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Lets be Honest

Novasina AG
Freitag, 8.3.2019

Vor- und Nachteile der Wasseraktivitäts-Methoden schonungslos erklärt  

Dr. Brady Carter erklärt in seinem Video: "Let's be Honest" welche Methoden aktuell genutzt werden, um den aW-Wert von Produkten zu analysieren. Diese Methoden beinhalten resistive elektrolytische Sensoren, Taupunkt, Diodenlaser und kapazitive Sensoren. Er zeigt schonungslos die Vor- und Nachteile jeder Methode anhand von sechs Kriterien auf, welche für den Entscheidungsprozess unumgänglich sind. Profitieren Sie von der gut 15 jährigen Erfahrung, die sich Dr. Carter im Bereich der Wasseraktivität angeeignet hat. 

 

 

 

Dr. Carter hat uns, das Novasina Team, mit diesem Beitrag gefesselt. Seine offene und ehrliche Art über all diese Methoden zu sprechen ist für jeden Zuhörer einen Mehrwert und bietet die Möglichkeit, über den Tellerrand hinaus zu blicken und tiefer zu verstehen, was bei der Messung der Wasseraktivität wichtig ist und berücksichtigt werden muss. 

Unbearbeiteter Volltext zum Beitrag: 
An Honest Discussion of Water Activity Methods
By Dr. Brady Carter 
Do you ever feel confused by all the possible water activity measurement methods? Do you wish you could find a straight forward explanation of how the methods are different? The objective of this discussion is to honestly investigate the real differences between water activity methods. The outcome will hopefully be a helpful resource to use when considering a water activity instrument.The most important distinguishing factors to consider when comparing water activity testing methods include stability, analysis time, accuracy and precision, maintenance, interference by volatiles, and cost. While many different methods have been used to measure water activity, this discussion will be limited to the current most commonly used methods. These methods include Resistive Electrolytic (RE), Dew Point (DP), Tuneable Diode Laser (TDL), and Capacitance Polymer (CP). 
WATER ACTIVITY METHODS
All methods utilize the same measurement concept of determining the relative humidity of the headspace of a sealed chamber that is in equilibrium with the test sample. The RE and CP methods both measure changes in the electrical properties of a sensor in response to changes in relative humidity. The RE method tracks changes in the electrical resistivity of an electrolytic sensor, while the CP sensor tracks the changes in the electrical capacitance of a hygroscopic polymer. The DP method measures the equilibrium relative humidity in the headspace by determining the dewpoint temperature and sample temperature. The TDL method determines equilibrium relative humidity by tracking changes in the vapor density of the headspace and the sample temperature.
STABILITY
The first distinguishing factor in our discussion is method stability, specifically how dependable is each method over time. To be considered stable, the method needs to provide the same water activity value for a given sample each time it is tested, regardless of outside factors. This is critical in providing the user with confidence in their water activity values. Outside factors that could impact test results include contamination of the sensor due to user testing practices, and changes in lab environmental conditions. Changes in lab temperature and humidity can impact samples prior to testing but can be mitigated for any of the methods through temperature control and a tight sealing chamber. Changes in atmospheric pressure due to elevation or weather changes typically do not impact test results, but the TDL method has shown to be particularly sensitive to changes in pressure, requiring a pressure adjustment to each reading. While cleanliness is important for all types of instruments, the dew point method, which utilizes a chilled mirror to determine the dew point temperature, and TDL, which utilizes optics for measurement, are particularly sensitive to contamination and require daily verification to determine if testing practices have resulted in contamination. If the mirror or optics are dirty, the correct dew point temperature or vapor density will not be measured, resulting in variable readings. RE and CP methods are less sensitive to contamination because the sensor is typically isolated from the sample chamber by a filter, but the CP sensor can give unreliable results over time due to changes in the polymer during repeated wetting and drying events. During exposures to high humidity conditions, the CP sensor will continue to absorb water and as a result, the readings will continue to drift up. If after wetting, the CP sensor is then exposed to low humidity, residual moisture will remain in the sensor causing the readings to be too high. In terms of stability, the RE method has the fewest issues, followed by CP, with DP, and TDL in last place due to the need for cleanliness and adjustment for changes in pressure.Download Full Text
ANALYSIS TIME
The second distinguishing factor in our discussion is analysis time. All water activity methods utilize the same basic principal of measuring the relative humidity of the headspace that is in equilibrium with the sample because no method currently exists that can directly measure water activity in the sample. Consequently, any arguments about which methods are primary or secondary is moot because none of the methods directly measure water activity. Since equilibrium is required to measure water activity, the first time-determining process for any of the methods is to achieve equilibrium between the relative humidity of the headspace and the water activity of the sample. Further, the sample, the headspace, and the sensor all must be at the same temperature or any difference in temperature between the sensor and the sample must be factored into the final value. Finally, when properties of the sensor are being tracked, the water activity of the sensor itself must be in equilibrium with the relative humidity of the headspace and this can only happen after the headspace is at equilibrium with the sample. Each of the methods determines the achievement of vapor equilibrium by tracking changes in measurement values over time until the variation reaches a minimum level over a specified number of readings or time period. Differences in the stability settings between methods will result in differences in analysis time and as a rule, the more stringent the stability settings, the more reliable the result will be. Setting the most stringent stability requirements could extend test time to 40 minutes or more but assures equilibrium has been achieved, while less stringent settings will speed up the analysis time but also increase the possibility that the true water activity value has not been achieved.  Most high-end iterations of the methods offer the ability to adjust equilibrium settings to more or less stringent requirements although default settings will vary with instrument. Some lower cost instruments utilizing the CP method with accuracy specifications lower than 0.01 aw offer timed or predicted results to keep analysis time to 5 minutes or less.The time required to reach equilibrium between the headspace and sample varies with sample type. Some samples, particularly those with high fat content, can take to 20-40 minutes, but most samples require 10 minutes or less, with some even needing less than 5 minutes. Once vapor equilibrium is achieved, the equilibrium relative humidity can be measured. For the DP and TDL methods, because properties of the sensor itself are not being measured and instead are measuring a property of the headspace, the water activity value can be obtained as soon as vapor equilibrium is achieved (assuming temperature equilibrium). For the RE and CP methods, additional test time is required for the sensors themselves to come into equilibrium with the headspace after vapor equilibrium is achieved. The time that is required for this additional equilibrium step varies by sensor type, but it means that if all other testing conditions are the same, including sample type and end-test stability settings, the RE and CP methods will never be as fast as the DP and TDL methods. That said, the additional time for RE sensor equilibration has been reduced in the latest iterations through updates in chamber design. All methods in their latest iterations measure sample temperature to account for any differences between the sample temperature and sensor temperature, eliminating the need for absolute temperature equilibrium. However, placing a sample in the chamber at a temperature different from the instrument set temperature can extend test times due to instability in water activity readings as the sample moves to thermal equilibrium with the instrument. In addition, reading samples with drastically different water activities in succession can extend analysis times. While test times will vary for all methods based on varying conditions, the DP and TDL method, given the same set of conditions, will be the fastest, followed by RE and CP methods (when not in predictive mode).
ACCURACY AND PRECISION
The next distinguishing factor for discussion is accuracy and precision. These terms are sometimes used interchangeably but are not the same thing. Precision is the degree to which a measurement gives the same value during repeated measurements while accuracy is a determination of whether the measurement value is correct. When thinking of a target, precision refers to the grouping of shots while accuracy is determined by hitting the bullseye. Both accuracy and precision are typically reported as a +/- value and the smaller this value the better. For accuracy, this value represents the range around the measured value that confidently contains the actual value. If the reported accuracy is +/-0.003aw, then for a measured value of 0.840aw, the actual value is somewhere between 0.837aw and 0.843aw. For precision, the value represents the range around a measured value that is expected to contain 95% of all repeated measurements on that sample. A reported precision of +/-0.001aw then indicates that for a measured value of 0.840, any other measurement made on that sample will be between 0.839 and 0.841. The importance of accuracy and precision are determined by the level of confidence required of water activity readings, with the expectation that instruments with better accuracy and precision are more expensive. A practical implication of accuracy and precision can be seen in a scenario where the safe water activity for a product has been identified as 0.700 aw. If the accuracy of the water activity measurement is +/-0.02 aw, the production specification would have to be set to 0.680, while for an accuracy of +/-0.003 aw, the specification could be set to 0.697. The reported accuracy for both the DP and RE methods is the same at +/- 0.003 aw, with precision at +/-0.002 aw for RE and +/-0.001 for DP. The TDL accuracy is +/-0.005 aw with a precision of +/-0.002 aw while the accuracy for CP is less than +/-0.01 aw with a similar precision. When considering achievable accuracy and precision under controlled conditions, the RE and DP methods are the best, followed by TDL and then CP.
MAINTENANCE
The next distinguishing factor for discussion is maintenance. All water activity instruments are easy to use as the sample is typically just placed in a chamber and a button pushed to start a read, but higher end versions of the methods offer additional features such as connectivity for data transfer and large touch screen interfaces. That said, the level of maintenance needed to maintain performance does vary by method. For water activity measurements, accuracy can only be reported for measurements made on samples of known water activity. Salt solutions, either saturated or unsaturated, have known water activity values, so accuracy and precision values reported for water activity methods represent their performance on these samples at 25°C under ideal conditions. Since the actual values are not known for other sample types, the instruments must be verified with salt solutions routinely to ensure that they are still performing in specification. When verification results do not fall within acceptable limits, the instrument must be updated through either cleaning or calibration. Water activity standards used for verification are available as either unsaturated salt solutions or saturated salt slurries. The advantage of saturated slurries is that they can be re-used for several years if maintained while the disadvantage is that if not maintained, they can give incorrect readings. The advantage of unsaturated salt solutions is that they require no maintenance, but the disadvantage is that they can only be used once. While verification is checking the instrument performance, calibration is making changes to instrument to correct readings that are not meeting specification. For the DP and TDL method, verification is required at least daily and when readings are not in specification, the sensors must be cleaned, reverified, and then a calibration offset applied if cleaning doesn’t resolve the problem. The frequency of verifications not meeting specification for DP and TDL, thus requiring some type of action, vary depending on the sample type and the quality of testing practices, but on average some action is needed every couple of days of testing. As mentioned earlier, because the RE and CP sensors are isolated, they require very little cleaning maintenance. However, the susceptibility of CP sensors to shifts in the electrical properties of the polymer still necessitates daily verification and frequent calibration. The stability of the RE method and its lack of sensitivity to contamination does not necessitate frequent adjustments and calibration is only needed every 1-2 months. The low level of maintenance needed to achieve reliable results with the RE method makes it the clear winner, followed by the CP and TDL methods, with DP having the lowest ranking due to the need of frequent cleaning.
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INTERFERENCE BY VOLATILES
The next distinguishing factor for discussion is interference by volatiles. The headspace of a water activity testing chamber will contain water vapor, but it will also contain any other volatile material in the sample. These volatile materials will interfere with most methods because they either co-condense on the mirror or are absorbed by the sensor itself. Only the TDL method is unaffected by the presence of volatiles because it utilizes optics that are tuned to wavelengths only absorbed by water molecules. The chilled mirror used for the DP method is particularly sensitive to specific types of volatiles such as propylene glycol and ethanol because as they co-condense on the mirror, test results are impacted immediately and are erroneously high. The RE method is also quite sensitive due to changes in resistivity as volatile ions are absorbed and, in some cases, this change is irreversible. The effect on the CP method requires longer exposure than the RE method, but it too will eventually be irreversibly changed. As a solution, some DP models offer both a DP and CP sensor, where the CP sensor is used on samples containing volatiles due to its lower sensitivity (the mirror is not cooled when testing with the CP senor to avoid cross contamination) but at the cost of lower accuracy and precision. The solution for RE models is to offer filters that allow water vapor to pass through to the electrolytic sensor but restrict movement of other volatiles at the cost of extended test time due to the water vapor moving through the filter. With its complete lack of sensitivity to volatiles, TDL is the clear winner in this category, followed by the CP, RE, and DP methods, with DP the lowest ranked due to its only solution for volatiles being to not use the sensor.
COST
The final distinguishing factor in our discussion is cost. Some buyers may claim that this is the most important factor. However, since instruments utilizing each method are offered at prices ranging from around $2,000 up to $12,000, with the price difference more a reflectance of accuracy and feature differences, cost is not as effective as the others in distinguishing between methods. That said, the CP method tends to be the least expensive while the TDL method is the most expensive.
SUMMARY
In conclusion, this discussion has used an honest approach that cuts through marketing jargon and company claims to differentiate between water activity testing methods using six important distinguishing factors. The resistive electrolytic method is stable and gives reliable results with almost no maintenance, but it needs filters to read samples containing volatiles and it has longer analysis times than DP and TDL due to the nature of its measurement method. The dew point method can give faster analysis times, but obtaining the specified accuracy and precision requires high maintenance and it cannot read samples with volatiles. The TDL method can read any samples with no limitations due to volatiles, but the method is expensive, unproven, and susceptible to changes in atmospheric pressure. The CP method is the least expensive and does not require much maintenance, but it is unreliable due to changes in the polymer and can’t achieve the high accuracy of the other methods. When weighing performance rankings for all factors and with no instantaneous test available to truly differentiate analysis time, the preferred method would be RE due to its reliability and low maintenance, followed by DP due to its speed, TDL due to its insensitivity to volatiles, and finally CP due to its low cost (Table 1).  Another factor that is not specific to the method, but just as important to consider when comparing water activity meters is the level of support provided by the manufacturer. All the methods discussed here can provide a water activity value, but it is also important to know what the value means and how it can it be best utilized. Answering these questions necessitates choosing a manufacturer that also knows the science of water activity and can provide the additional information and training needed to optimize water activity utilization for specific applications. Consequently, the expertise, application knowledge, and customer service of the manufacturer may play a larger role in who you choose as your water activity provider. This discussion hopefully has provided insights into the pros and cons of common water activity testing methods but also look for more discussions on how to effectively utilize your water activity testing results as well.
 
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