Psychrometry and Instruments

Quick Answer: Ambient condition measurement is the coatings inspector's first pre-application duty. A sling psychrometer (whirling wet/dry bulb hygrometer) gives two readings — dry-bulb and wet-bulb temperatures — from which relative humidity and dew point are derived using a psychrometric chart or calculator. Surface temperature is measured separately with a magnetic dial thermometer, electronic thermistor probe, or infrared (IR) thermometer. The inspector compares surface temperature to dew point before any coating is applied.

Sling Psychrometer Operation

The sling psychrometer — also called a whirling hygrometer — is the field instrument specified by AMPP CIP Level 1 for ambient moisture measurement. It consists of two thermometers mounted side-by-side on a frame with a swivel handle. One thermometer has a clean muslin wick (sock) fitted over its bulb and is wetted with distilled or deionized water; this is the wet-bulb thermometer. The other is bare and reads the dry-bulb temperature. The inspector whirls the assembly through the air for approximately 90 seconds (or until the wet-bulb reading stabilizes at its lowest point), then reads both thermometers immediately.

Wet-Bulb and Dry-Bulb Temperatures

• Dry-bulb temperature (DBT): The actual ambient air temperature from the bare thermometer — the same reading any ordinary thermometer gives.
• Wet-bulb temperature (WBT): The lowest temperature achievable by evaporative cooling of water into the air. The wet wick cools as water evaporates; the drier the air, the more evaporation occurs and the lower the wet-bulb reading.
• Wet-bulb depression: DBT minus WBT. A large depression means low humidity; a depression of zero means the air is saturated (100% RH, wet-bulb equals dry-bulb, and dew point equals air temperature).

The depression is the raw input to a psychrometric chart or calculator that yields RH and dew point.

Relative Humidity vs Absolute Humidity

RH is what matters for coating work because it governs condensation. Fifty percent RH at 90°F holds far more actual water vapor than 50% RH at 50°F — so absolute humidity is not the operative metric for coating inspection.

Psychrometric Chart Use and Worked Dew Point Calculation

The psychrometric chart plots dry-bulb temperature on the horizontal axis, with wet-bulb lines sloping diagonally and RH curves arching across the chart. To use it:

1. Locate the dry-bulb temperature on the horizontal axis.
2. Follow the sloping wet-bulb line from that point until it intersects the wet-bulb reading.
3. Read RH from the curved RH lines passing through the intersection (50%, 70%, 85%, etc.).
4. Find the dew point by following the horizontal line left from the intersection to the 100% RH (saturation) curve, then read down to the temperature axis.

Worked Example

A sling psychrometer reads dry-bulb 70°F and wet-bulb 60°F — a 10°F depression. On the psychrometric chart, the intersection of 70°F dry-bulb and 60°F wet-bulb gives RH approximately 55%; following the horizontal line left to the saturation curve gives dew point approximately 53°F.

Interpretation: condensation forms on any surface at or below 53°F. A steel surface at 58°F gives a 5°F margin — application may proceed. A steel surface at 52°F is 1°F below the dew point — coating must not begin.

Surface Temperature Instruments

Surface temperature must be measured on the substrate itself — it cannot be inferred from air temperature, because steel radiatively cools below air temperature at night. Three instruments are recognized in CIP Level 1:

Field Procedure Notes

• Allow dial thermometers at least 2-3 minutes to stabilize before reading.
• Take surface temperature on the actual area to be coated, not a nearby rail or scaffold.
• IR readings of bare blasted steel are unreliable without emissivity correction; black tape on the steel gives a known-emissivity target.
• Record the instrument used — auditors and spec writers may require a specific type.

Exam Traps

• The wick must be wetted with distilled or deionized water — tap water leaves mineral deposits that skew wet-bulb readings.
• The sling must be whirled long enough (approximately 90 seconds or until the wet-bulb stabilizes). A few swings give a falsely high wet-bulb reading, inflating calculated RH and dew point.
• Avoid whirling in direct sunlight or near heat sources — radiant heat inflates the dry-bulb and corrupts the depression.
• Magnetic dial thermometers only work on ferrous steel; they cannot measure aluminum, galvanized, or non-ferrous substrates.
• Dew point is the temperature at which condensation begins — it is NOT the same as the wet-bulb temperature (dew point is always between the wet-bulb and dry-bulb when RH is below 100%).