How do we measure water activity?

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What is water activity?

As described in a previous blog (Water Activity and Moisture Content: What is the Difference?) water activity is the the measurement of the free water in a substance. The free water is any water that is not chemically bound to anything and is free to move in and out of a sample. The ability of the water to move in such a way is what allows us to measure water activity.

Partial pressure of water vapour.

Air is made up of a mixture of gasses. Each of the gasses is responsible for part of the overall air pressure. For example, at 23 °C / 35 %rh the water vapour in the air is responsible for just under 1% of the overall air pressure – around 9.8 hPa is exerted by water vapour out of 1013.2 hPa for the total mixture of gasses. The partial pressure of water vapour is in a constant state of flux and is influenced by water moving in and out of any materials exposed to the air. The air can only hold so much water. At 23 °C, the partial pressure of water vapour can only go as high as 28.1 hPa (approximately 2.8%) before the air is saturated.

What is diffusion?

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Diffusion is the movement of particles of liquid or gas from an area of high concentration to an area of lower concentration. The particles will continue to move until an equilibrium is reached and the concentration is consistent and stable across the whole available space. In terms of water vapour, think of drying wet clothes. When you hang wet cloths to dry them, the concentration of water in the clothes is much higher than the concentration in the air around them. Because the water in the clothes is not chemically bonded to the material, it is able to move out of the clothes into the air around them until the amount of water in the air is the same as the amount in the fabric, at which point an equilibrium has been reached. This will increase the overall partial pressure of water vapour in the air. When you hang your clothes outside on a dry day, there is plenty of space for the water to move into, but if you hang them in a small room you will notice the humidity in that room increases, possibly even to a point where the clothes will not dry properly. It is this property that allows us to measure water activity. 

Diffusion
Figure 1: Diffusion is the process by which particles move from an area of high concentration to an area of low concentration.

What has drying clothes got to do with measuring water activity?

In the same way that water will move in and out of your clothes, it will also move in and out of everything else to a different degree. If you cut an apple in half and leave it to one side, you will notice that it will dry out. This is because all of the free water in the apple is escaping into the air around it. Again, this will continue to happen until moisture levels are the same in the air around the apple as in the apple. In a large room this will essentially mean that most of the water will eventually leave the apple. If however, you were to put the sliced apple into a small container you will notice that it does not dry out in the same way. This is because the air space is much smaller, so it takes much less water from the apple to fill the air until an equilibrium is reached. If you were to measure the relative humidity in the container, you would notice that it will quickly increase before levelling off. Divide this final relative humidity by 100, and you have a good idea of the water activity of the slice of apple.

How does this apply to water activity measurements in industry?

Generally, a water activity measurement device will simply be made of some kind of relative humidity sensor and a small, sealed sample container. When you put a sample into the container, water will move in and out of the sample until an equilibrium is reached, and the sensor will read the relative humidity. Once the temperature and relative humidity are stable, the machine will give its final reading.

Water activity
Figure 2: Water activity devices measure the relative humidity of the air around a sample after an equilibrium has been reached

What has temperature got to do with it?

Just as temperature is a key influencing factor on relative humidity, it can also effect water activity. The exact effect that a change in temperature has on the water activity of a specific sample is impossible to know without testing the sample at a number of different temperatures. As such, when measuring water activity it is vital to make sure that sample temperature is consistent and stable.

The scale of water activity.

Water activity is measured on a scale of 0 to 1. A water activity of 1 would occur if you measured absolutely pure water, almost an impossibility – The air above completely pure water will have a relative humidity of 100%. To get a water activity of 0, you would need to measure something with no water at all. Given that there is always water in the air, the likelihood is that if something has been exposed to the air it will contain at least some water, so a water activity of 0 is again more or less impossible.

If you have any questions about anything in this blog, or would like to discuss water activity measurement in general and how it can be applied, please don’t hesitate to give us a call on 01293 571000 or email instruments@rotronic.co.uk.

Rotronic UK

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