WATER ACTIVITY MEASUREMENT TECHNOLOGIES COMPARED: CHILLED MIRROR, ELECTROLYTIC, CAPACITIVE AND TDL SENSORS
Choosing a water activity meter is not simply a matter of selecting a sensor technology. Laboratories usually need to answer much more practical questions. How accurate and repeatable will the measurement be around the specification that matters? How stable will the system remain over years of routine use? What happens when products contain alcohols, organic acids, aromas or other volatile compounds? How much cleaning and maintenance will be required, and how easily can the laboratory verify that the instrument is still performing correctly?
These questions are more useful than dividing technologies into simple categories such as “direct” and “indirect”. Chilled mirror, resistive electrolytic, capacitive and tunable diode laser sensors determine the humidity condition above a sample using different physical principles, but the sensor principle alone does not define the performance of the complete water activity instrument.
ISO 18787 reflects this point by recognizing several measurement principles for the determination of water activity, including dew point, changes in the electrical conductivity of an electrolyte and changes in the permittivity of a polymer. The relevant question for the laboratory is therefore not which technology has the most attractive label, but which complete measurement system provides the accuracy, repeatability, stability and application robustness required for the samples being tested.
What all water activity instruments have in common
Regardless of sensor technology, water activity measurement begins with the sample. Once the sample is placed in a closed measurement chamber, water vapor moves between the product and the headspace until a sufficiently stable equilibrium condition has been established. The instrument then determines the equilibrium relative humidity and converts this into water activity at the defined temperature.
This means that measurement speed cannot be considered independently of the sample. A powder with a large exposed surface may equilibrate relatively quickly, while a dense confectionery product, coated sample or high-fat matrix can require more time. Sample preparation, temperature and the end-of-test criterion therefore influence measurement time regardless of which sensor technology is used.
The sensor technologies mainly differ in how they determine the humidity condition once this equilibrium is established. That difference affects practical aspects such as susceptibility to certain contaminants, sensor protection, maintenance and suitability for volatile-containing samples, but it does not by itself determine whether an instrument will be accurate or repeatable.
Resistive electrolytic sensing: high precision with a protected sensor concept
A resistive electrolytic sensor uses a hygroscopic electrolyte whose electrical resistance changes according to the equilibrium humidity in the measurement chamber. This response can be measured with very high sensitivity and converted into water activity.
One of the major practical advantages of this principle is that the sensing element does not need to be directly exposed to the sample environment. In Novasina systems, the sensor sits behind a mechanical protection filter, and application-specific chemical filters can be added when the sample contains potentially harmful volatile compounds. This creates a physical barrier between the product and the sensitive measurement cell while still allowing water vapor to reach the sensor.
The result is a particularly robust design for laboratories working with real products rather than ideal laboratory standards. Powders, fats, acids, alcohols, aromas and other compounds can all create contamination challenges for humidity sensors. Keeping the actual sensing element protected significantly reduces its direct exposure to these substances.
This protection concept also changes the maintenance requirements. The resistive electrolytic sensor itself does not require routine cleaning. Normal maintenance is therefore centred on keeping the sample chamber clean, periodically verifying the instrument against humidity standards and replacing the protective filter when its capacity has been reached.
For laboratories performing large numbers of routine measurements, this translates into a very low-maintenance workflow. There are no reagents or other measurement chemicals, no optical sensing surface that requires routine attention as part of the measurement principle, and the protected sensor does not require regular cleaning. The primary recurring items are reference standards and, where chemically challenging samples are measured, specific filters.
Protection against volatile compounds is a design feature, not simply an accessory
Volatile-containing samples are frequently presented as a binary problem: either a sensor can measure them or it cannot. In practice, the situation is considerably more complex because the type and concentration of the volatile compound matter.
A trace amount of aroma in a finished food presents a different challenge from concentrated ethanol, essential oils or aggressive organic acids. Rather than treating all of these samples identically, Novasina uses different chemical protection strategies depending on the expected exposure. The filter material is designed to adsorb the interfering or potentially damaging volatile compounds while allowing water vapor to pass through to the sensor.
This approach considerably broadens the application range of the resistive electrolytic sensor. Instead of exposing the measuring element directly and dealing with contamination afterwards, the objective is to prevent that contamination from reaching the sensor in the first place.
Practical work with demanding matrices illustrates the value of this approach. In tobacco, for example, comparative measurements using a protected resistive electrolytic sensor produced results that were statistically equivalent to TDL measurements across the four tobacco products tested. The application is particularly demanding because tobacco contains numerous aromatic and volatile compounds, yet the protected resistive electrolytic system was able to provide comparable water activity results with a substantially simpler and lower-maintenance measurement concept.
The same principle is applied to cannabis and other aromatic products, where terpenes and related compounds can influence or damage humidity sensors over time. Novasina therefore recommends the appropriate protective filter combination when such products are measured.
This does not mean that every volatile-containing product should automatically be measured with the same setup. Extremely aggressive matrices or products dominated by very high concentrations of volatile compounds should still be assessed individually. The strength of the protected resistive electrolytic concept is that the sensor can be adapted to a very broad range of real-world samples without exposing the measuring element unnecessarily.
Why high repeatability matters more
When laboratories work close to a specification limit, repeatability becomes one of the most important instrument characteristics. An accuracy specification tells you how closely an instrument should agree with a reference value, while repeatability tells you how consistently the instrument can reproduce its own measurement under the same conditions.
The LabMaster-aw neo is specified with an accuracy of ±0.003 aw and a typical repeatability of ±0.001 aw, together with a resolution of 0.0001 aw and full temperature control from 0 to 60°C.
A performance evaluation report provides additional evidence beyond the published specification. Repeated measurements were carried out across several SAL-T reference points spanning approximately 0.11 to 0.97 aw. The reported accuracy values in the summary results were between approximately 0.0005 and 0.0010 aw, while repeatability values were also between approximately 0.0005 and 0.0010 aw. The instrument therefore met and, at the tested reference points, exceeded its published performance specifications.
This is a much more useful basis for evaluating a measurement system than describing one sensor technology as inherently superior to another. The classification of a sensor explains how it produces its signal; repeated performance data show how well the complete instrument actually measures.
For laboratories carrying out release testing, stability studies or formulation development, this level of repeatability is particularly valuable because small changes in water activity can be distinguished from normal instrument variation. It also provides confidence when measurements are repeated across operators, days or different stages of a product study, provided that sampling and measurement conditions are controlled consistently.
Chilled mirror sensing: precise, but dependent on the condition of the optical surface
Chilled-mirror systems determine water activity by cooling an optical surface until condensation is detected. This principle can provide highly accurate measurements, but it also creates an important practical dependency: the mirror itself is part of the measurement and its condition must remain controlled. The optical system has to recognize the onset of water condensation correctly, which requires the mirror surface to be sufficiently clean and free from residues that could alter the optical response. For pharmaceutical applications in particular, where powders and excipients can generate fine dust and formulations may contain volatile compounds, the optical mirror must remain sufficiently clean and free from deposits or co-condensing substances to ensure reliable detection of water condensation.
This becomes particularly relevant in routine laboratories where the instrument is exposed to powders, fats, aromas or volatile compounds. A contaminated mirror can influence the measurement, and the problem may not always be obvious simply from the reported aw value. Routine verification and cleaning are therefore important to confirm that the system is still measuring correctly. Manufacturers of chilled-mirror instruments themselves identify insufficient cleaning as a common cause of measurement problems and recommend frequent verification when instruments are used daily or in demanding environments.
Volatile compounds create an additional challenge because the optical system detects the formation of condensate on the mirror; it does not chemically analyse whether that condensate consists exclusively of water. If another volatile compound co-condenses, the detected condensation event can therefore shift the apparent dew point and lead to an inaccurate water activity result. Published work confirms that chilled-mirror measurements can become unreliable in the presence of significant concentrations of non-aqueous volatiles. Technical guidance for chilled-mirror systems explains that certain volatile compounds can condense on the mirror surface and interfere with the dew-point determination. [1]
The practical concern is not only that volatile interference can lead to an inaccurate result, but that the reported aw value alone does not necessarily reveal whether such interference has occurred. Earlier chilled-mirror guidance therefore recommended comparing the dew-point result with a second sensor principle when volatile interference was suspected. A significant difference between the two measurements was used as an indication that non-water volatiles were affecting the mirror measurement. [2]
For laboratories measuring clean, non-volatile samples and maintaining the mirror correctly, chilled-mirror technology can provide reliable performance. For laboratories processing a broad range of real-world samples, however, the requirement to keep the active optical surface clean and the possibility of undetected co-condensation should be considered as part of the routine measurement risk and maintenance effort.
Capacitive sensors: practical where the required performance matches the application
Capacitive humidity sensors use a hygroscopic polymer whose dielectric properties change as the surrounding humidity changes. The principle is widely established and allows relatively simple and compact measurement systems.
As with any sensor technology, the instrument specification matters more than the generic sensor category. Polymer composition, electronics, temperature compensation, calibration strategy and exposure to chemical compounds can all influence long-term performance.
For routine applications where a wider measurement tolerance is acceptable, capacitive systems can provide a practical solution. Laboratories working with narrow release specifications or looking for very high repeatability should compare the actual accuracy and repeatability of the complete instrument rather than assuming that all humidity sensors provide equivalent performance.
TDL sensing: a specialized solution for highly volatile matrices
Tunable diode laser systems use laser spectroscopy to selectively quantify water vapor. This selectivity is particularly attractive when the sample headspace contains very high concentrations of volatile compounds that would strongly interfere with other measurement principles.
For specific matrices such as very high-alcohol formulations, concentrated aroma systems or essential oils, this can be a significant advantage. The trade-off is that TDL is a more specialized measurement concept and may involve higher acquisition and maintenance costs than are necessary for a conventional food or pharmaceutical QC laboratory.
For most laboratories, the decision should therefore be driven by the actual sample portfolio. A specialized volatile-resistant technology provides little additional value if almost all routine samples are conventional powders, bakery products, meat products or pharmaceutical solids. Conversely, where strongly volatile matrices dominate the workload, that additional selectivity can justify the specialized technology.
Maintenance and operating cost should be considered over the lifetime of the instrument
Purchase price is only one part of the cost of a laboratory instrument. Routine maintenance, sensor cleaning, consumables, verification standards, replacement parts, downtime and the amount of analyst intervention required over several years can be equally important.
This is another area where a protected resistive electrolytic sensor has a practical advantage. The sensor itself requires no routine cleaning, and its exposure to both physical contamination and selected volatile compounds can be reduced through mechanical and chemical protection filters. Periodic verification and calibration with reusable humidity standards and replacement of an exhausted filter are straightforward procedures that can be performed by the laboratory user.
This keeps the routine operating burden low and cost-effective. There is no measurement reagent, there is no routine sensor-cleaning procedure, and filters are only required according to the sample chemistry and measurement frequency.
For laboratories evaluating total cost of ownership, this deserves as much attention as headline measurement speed. An instrument that remains stable, protected and easy to verify over years of routine use can be more economical than a system whose sensing surface requires greater attention after exposure to challenging samples.
Measurement speed should always be considered together with equilibrium
A rapid measurement is valuable when many samples have to be released every day, but water activity remains an equilibrium measurement regardless of the sensor technology used.
The instrument can only measure the conditions created by the sample. Products with very different moisture mobility will therefore equilibrate at different rates. Sample size, exposed surface area, coatings, fat content, sugar concentration and temperature can all influence how quickly the headspace becomes sufficiently stable.
This is why a universal measurement-time claim should be interpreted carefully. The meaningful comparison is not simply which instrument produces the earliest number, but whether that number remains stable as the sample continues to equilibrate.
Novasina has investigated this with real products including crème filling, gummies and beef jerky. In these tests, early measurements from a rapid dew-point approach were obtained sooner, but subsequent readings differed from the first result by more than the instrument’s published accuracy. The LabMaster-aw neo took longer under its defined stability criterion but produced a result based on a more stable equilibrium condition.
For routine QC, the most useful instrument is therefore the one that offers the appropriate balance between throughput and confidence in the final result. Different stability settings can be used depending on the purpose of the measurement, and ISO 18787 measurement criteria are available when a standardized equilibrium endpoint is required.
Temperature control is part of measurement quality
Water activity changes with temperature, which means that temperature capability should be considered together with the humidity sensor when comparing instruments. For simple routine measurements carried out under stable laboratory conditions, extensive temperature control may not always be necessary. For formulation studies, shelf-life research, regulated work or products tested across different environmental conditions, however, it can become critical.
The LabMaster-aw neo combines the resistive electrolytic sensor with full programmable temperature control from 0 to 60°C, allowing samples to be measured under defined and reproducible conditions across a broad range.
This illustrates another reason why sensor-only comparisons can be misleading. The performance of a water activity system depends on the complete measurement environment, and temperature control is a significant part of that environment.
Which water activity sensor technology should you choose?
There is no reason to select an instrument simply because its sensor is described as direct, indirect, primary or secondary. Those terms describe the measurement mechanism but tell the user relatively little about how the complete system will perform with an actual product.
For a laboratory that requires high accuracy and excellent repeatability across a broad range of food and pharmaceutical products, resistive electrolytic technology offers a particularly strong combination. It can achieve high-end measurement performance, the sensor can be physically and chemically protected from the sample environment, the measuring element does not require routine cleaning, temperature can be tightly controlled, maintenance requirements remain low and at a low total cost of ownership.
The protection concept becomes particularly valuable when a single instrument has to handle very different sample types. Rather than exposing the sensing element directly and dealing with contamination afterwards, suitable filters can be selected for many volatile-containing matrices before they reach the sensor.
For laboratories dominated by extremely high volatile concentrations, a specialized spectroscopic technology may still be appropriate, while simpler capacitive instruments can be sufficient when the required accuracy is less demanding. Chilled-mirror systems remain an established high-accuracy option for compatible samples.
The decision should therefore be based on the laboratory’s actual workload rather than on a theoretical ranking of sensor principles. Accuracy, repeatability, temperature control, sensor protection, sample compatibility, verification/calibration, maintenance and total operating effort should all be considered together.
Compare overall performance, not technologies
A good water activity instrument should continue to provide trustworthy results long after the initial purchasing decision. This requires more than an attractive sensor description. It requires stable accuracy, high repeatability, controlled measurement conditions, a sensible approach to contamination and a practical method of verifying performance over time.
Resistive electrolytic sensing demonstrates particularly well why measurement principles should not be confused with measurement quality. The technology is explicitly recognized within ISO 18787, can achieve ±0.003 aw instrument accuracy and ±0.001 aw published repeatability, and allows the sensitive measuring element to remain protected behind mechanical and application-specific chemical barriers. Performance testing across a broad aw range further demonstrated repeatability and accuracy down to approximately 0.0005–0.0010 aw at the tested reference points.
Combined with full temperature control, a sensor that requires no routine cleaning and a low-maintenance filter concept for challenging samples, this provides a robust measurement platform for laboratories that need reliable results day after day.
Ultimately, the best sensor technology is the one that continues to deliver the measurement performance your laboratory requires with the samples you actually measure.
FAQ for SEO
Is a resistive electrolytic water activity sensor accurate?
Yes. High-performance resistive electrolytic systems can achieve very high accuracy and repeatability. The LabMaster-aw neo, for example, is specified at ±0.003 aw accuracy with typical repeatability of ±0.001 aw. A performance evaluation reported accuracy and repeatability values of approximately 0.0005–0.0010 aw across the tested reference points.
Do resistive electrolytic water activity sensors need cleaning?
The protected sensing element itself does not require routine cleaning and should not be cleaned or touched. The measurement chamber should still be kept clean as part of normal laboratory practice. Protective filters reduce direct exposure of the sensor to sample material and selected volatile compounds.
Can resistive electrolytic sensors measure samples containing volatiles?
Many volatile-containing samples can be measured using the appropriate chemical protection filter. Novasina offers protection for applications involving substances such as alcohols, organic acids, glycerin, PEG, strong aromas and essential oils. The exact sample chemistry and concentration should always be considered when selecting the appropriate protection.
Do protective filters increase maintenance?
Filters are consumable protection elements, but replacement is straightforward and helps prevent the more disruptive maintenance associated with direct sensor contamination. The appropriate replacement frequency depends on sample exposure and measurement volume.
What matters more: direct or indirect water activity measurement?
The distinction explains how the sensor generates its signal but does not determine instrument performance on its own. Accuracy, repeatability, temperature control, sample equilibration, contamination protection and verification of the complete measurement system are more meaningful criteria when selecting a water activity meter.
Sources
[1] Campbell, G. S., Galloway, M. & Campbell, Z. (2022). “Measurement of Water Activity in the Presence of High Volatile Concentrations Using a Tunable Diode Laser – Single Laboratory Validation, First Action 2021.04.” Journal of AOAC International, 105(3), 649–656. DOI: 10.1093/jaoacint/qsac003.
[2] Decagon Devices, Inc. AquaLab 4TE Operator’s Manual, “Volatile Samples”, p. 49. Guidance on volatile co-condensation on the chilled mirror and comparison with capacitance measurement to identify interference.
