WATER ACTIVITY BUYING GUIDE

HOW TO CHOOSE A WATER ACTIVITY METER?

When it comes to water activity instrumentation, there are lot of choices out there. The one that is best for you will depend on several factors. If you just want a number, there are cheap options that will give a number, but come with no support. If you pride yourself on reliability, but do not want to spend all your time maintaining the instrument, there are options as well. The purpose of this guide is not to bombard you with reasons why one specific instrument is the best, but instead honestly present the strengths and weaknesses of each sensor type so you can make an educated decision based on your own priorities. Since within each sensor type there are instruments covering a range of accuracy, options and price, this guide will focus on sensor type and not specific instruments.

SENSOR TYPES

There are 4 main sensors that are used to determine the equilibrium relative humidity in the chamber:

PRIMARY VS. SECONDARY METHODS

Water activity is a measure of the energy status of water and is calculated as the partial vapor pressure divided by the saturated pressure at the same temperature. In practice, since the vapor pressure cannot be directly measured in the product, all water activity methods rely on placing the sample in a chamber and then waiting for the headspace to come to equilibrium with the sample so that the partial pressure in headspace is the same as the partial pressure in the sample. Then, the equilibrium relative humidity (ERH) of the headspace is measured and when divided by 100, gives the water activity

The various sensors used to measure water activity then only differ in how they determine the equilibrium relative humidity, making none of them a primary measurement of the vapor pressure in the sample, but all a primary measurement of equilibrium relative humidity. Consequently, any claims made of one sensor being primary vs. another being secondary is not critical to performance, but just marketing jargon.

READING VOLATILES

There is some confusion in the literature concerning the water activity testing of samples containing volatiles. The chilled mirror sensor in particular struggles with measuring volatile containing samples and there is not way to overcome this. The only choice is to use a different sensor. The same is true for capacitance sensors, although the impact is more delayed as the sensor becomes poisoned by absorbing the volatile. The electrolytic sensor will also be poisoned by volatiles and only the tunable-diode laser can handle volatile containing samples.

However, the electrolytic sensor from Novasina can be protected from volatiles using filters, making it possible to read most volatile containing samples. That means that the only sensor types that can actually read samples containing volatiles are the resistive electrolytic sensors from Novasina (with filters) and the TDL.

ISSUES WITH 5 MINUTES TEST TIME CLAIMS

There can be an abundance of confusion with water activity sensors concerning test time. Some chilled mirror and TDL sensor instruments claim a 5-minute test time while others offer fast or quick modes. Since water activity is an equilibrium measurement, a reading is not complete until equilibrium has been achieved and this process cannot be sped up, so any claim to a specific test time is illogical and would only be true for select samples. The reality is that most types of samples require a minimum of 5 minutes or more to reach true equilibrium and test times that are faster than that are either using a prediction or the system uses end-of-test settings that are not stringent enough to achieve true vapor equilibrium.

EXAMPLE

Low moisture, glassy food powders are an example of samples that require a surprisingly long time to reach true vapor equilibrium. A test run with a chilled mirror sensor or TDL will provide a completed reading in about 5 minutes versus an electrolytic sensor using the average mode stability setting will take about 30 minutes.

WHY IS THE OTHER SENSOR SO MUCH FASTER?

This can be discovered by simply repeating the test with the chilled mirror or TDL sensor for 30 minutes while tracking the result and an average of 0.01 aw drift can be observed

This indicates that while a result was given by the other sensors after 5 minutes, it was not the true water activity and was prematurely ending the test.

The cause of these premature readings is that the end-of-test setting for the other sensors, which is set by the manufacturer and cannot be altered, is not stringent enough and can be too easily achieved. Conversely, the end-of-test setting for the Novasina electrolytic sensor can be adjusted by the user to be made either more or less stringent. For example, the average setting in the Novasina instrument requires no change in water activity greater than 0.001 for 4 minutes and repeated measurements using the electrolytic sensor will not show the drift seen with the other sensors because the setting is stringent enough to ensure true vapor equilibrium. The more stringent settings of the Novasina can result in longer test times, but they also provide a true water activity.

ANY INSTRUMENT CAN BE MADE TO PROVIDE FAST READ TIMES BY MAKING THE TEST REQUIREMENTS LESS STRINGENT BUT AT WHAT COST?

CLEANING INSTRUMENTS

It is an undisputable fact that the chilled mirror and tunable- diode laser sensors, based on how they function, must be clean to provide good readings. Contamination will result in false determinations of vapor pressure and these sensors must be verified at least daily if not more often to make sure no sensor contamination has occurred that could be altering the readings. The electrolytic senor form Novasina, and to a lesser extent the capacitance sensor, do not have these same challenges because reflection and emissivity are not part of the measurement process. In addition, the Novasina electrolytic sensor is isolated from the chamber behind a protective filter that prevents access to the sensor by the contaminates.

Recently, a claim was made that a dirty chamber caused more problems for the electrolytic sensor than it did for a chilled mirror sensor. The key here is a dirty chamber, not a dirty sensor. A dirty chamber containing contaminates will in fact impact the results of any water activity instrument by altering the vapor pressure in the chamber. But that is a much different scenario than just the sensor being dirty.

When the study is altered and the contamination is applied to the sensor, the chilled mirror and TDL sensors are no longer able to read the standards in tolerance, while the electrolytic sensor from Novasina is not impacted because the sensor is protected.

SENSOR STRENGTH AND WEAKNESSES

SUMMARY

Based on this investigation of the various sensors for measuring water activity, each has its own strength and weaknesses and no one sensor type was ranked first in every category. When deciding which water activity sensor will work the best for your application, it is important to weigh the strengths and weaknesses of each sensor and choose the one that best aligns with your needs. If an average of all rankings is taken, the electrolytic sensor from Novasina does line up as the top ranked sensor across all the categories considered important when deciding which water activity instrument to buy.

All Novasina devices contain the electrolytic sensor. Learn more! www.novasina.ch