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Relative Humidity

Definition:

Humidity is a term describing the amount of water vapor or moisture being carried by air. Measurement can be either absolute or relative; absolute measurement deals with the weight per unit of volume of water vapor whereas relative measurement deals with the condition of saturation, usually stated in percent. At 100% RH, the air is completely saturated with water and no evaporation is taking place.

Relative Humidity is a relative measurement! The total amount of water vapor that air can hold is dependent upon its pressure and temperature. At a pressure of 30.11 Hg, air at O deg F will hold about 1/2 grain of water vapor per cubic foot; at 32 deg F, about 2 grains, at 70 deg F, about 8 grains; and at 100 deg F, about 20 grains. A grain is 1/7000th part of a lb of water.

Under normal conditions of pressure at 70 deg F, a cubic foot of space can hold 8 grains of water vapor. Therefore, air with only 2 grains has only one-fourth of its maximum capacity, or a RH of 25%.

Dewpoint is a temperature at which air in the atmosphere becomes saturated with water vapor and starts to condense. Any lowering of temperature below the dew point results in condensation of some of the water pressure present. Relative Humidity at the dewpoint is always 100%.

Measurement Considerations and Solutions

Wet bulb- Drybulb:

Time honored method whereby one of a matched set of thermometers is constantly wetted by a fabric wick connected to a cistern filled with distilled water and is compared to the other thermometer. The wet-bulb draws water to it by capillary action, and evaporation takes place from around the bulb of this thermometer, the rate being governed by the dryness of the surrounding air, and heat is given up, with the result that the dry-bulb thermometer reads higher than the wet-bulb thermometer. The difference between the two matched thermometers determines the relative humidity from pyschrometric charts. The thermometers should be located where there is a reasonable amount of air circulation. Any device using this technology should have forced air fan action of about 600 FPM; applications having less air circulation will have a slower response and lessened ability of the wetbulb/drybulb to respond to relative humidity changes. Care should be taken that the wetbulb/drybulb is not placed near heat sources, including direct sunlight.

Important Considerations:

Care must be taken to keep the wick clean and supplied with clean, preferably distilled, water. Soluble salts and dirt particles accumulated on the wick adversely affect the rate of evaporation and capillary causing inaccuracies in the wet-bulb readings. Wash the wicks frequently; discard and replace when the material becomes hard.

Fixed-Position Hygrometers: Rapid circulation of air around the bulbs just prior to reading is absolutely vital for accuracy. Use a hand-fan or other device for one minute or more, then immediately take the reading. Make sure that the reading is done at eye-level so that the thermometer is correctly addressed. Read the WET-BULB first, then the DRY-BULB.

Typical instruments:

The Taylor Mason's Hygrometer is a wall mount wet/dry bulb instrument.

The Sling Psychrometer has a folding swivel handle for use to "sling" around to mimic rapid air circulation. To use it, one first saturates the wicking of the wetbulb with water at room temperature. The Psychrometer is whirled for 15 to 20 seconds, holding it away from the body and from any objects it might hit. Immediately read the wetbulb and then the drybulb thermometers and note; repeat until two or more readings agree at the lowest indication obtainable. The wicking must be kept perfectly clean and thoroughly saturated to secure accurate indication.

For remote reading, computer input, digital displays, or very high temperature applications, matched 100 ohm Platinum RTD's with transmitters can be used instead of thermometers and the outputs put into an adder-subtracter transmitter for a net output. Our Digital Panel Meters can be calibrated to read out directly in relative humidity. We even offer a round chart recorder from Partlow that will take the wetbulb/drybulb inputs directly from RTD'S, Thermocouples or Temperature Transmitters. It uses an algorithm to automatically calculate relative humidity.

Direct Reading, dial indicators using either Goat's hair or synthetics which are hydroscopic are readily available. These devices have carefully chosen materials which change length in the presence of moisture, and convert this lateral movement into a rotary motion of the hand on the dial. Direct reading types have the advantage of fast response. The disadvantage is in the higher cost and, in general, lower accuracy, and a narrow operating temperature range.

Digital Hand Held Instruments utilize hydroscopic capacitance polymers which complete a variable voltage electronic circuit. Advantages are the ability to store 16 readings ( Model THIOO ), rapid response, temperature, humidity and dewpoint readings, and the unit can be calibrated @ 12% and 75% with the A834 Calibration Kit.

Data Loggers with Software perform months of unattended monitoring and are available (with or without sensors) from stock.

Round Chart Recorders, either self-contained or with remote sensors, are available from stock. These can be either mechanical devices or utilize electronic capacitance technology.

R.H. and Temperature Transmitters are stock items too.

Whatever your relative humidity requirements are, give Branom a call!

Relative Humidity Tables (Publication Number PB257-023/2) can be ordered from:

National Technical Information Service
Department of Commerce
Springfield, Va 22161

Re: 200 Degrees F Relative Humidity (September 11, 1998)

Beyond the capabilities of RH Transmitters

One way to attack the high temperature problem with relative humidity is to take a modern day twist on the age-old wet/dry bulb method of measuring relative humidity.

The first thing you need is a Taylor Water Cistern #5522N and some Taylor Wicks, or any kind of semi-enclosed water vessel and cotton wick.

The wetbulb must, of course, be kept wet, either through manual filling or direct coupled to some tubing with a water source and a needle valve. A quality wick, such as the Taylor, will pull the water from the cistern via capillary action, and keep the wetbulb wet.

Then we need two matching output RTD's, which must be tested at 200 Deg F. We normally would utilize the heavy duty Gordon # RFJBOTK14OBB300 with 30 feet of Extension Wire.

The output of the two RTD's would become the inputs for the API Transmitter, a two input (RTD, T/C, linear etc) device with advanced math capabilities. It will take the two RTD inputs, and using the level one math will deduct the wetbulb temperature from the dry bulb, and then the remainder would be massaged to the relative humidity table (see below) for the temperatures to be measured, i.e. 190-210 degrees F. This would then be converted in the second level to a direct output of 4-20 mA DC corresponding to Relative Humidity. The 4-20 mA Signal then would become the input to the Red Lion PAXP Digital Panel Meter, which has 16 points of linearization, whereby the non-linear 4-20 mA signal can be linearized. The PAXP is available with up to 4 alarms, retransmitted 4-20 mA or DCV outputs, or RS232, RS485 or DeviceNet. Calibrated. For recording, Partlow makes a 12" Round Chart direct reading from RTD wetbulb/drybulb Recorder #MRC7700.

All of these items, except the Partlow, are normally Seattle Stock; with calibration and setup, possibly two days or so maximum delivery. The Partlow is usually one week ARO.

The Method of Dewpoint Temperature

When measuring Humidity, it is often necessary to determine the dew point temperature.
The following table shows the relationship between Relative Humitidy and Temperature.

Example: Temp 32 deg F, Humidity 50%RH  --> Dew Point is 15.3 deg F