Published with permission from Vaisala
Humidity measurement and control is called for in a wide variety of industrial applications. Each application has a different set of requirements for humidity instruments, such as required measurement range, tolerance to extreme temperature and pressure conditions, ability to recover from condensation, ability to operate in hazardous environments, and options for installation and calibration. There is no single device that is suitable for all needs. In fact, the range of available equipment is quite large, varying both in cost and quality.
This paper discusses the following topics in order to help in selecting the right humidity instrument:
Humidity is simply water in its gaseous phase, properly called water vapor. Because water vapor is a gas, most of the common gas laws apply to it, including Dalton's law of partial pressures. Dalton's law says that the total pressure of a gas is equal to the sum of the partial pressures of each of the component gases:
Ptotal = P1 + P2 + P3…
If we consider air, the equation means that the total atmospheric pressure of 1.013 bar (14.7 psia) is the sum of the partial pressures of nitrogen, oxygen, water vapor, argon, carbon dioxide, and various other gases in trace amounts.
Definition of water vapor pressure
Water vapor pressure (Pw) is the pressure exerted by the water vapor present in air or a gas. Temperature dictates the maximum partial pressure of water vapor. This maximum pressure is known as saturation vapor pressure (Pws). The higher the temperature, the higher the saturation vapor pressure and the more water vapor the air can hold. Thus, warm air has a greater capacity for water vapor than cold air.
If saturation vapor pressure is reached in air or in a gas mixture, the introduction of additional water vapor requires that an equal amount condenses out of the gas as a liquid or a solid. A psychrometric chart shows graphically the relation between saturation vapor pressure and temperature. In addition, vapor pressure tables can be used to see the saturation vapor pressure at any temperature, and there are also a number of computer-based calculation programs available.
Effect of pressure on humidity
Dalton's law states that a change in the total pressure of a gas must have an effect on the partial pressures of all of the component gases, including water vapor. If, for example, the total pressure is doubled, the partial pressures of all component gases are doubled as well. In air compressors, a pressure increase "squeezes" water out of the air as it is compressed.
This happens because the partial pressure of water vapor (Pw) is increased, but the saturation vapor pressure is still only a function of temperature. As pressure builds in a receiver tank and Pw reaches Pws, water condenses into liquid and must ultimately be drained from the tank.
When thinking conceptually of water vapor as a gas, it's easy to define relative humidity. Relative humidity (RH) can be defined as the ratio of the partial water vapor pressure (Pw) to the water vapor saturation pressure (Pws) at a particular temperature:
%RH = 100% × Pw / Pws
Relative humidity is strongly temperature dependent as the denominator in the definition (Pws) is a function of temperature. For example, 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 the water vapor remains the same.
Pressure will also change relative humidity. For example, if a process is kept at a constant temperature, relative humidity will increase by a factor of two if the process pressure is doubled.
Dew point temperature
If a gas is cooled and gaseous water vapor begins to condense in the liquid phase, the temperature at which condensation occurs is defined as the dew point temperature (Td). At 100%RH the ambient temperature equals the dew point temperature. The further negative the dew point is from the ambient temperature, the smaller the risk for condensation and the drier the air.
Dew point directly correlates with saturation vapor pressure (Pws). The partial pressure of water vapor associated with any dew point can be easily calculated. Unlike RH, dew point is not temperature dependent but it is affected by pressure. Typical applications for dew point measurement include various drying processes, dry air applications, and compressed air drying.
Frost point temperature
If the dew point temperature is below freezing – which is the case in dry gas applications – the term frost point (Tf) is sometimes used to explicitly state that the condensing phase is ice. The frost point is always slightly higher than the dew point below 0°C as the water vapor saturation pressure of ice is different to water. People also often refer to dew point for subzero values, even though they mean frost point. Ask for clarification if you are not certain.
Parts per million
Unit parts per million (ppm) is sometimes used for low levels of humidity. It is the ratio of water vapor to dry gas or total (moist) gas, and is expressed either by volume/volume (ppmvol) or mass/weight (ppmw). Parts per million (ppmvol) can be quantitatively expressed as follows:
ppmvol = [Pw /(P - Pws)] × 106
The ppm parameter is typically used when defining the water vapor content of pressurized and dry pure gases.
The mixing ratio (x) is the ratio of water vapor mass to the mass of dry gas. It is dimensionless but often expressed in grams per kilogram of dry air. The mixing ratio is mainly used in drying processes and HVAC applications for calculating water content when the mass flow of air is known.
Wet bulb temperature
Traditionally, the wet bulb temperature (Tw) is the temperature indicated by a thermometer wrapped in a wet cotton sheath. The wet bulb and ambient temperatures can be used together to calculate relative humidity or dew point. For example, the wet bulb temperature is used in air conditioning applications where it is compared to the dry bulb temperature to determine the cooling capacity of evaporative coolers.
Absolute humidity (a) refers to the mass of water in a unit volume of moist air at a given temperature and pressure. It is usually expressed as grams per cubic meter of air. Absolute humidity is a typical parameter in process control and drying applications.
Water activity (aw) is similar to equilibrium relative humidity and uses a scale of 0 to 1, instead of 0% to 100%.
Enthalpy is the amount of energy required to bring a gas to its current state from a dry gas at 0°C. It is used in air conditioning calculations.
Environmental conditions can have a significant effect on humidity and dew point measurements. Take the following environmental factors into consideration to achieve the best possible measurement result:
Select a representative measurement location
Always choose a measuring point that is representative of the environment being measured, avoiding any hot or cold spots. A transmitter mounted near a door, humidifier, heat source, or air conditioning inlet will be subject to rapid humidity changes and may appear unstable.
As relative humidity is strongly temperature dependent, it is very important that the humidity sensor is at the same temperature as the measured air or gas. When comparing the humidity readings of two different instruments, the thermal equilibrium between the units/probes and the measured gas is particularly crucial.
Unlike relative humidity, dew point measurement is independent of temperature. However, when measuring dew point, pressure conditions must be taken into account.
Beware of temperature differences
When mounting a humidity probe into a process, avoid temperature drops along the probe body. When there is a large temperature difference between the probe and the external environment, the whole probe should be mounted within the process and the cable entry point should be insulated.
When there is a risk of condensation, the probe should be mounted horizontally to avoid water dripping down the probe/cable and saturating the filter (see figure 1).
Ensure that air is allowed to flow around the sensor. Free air flow ensures that the sensor is in equilibrium with the process temperature. At 20°C and 50%RH, 1°C difference between the sensor and the measurement zone will cause an error of 3%RH. At 100%RH the error is 6%RH (see figure 2).
The right instrument for high humidity
Environments with >90%RH are defined here as high humidity environments. At 90%RH a difference of 2°C can cause water to condense on the sensor, which in an unventilated space may take hours to dry. Vaisala humidity sensors will recover from condensation. However, if the condensed water is contaminated, the instrument accuracy can be affected due to deposits on the sensor, especially salt deposits. Even the life of the sensor may be shortened. In applications with high humidity where condensation can occur, a warmed sensor head probe such as the Vaisala HUMICAP® Humidity and Temperature Transmitter HMT337 should be used.
The right instrument for low humidity
Environments with <10%RH are defined here as low humidity environments. At low humidities, the calibration accuracy of instruments measuring relative humidity may not be adequate. Instead, measuring dew point will provide a good indication of humidity. For example, Vaisala DRYCAP® products are designed for measuring dew point.
If a dryer fails in a compressed air system, water condensation may appear and the instrument will need to recover. Many dew point sensors are damaged or destroyed in such situations, but Vaisala DRYCAP® dew point sensors withstand high humidity – and even water spikes.
The right instrument for extreme temperature and pressure conditions
Continuous exposure to extreme temperatures may affect sensor and probe materials over time. It is therefore very important to select a suitable product for demanding environments. In temperatures above 60°C the transmitter electronics should be mounted outside the process and only a suitable high temperature probe should be inserted into the high temperature environment. Moreover, built-in temperature compensation is required to minimize the errors caused by large temperature swings or operation at temperature extremes.
When measuring humidity in processes operating at around ambient pressure, a small leak may be tolerable and can be reduced by sealing around the probe or cable. However, when the process needs to be isolated, or when there is a large pressure difference between the process and the external environment, a sealed probe head with appropriate mounting must be used. Pressure leaks at the point of entry will alter the local humidity and result in false readings.
In many applications it is advisable to isolate the probe from the process with a ball valve to enable the removal of the probe for maintenance without shutting down the process (see figure 3).
When is a sampling system needed for dew point measurement?
Wherever possible, the probe should be mounted in the actual process to achieve the most accurate measurements and a rapid response time. However, direct installations are not always feasible. In such situations, sample cells installed in-line provide an entry point for a suitable measurement probe.
Note that external sampling systems should not be used to measure relative humidity because the change in temperature will affect the measurement. Sampling systems can instead be used with dew point probes. When measuring dew point, sampling systems are typically used to lower the temperature of the process gas, to protect the probe against particulate contamination, or to enable easy connection and disconnection of the instrument without ramping down the process.
The simplest dew point sampling setup consists of a dew point transmitter connected to a sampling cell. Vaisala has several models suitable for the most common applications and sampling needs. For example, the easy to install DSC74 sampling cell is designed for the flow and pressure conditions in compressed air applications.
In demanding process conditions, sampling systems must be designed carefully. As dew point is pressure dependent, a flow meter, pressure gauge, special non-porous tubing, filters, and pump may be needed. As an example, a flow chart showing the Vaisala DRYCAP® Portable Sampling System DSS70A for DM70 is shown in figure 4.
In a pressurized system a sample pump isn’t needed as the process pressure induces a large enough flow to the sampling cell.
When measuring dew point with a sampling system, trace heating should be used when the ambient temperature around the cooling coil or connecting tube is within 10°C of the dew point temperature. This prevents condensation in the tubing that connects the dew point instrument to the process.
Only products with appropriate certification can be used in potentially explosive areas. For example, in Europe products must comply with the ATEX100a directive, which has been mandatory since 2003. Intrinsically safe products are designed in such a manner that even in the event of failure they do not generate enough energy to ignite certain classes of gas. The wiring from the intrinsically safe product into the safe area must be isolated via a safety barrier. For example, the Vaisala HMT360 series of intrinsically safe humidity transmitters are specially designed for use in hazardous environments.
Shock and vibration
When the probe will be subject to excessive shock or vibration, the choice of probe, mounting method, and installation location needs careful consideration.