Category Archives: Water Activity

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.

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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

Water Activity – A critical measurement for sensitive seeds

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Water activity measurement plays an integral role at the Tree Seed Centre in British Columbia.

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The Tree Seed Working Group has four objectives related to promoting tree seed science and technology through:
• Seed research from bud initiation to seed use
• Identification of seed problems relating to seed improvement and forest management
• Exchange of information on seed related problems
• Consulting on implementation practices

Water activity continues to be an area of interest to the Tree Seed Working Group, especially with their genetic conservation collection.

Rotronic has become the standard for water activity measurement for many seed storage facilities throughout the world. Although water activity is a relatively new technology to the Tree Seed Centre, it already plays an important role. Water activity measurement devices from Rotronic offer quick, non destructive tools for evaluating the free moisture in a sample of seeds. The meter outputs a value between 0 and 1, which is equivalent to equilibrium relative humidity (eRH). Aw or eRH differs from a moisture content test in that it only measures the free water rather than the total water content in a sample of seed. Free water is often far more relevant to germination, spoilage and shelf life of seeds. The amount of free water in a seed will depend on its relative composition of lipids, starches, and proteins. Storing seed in the freezer with a water activity measurement that is either too high or too low will decrease the longevity of the seed. The ideal level ultimately depends on the species; however, most people now accept the concept of a universal value. At the Tree Seed Centre, they keep within a range of 0.35 ± 0.05”.
The role of water activity measurement

Currently, water activity is used primarily for testing seed collected for genetic conservation and research. The seeds are often rare and valuable; thus a non destructive method of assessment is imperative. Collections intended for the seedbank arrive at the Tree Seed Centre grouped by population (noted by location), with each population containing,on average, a sample of 10 parent trees. Populations are stored in the cooler at 2°C to await an initial water activity assessment. If the water activity is too high, the seeds are dried back and retested prior to placement in the freezer at-18 °C to maximise seed longevity.

In the UK we have worked closely with Kew Gardens at the Millennium Seed Bank where a bank of Rotronic devices with direct PC connection are used to screen and test all seeds that are stored in their highly secure underground bunker.

Read the Kew Gardens article titled “A day in the life of a Seed Processing Assistant” here!

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Six Rotronic eRH devices working in tandem with HW4 software at Kew Gardens.

For information on any of our products please contact us on 01293 571000 or email instruments@rotronic.co.uk or visit our website by clicking Here

12 Slides explaining Water Activity; what it is and how to use it!

Water activity is a useful parameter for many industries. Measuring water activity takes only a few minutes so can provide results quickly, ideal for critical control points.

However water activity is a qualitative measure therefore interpreting results can be complex. The following slides provide an overview to water activity and how to utilise Aw measurements.

For further information please do not hesitate to contact us or visit our webpage to download our latest AW White Paper!

Dr Jeremy Wingate
Rotronic UK

 

 

Five Applications where Water Activity is critical and why!

Not clear on what water activity is? Check out our other posts on water activity here!

Water activity affects a wide range of products from pharmaceuticals to foods, here we take a brief look at five typical applications where water activity measurements are used.

Remember water activity affects biological, chemical and physical properties of products.

Application 1 – Food

Water activity is critical in food production, reducing water activity reduces the available moisture for spoilage organisms to utilise as such low Aw can limit or prevent growth and spoilage.

Aw is often used to screen raw ingredients or end products to confirm that they are within tolerance. Aw measurements only take a few minutes and can therefore provide immediate feedback. This can save time, money and prevent health risks or product spoilage.

Aw can be used as an excellent indicator of shelf life. Once a production process has been fine tuned to deliver products with suitable shelf-life, online aw measurements can confirm production consistency.

Classic methods to control Aw in food are listed below.Slide14

Application 2 – Paper

Water activity or %equilibrium relative humidity in paper may not be your first thought, but the impact of poor or changing ERH in paper can be very costly. Changes in ERH can cause paper to curl or lead to increased static both of which can cause paper jams. For an industrial printing company this can lead to costly downtime. ERH also affects the binding of inks, drying times and ultimately spoilage by moulds and fungi.Slide15

 

Application 3 – Pharmaceutical Products

Drug production and storage is one of the most highly regulated manufacturing sectors. All stages of drug production, storage and delivery rely on critical control of many factors including water activity. Most drugs must be stored in stable conditions hence the advanced moisture impermeable packaging used. Aw readings as well as humidity monitoring and control are utilised to ensure free water is maintained within tolerance.

Slide16

 

Application 4 – Seed preservation and storage

Seeds present an interesting challenge, if Aw is too low the seeds quickly become non viable, where as too high and they may germinate or suffer spoilage through mould or fungal growth. One of the most critical seed preservation projects is the Millennium Seed Bank project. More details about how Rotronic AW meters were used is avaialble on the Kew Gardens websiteSlide17

 

Application 5 – Soaps and powders

Finally we come to good old washing powder, even with powders and liquids now packed into clever tablets and pods Aw remains critical. The materials must be stored in a way that ensures safe delivery to the customer but can be released once in contact with water in your washing machine. The science of behind the latest washing powders and liquids relies heavily on the measurement and conrtol of water activity and %ERH!Slide18

 

Hopefully this brief in site into how water activity affects so many day to day products has been useful. For more information visit our website or contact us to discuss!

Dr Jeremy Wingate
Rotronic UK

Durable, Adaptable, Accurate

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Campbell Scientific (CSC) are an ISO 9001 certified company who are a leading manufacturer in a variety of applications related to weather, water, energy, gas flux and turbulence, infrastructure, and soil. Campbell Scientific, are committed to satisfying the instrumentation needs of their customers, and are internationally recognised in the measurement and control industry for producing accurate and dependable instruments

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HC2A-S3

 

CSC systems are acclaimed for their dependability, which they demonstrate even in the most extreme weather climates. Their attributes include wide operating ranges, low energy usage, many communications options, and the flexibility to support a wide variety of measurement  and control applications. Applications include, agriculture, air quality, fire warning, water quality, weather and climate recording, structural monitoring, Geo-technical monitoring and mining.

Rotronic and CSC have been business partners for many years, CSC uses the standard Rotronic meteo probe in many applications. Recently CSC installed the probe in a network of Road Weather Information Systems  in Kelowna, British Columbia. CSC selected the probe because of its reliability, ease of use and accuracy. The HC2A-S3 is also highly regarded for its ability to function in extreme temperatures, this makes it good for the  Canadian climate, and a perfect complement to Campbell Scientific systems.

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” We value the Rotronic HC2A-S3 probe for its ability to function at extreme temperatures.” Mike Ryder Campbell Scientific, Canada

For more information on the latest HC2A-S click here ,or for any of our products please visit the Rotronic website.

Meteorology Numerical Weather Prediction

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The calculation of weather data

What is the weather going to be like tomorrow?

For a long time, people have tried to predict weather conditions using the hydrologic climate cycle.In the early 1920’s scientists were able to compile a six hour forecast, back then it took six weeks to analyse weather data collected at only two points in Europe and calculate, by hand, a useful illustrative model. Today, supercomputers are used to predict the weather for a period of several weeks. The complex modelling programs require several million data points for parameters such as temperature, humidity, pressure, vertical & horizontal wind velocity with time stamps and absolute coordinates. To create a correlation between the data and the environment, scientists “slice” the atmosphere virtually into smaller horizontal &vertical parts—this process is called discretisation. It is more useful to compute the chronological change of the parameters using this model.

watercycle-pageClockwise from top left: Map of the average temperature over 30 years. .Weather station on Mount Vesuvius. .Water cycle summary

Meteorological events that are too “small” such as a single thunderhead, layer clouds or smaller turbulence’s will be parameterised through variables. This parameterisation is a science of its own that aims to reduce uncertainties as best as possible. Every forecast calculation starts with the current weather conditions. The quality of this input is crucial for the accuracy of the final forecast. Meteorologists link the forecast of yesterday’s weather with the actual measured parameters. Only large data centres are capable of computing this data assimilation. The overall result is a best possible calculation basis predict the weather for the next day. If this groundwork is flawed the forecast may be incorrect. For example, it could report rain at the wrong location. Today’s meteorological mathematicians also take parameters into account that change extremely slowly compared to the other factors. Growth and the reduction of polar ice, or the temperature of the oceans are summarised as boundary values. After a model is run using all the available data, meteorologists process and customise reports for a wide range of target groups such as public authorities, flight control centres, energy producers, industries and many more. These reports also include specific weather warnings.

Why the need to measure humidity?

Atmosphere_composition_diagramAtmosphere composition diagram

As described above, the daily weather forecast relies on the precise measurement of weather parameters. The science of numerical weather prediction aims to describe the daily hydrologic cycle in numbers. Humidity plays an  important role. Typically, data errors will multiply during calculations. Humidity values influence weather calculations e.g. through the water vapour balance equation— this formula expresses the influence of humidity through rain & condensation, and vice versa. Incorrect measurement or incomplete humidity data directly leads to wrong predictions of a huge number of weather phenomena such as the condensation altitude of clouds, locations of hyetal regions, fog layers and storms. In 1999, incorrect data sent by a weather station in Nova Scotia, Canada led to a incorrect forecast for Hurricane Lothar two days before it hit Central Europe. Authorities were insufficiently prepared to alert people in time. The prediction of rain and snowfall is still challenging for meteorologists. Only more extensive networks of weather stations and enhanced mathematical models will reduce problems due to unknown factors.

Facts & Figures

  • 7 inches is the diameter of the largest hailstone ever recorded.
  • Sukkur City in Pakistan is one of the most humid places in the world with
    30 °C dew point & a felt air temperature of 65 °C.
  • A study showed that a small thunderstorm system holds more than 10 million tons of water.
  • No two weather patterns are completely alike.
  • Some weather models assimilate data obtained from more than 25,000 weather stations.

Rotronic International Sales Meeting 2014 Grindelwald Photo Gallery

Great to see everyone at the 2014 ISM in Grindelwald!