1 August 2002

Getting a grip on humidity

By James Tennermann

Humidity measurement and control is called for in a wide variety of industrial applications.

One of the more challenging aspects of humidity measurement is the proliferation of humidity parameters. There are at least half a dozen in common use, including dew point temperature, relative humidity, parts per million, wet bulb, grains per pound, and absolute humidity.

Understanding the relationships among humidity, temperature, and pressure is the first step toward sorting out measurement and control issues.

Gas properties: Humidity is simply water that is in the gaseous phase, properly called water vapor. Virtually all humidity measurements come down to the question of: "How much of this gas is water vapor?"

Because water vapor is a gas, most of the common gas laws apply to it, including Dalton's law. Dalton's law is particularly useful when thinking about humidity. It says the total pressure of a gas is equal to the sum of the partial pressures of each of the component gases:

P_total = P₁ + P₂ + P₃ . . . + P_n

If we consider air, this means the total atmospheric pressure of 1.013 bars (14.7 pounds per square inch absolute) is the sum of the partial pressures of all its constituents: nitrogen, oxygen, water vapor, argon, carbon dioxide, and various other gases in trace amounts.

Water vapor pressure: Temperature dictates the maximum partial pressure of water vapor. This maximum pressure is the saturation vapor pressure. The higher the temperature, the higher the saturation vapor pressure.

Thus, warm air has a greater capacity for water vapor than cold air. If saturation vapor pressure has occurred in air or a gas mixture, introducing additional water vapor requires that an equal amount of water vapor condense out of the mixture as a liquid or solid.

A psychrometric chart graphically relates saturation vapor pressure to temperature. Vapor pressure tables also enable one to look up the saturation vapor pressure for any temperature, and of course there are a number of water vapor calculation programs available for PCs.

Relative humidity: Now that we conceptually think of water vapor as a gas, it is easy to define relative humidity. First, we'll introduce the symbol e to denote "partial pressure of water vapor" and esat to denote "saturation vapor pressure." Relative humidity is then:

(e/e_sat) • (100) = %RH

In plain English, relative humidity (RH) is the ratio of the partial pressure of water vapor to the saturation vapor pressure at the gas temperature, or "how much water vapor is in this air vs. how much could possibly get in the air at this temperature."

Note that temperature is critical to relative humidity, as the denominator in the definition, esat, is a function of temperature. Therefore, in a room with an RH of 50% and a temperature of 20°C, increasing the temperature of the room to 25°C will decrease the RH to about 37%, even though the partial pressure of water vapor remains the same.

Dew point temperature: If a gas is cooled and water begins to condense in the liquid phase, the temperature at which condensation occurs is defined as the dew point temperature.

Dew point is useful because it correlates directly to saturation vapor pressure, or esat. We can easily look up or calculate the partial pressure of water vapor associated with any dew point. Unlike RH, dew point is not a temperature-sensitive parameter.

In our hypothetical room where the RH is 50% and it's 20°C, the dew point temperature is about 9.3°C. Increasing the room temperature to 25°C has no effect on the dew point temperature.

Parts per million by volume (ppmv): A ppmv measurement is for low levels of humidity.

Pressure effects: Consider Dalton's law again. We see that a change in the total pressure of a gas must effect the partial pressures of all the component gases, including water vapor.

If, for example, the total pressure doubles, then the partial pressures all double as well. In air compressors, this has the interesting effect of "squeezing" water out of the air as it is compressed.

This happens because the partial pressure of water vapor, e, increases, but the saturation vapor pressure is still a function only of temperature. As pressure builds in a receiver tank and e reaches esat, water condenses into the liquid phase and must ultimately drain from the tank.

Practical implications for measurement: With the basics in hand, we can begin to formulate some general guidelines for humidity measurement. For all applications, we will need to know or decide which humidity parameter is of greatest utility.

We must know both the total range of gas temperatures and the expected water vapor levels. For process measurements, we require the process pressure, and we must consider whether the measurement takes place at this pressure or at some other pressure value.

For gases other than air, knowledge of gas composition is required. We should establish desired measurement accuracy. Finally, generic instrumentation issues (outputs, form factor, electrical requirements, etc.) must be taken into account.

Instrumentation for humidity measurement: There is no single device suitable for all humidity measurements. The range of available equipment is quite large, varying in cost from a couple of dollars to $50,000 or more per measurement point.

Technology for humidity measurement ranges from the "hair hygrometer" (which actually depends on the lengthening of hair as it correlates to relative humidity) to surface acoustic wave dew point detectors to laser ring-down spectroscopy.

Given enough time, the diligent engineer will be able to find and evaluate all of these options, if well prepared. An alternative is to contact one of the many companies that design and manufacture this type of instrumentation.

It may serve you well to contact more than one to avoid commercial bias. Try to find technical staffers that are familiar with your intended application. IT

James Tennermann has worked in the instrumentation field for 17 years. He is the business development manager at Vaisala, Inc. Tennermann is an ISA member, a National Conference of Standards Laboratories delegate, and a Society for Applied Anthropology member. Write him at james.tennermann@vaisala.com.

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