- ments vary, but most manufacturers indi- cate that the degree of accuracy is ± 3% of the indicated reading.

• The repeatability of the instrument will vary depending on the individual unit.

Routinely question readings when the instru- ment reading is not the actual wind speed. Always check local weather forecasts; this provides benchmark ranges for the day.

Common errors and causes of inaccurate readings from the wind speed monitor include:

• The user is not facing into the wind

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