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Defining Moisture Content in a Glove Box

Controlling the moisture content present in a glove box is vital. First, it helps maintain a stable environment for air-sensitive experiments. Second, it prevents moisture-sensitive materials from degrading in the workspace. Lastly, it can improve the accuracy and reliability of your results.

There are multiple ways of defining how much moisture is present in the air. Yet, it is difficult for people to quickly switch between these different definitions and values. With clear definitions, guidance on converting values, and a simple conversion table, you will easily be able to work with any of the available definitions to control the moisture content in your glove box environment.

Vapour Pressure

Vapour pressure is a measurement of the pressure exerted by water vapour in a gaseous form.

Vapour pressure in the evaporation process. Water particles in the liquid and vapoir phases exert pressure on eachother.
The interface between water in vapour phase and liquid phase

The water particles in the liquid phase and water phase exert a pressure on each other. If the pressure of the liquid phase is higher than that of the vapour phase, then moisture can evaporate and become part of the gas phase. As more moisture is present in the air the vapour pressure increases until an equilibrium is reached where the pressure exerted by moisture in the vapour phase is equal to that exerted by water in the liquid phase. This is known as the saturation vapour pressure and is the maximum moisture that air can contain before condensation occurs.

Relative Humidity

The most commonly used definition of moisture content is relative humidity. Relative humidity is a relative measurement based upon a scale from 0 to 100 percent. A value of 0 represents air with no moisture content, while 100 represents air that is fully saturated with water.

With relative humidity the saturation vapour pressure is taken as the maximum achievable relative humidity of 100%. By measuring the current water vapour pressure and comparing it to this maximum value, a relative saturation percentage can be calculated. This is shown below where Pw is the vapour pressure or water and Pws is the saturation vapour pressure.

Equation to calculate the relative humidity

The value of saturation vapour pressure is not constant and is dependent on the temperature of the water at any given time. There are several methods for calculating the saturation vapour pressure. The most commonly used approximation is the Arden Buck equation.

The Arden Buck equation

The relative humidity scale, as a function of temperature, is not constant and will vary. Air with the same amount of water in will have a variable relative humidity as the temperature changes over time.

There are various methods for measuring the vapour pressure of air to determine the relative humidity. Digital hygrometers work to measure the electrical properties of air directly or materials in contact with air.

Dew Point

An alternative method for measuring the moisture content of air in your glove box is to measure the dew point. This is the temperature to which air must be cooled for condensation to occur. It can also be defined as the temperature required to reach the saturation vapour pressure for a given level of moisture in the air. By cooling down a sample of air and observing when condensation occurs, you can easily determine the current dew point of any given volume of air.

Knowing the temperature of the air, you can obtain the saturation vapour pressure from the Arden Bucks equation. This will then be equal to the vapour pressure of moisture in the air sample given. Using the temperature of the air, you can calculate the saturation vapour pressure of that air and divide the vapour pressure of the condensed sample to give the relative humidity of the air.

Equation to calculate the relative humidity using the dew point

Where a, b, c, and d are constants determined by Arden Buck and are: a = 6.1121 mbar, b = 18.678, c = 234.5 °C, and d = 257.14 °C. Tdp is the dew point temperature and T is the temperature of the air the sample was taken from. Although this is a little more complex, there are advantages of using the dew point. Measurements of dew point are significantly more accurate than those taken using standard hygrometers. The use of cold stages to measure dew points down to below -90 °C can give water vapour pressures as low as 0.1 µBar of pressure.

Parts Per Million (PPM)

By measuring the relative humidity or the dew point of an air sample, the vapour pressure can be determined. From this value, parts per million (ppm) values for moisture content can be obtained. It is important to note that there are two ways of determining the ppm of a sample: the ppm by weight and the ppm by volume fraction.

Equation to calculate the ppm by volume

Where Pw is the water vapour pressure and Ptotal is the absolute pressure.

Equation to calculate the ppm by weight

Where Mw is the molecular mass of water and MD is the molecular mass of the dry gas, which could be air, nitrogen, or argon depending on the atmosphere you are measuring.

Moisture Content Conversions

Different glove boxes define moisture content in different ways. With the standard temperature, pressure, and Arden Buck approximations, we can create a quick conversion table for different relative humidity values, vapour pressure, dew point, and ppm.

Humidity (%) Pressure (mbar) Dew Point (C) PPM (volume) PPM (weight)
100 24.871695 21.00019944 25164.145 16179.13
90 22.3845255 19.29632381 22590.8825 14524.66
80 19.897356 17.41780975 20030.5059 12878.49
70 17.4101865 15.32091908 17482.9187 11240.53
60 14.923017 12.94250482 14948.0253 9610.737


12.4358475 10.18625957 12425.7311 7989.044


9.948678 6.893913923 9915.94228 6375.392


7.4615085 2.775633574 7418.56619 4769.72


4.974339 -2.799821685 4933.51094 3171.969


2.4871695 -11.75789851 2460.68556 1582.082


2.23845255 -13.06037343 2214.07219 1423.524


1.9897356 -14.49884475 1967.58013 1265.043


1.74101865 -16.10764253 1721.20928 1106.641


1.4923017 -17.93641026 1474.95957 948.3158


1.24358475 -20.06099891 1228.8309 790.069


0.9948678 -22.60632501 982.823172 631.8999


0.74615085 -25.8017308 736.93631 473.8085


0.4974339 -30.14834764 491.170219 315.7948


0.24871695 -37.18207612 245.524812 157.8586


0.124358475 -43.75722003 122.747337 78.91962


0.04974339 -51.81835899 49.0953192 31.56552


0.024871695 -57.48728061 24.547057 15.78237


0.012435848 -62.82271414 12.2733779 7.89109


0.004974339 -69.41210298 4.909315 3.156413


0.00248717 -74.07811105 2.45465147 1.578202


0.001243585 -78.49406292 1.22732423 0.7891


0.000497434 -83.98086105 0.49092933 0.31564


0.000248717 -87.88835032 0.24546461 0.15782

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Further Reading

Glove Box Controls Glove Box Standard Operating Procedure

Every glove box requires a good standard operating procedure to ensure proper use. Everyone with access to the glove box should agree to follow this procedure.

Ossila Glove Box in use How to Choose a Glove Box

It is important that you choose the most suitable glove box for your samples, experimental needs, budget, and lab environment. .


Contributing Authors

Written by

Dr. Jon Griffin

Product Developer

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