Humidity Units

Humidity in compressed air is one of the most problematic contaminants. It causes corrosion, damage to pneumatic equipment, and quality issues in processes.

Types of Measurement

Measure	Unit	What it indicates
Relative Humidity	% RH	Current saturation vs. maximum possible
Absolute Humidity	g/m³, gr/ft³	Mass of water per volume of air
Dew Point	°C, °F	Temperature at which condensation occurs
Pressure Dew Point	°C, °F	Dew point at line conditions

Relative Humidity (RH)

$$RH = \frac{P_{vapor}}{P_{saturation}} \times 100\%$$

• 100% RH = saturated air, any cooling causes condensation
• 50% RH = air contains half the vapor it could hold
• RH changes with temperature even if water content stays the same

note

A typical compressor's output is at 100% RH because air heats during compression and then cools down.

Absolute Humidity

Unit	Equivalence
1 g/m³	0.4354 gr/ft³
1 gr/ft³	2.297 g/m³
1 g/kg	ppm by mass

Unlike RH, absolute humidity indicates the actual amount of water in the air.

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Dew Point

The dew point is the temperature at which air becomes saturated and begins to condense water.

Key Concept

• If air is at 25°C with a dew point of 10°C → no condensation
• If that same air cools to 10°C → condensation begins
• If it cools to 5°C → all water corresponding to that 5°C difference condenses

Water Content vs. Dew Point Table

Dew Point	Water Content	Typical Application
+20°C (+68°F)	17.3 g/m³	Saturated ambient air
+10°C (+50°F)	9.4 g/m³	No treatment
+3°C (+37°F)	6.0 g/m³	Standard refrigerated dryer
-20°C (-4°F)	0.88 g/m³	Basic desiccant dryer
-40°C (-40°F)	0.12 g/m³	Typical desiccant dryer
-70°C (-94°F)	0.003 g/m³	Critical applications

Dew Point Conversion

$$°F = (°C \times 1.8) + 32$$

°C	°F
+3	+37
-20	-4
-40	-40
-70	-94

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Pressure Dew Point (PDP)

The Pressure Dew Point (PDP) is the dew point measured at the system's operating pressure.

Why It's Different

When you compress air, you concentrate the moisture. The same air has a different dew point depending on pressure.

$$PDP_{line} \neq DP_{atmospheric}$$

PDP ↔ Atmospheric Dew Point Conversion

PDP at 7 barg	Atmospheric DP
+3°C	-23°C
-20°C	-42°C
-40°C	-58°C
-70°C	-82°C

Watch the specifications

Always verify if a specification indicates PDP (at pressure) or atmospheric dew point. The difference is enormous.

Approximate Formula

To estimate atmospheric dew point from PDP:

$$DP_{atm} \approx PDP - 20°C \text{ (at 7 barg)}$$

The exact relationship depends on pressure:

Pressure	Approximate Depression
4 barg	~15°C
7 barg	~20°C
10 barg	~23°C
13 barg	~25°C

Why PDP Matters

PDP determines:

• Whether condensation will occur in piping
• Compatibility with sensitive equipment
• ISO 8573 compliance

ISO 8573-1 Humidity Classes

Class	Maximum PDP
Class 1	-70°C
Class 2	-40°C
Class 3	-20°C
Class 4	+3°C
Class 5	+7°C
Class 6	+10°C

Dryer Selection

• Class 4-6: Refrigerated dryer (PDP +3 to +10°C)
• Class 2-3: Desiccant dryer (PDP -20 to -40°C)
• Class 1: High-efficiency desiccant dryer (PDP -70°C)

Mental Calculator

Need	Rule of Thumb
Refrigerated dryer	PDP ≈ +3°C
Standard desiccant dryer	PDP ≈ -40°C
Reduction per 20°C lower PDP	~90% less water