METER AQUALAB PAWKIT User manual

PAWKIT

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TABLE OF CONTENTS
1. Introduction.............................................................................................. 1
2. Operation ...................................................................................................2
2.1 Setup .........................................................................................................2
2.1.1 Sample Preparation ......................................................................... 3
2.1.2 Taking a Measurement.....................................................................4
3. System......................................................................................................... 9
3.1 Specifications............................................................................................9
3.2 PAWKIT Components................................................................................ 10
3.3 Theory...................................................................................................... 11
3.3.1 Moisture Content........................................................................... 11
3.3.2 Water Activity ................................................................................ 12
3.3.3 Water Potential .............................................................................. 15
3.3.4 Sorption Isotherms ........................................................................ 15
4. Service....................................................................................................... 16
4.1 Verification and Calibration ..................................................................... 16
4.1.1 Verification Standards ................................................................... 16
4.1.2 Verify Calibration ........................................................................... 17
4.2 Maintenance............................................................................................ 19
4.2.1 Cleaning......................................................................................... 19
4.2.2 Filter Replacement ........................................................................ 21
4.2.3 Battery Replacement..................................................................... 22
4.3 Repairs .................................................................................................... 25
4.4 Troubleshooting ....................................................................................... 25
10366-03
6.12.2019

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4.5 Customer Support.................................................................................... 26
4.6 Terms and Conditions .............................................................................. 27
References .................................................................................................... 28
Index ................................................................................................................. 29

1
PAWKIT
1. INTRODUCTION
Thank you for choosing the AQUALAB PAWKIT Water Activity Meter from METER Group.
The PAWKIT is designed to be a simple, rapid, and portable system for water activity
measurement to ensure product safety. It is easy to use, durable, and requires
little maintenance.
This manual includes instructions for setting up, verifying the calibration, preparing samples,
and maintaining and caring for the PAWKIT. Please read these instructions before operating
the PAWKIT to ensure that the instrument performs to its full potential.
Verify that all PAWKIT components are included and appear in good condition:
• PAWKIT main unit
• Durable carrying case
• Disposable sample cups with lids
• Spare sensor filters
• Reusable stainless steel cup
• Two vials each of the following verification standards:
2.33 mol/kg NaCl (0.920 aw)
6.00 mol/kg NaCl (0.760 aw)
13.41 mol/kg LiCl (0.250 aw)
• AQUALAB Cleaning Kit
• Calibration certificate

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OPERATION
2. OPERATION
Please read all instructions before operating the PAWKIT to ensure it performs to its
full potential.
PRECAUTIONS
METER products are built to the highest standards, but misuse may damage the
product and possibly void the manufacturer’s warranty. Follow the recommended setup
instructions and arrange proper protections to safeguard products from damage.
2.1 SETUP
Follow the steps listed in Table 1 to set up the PAWKIT and start collecting data.
SAMPLING PRECAUTIONS
Sensor calibration can shift when the PAWKIT is exposed to a variety of volatile substances
or to samples with water activities near 1.00 over a long period of time. To prevent a sensor
calibration shift, always remove samples as soon as the PAWKIT is finished sampling (a beep
signals when it is finished) to avoid damage to the sensor. If a sample is accidentally left
in the chamber for an extended period of time, be sure to check the calibration when the
instrument is used next.
Table 1 Preparation
Tools Needed
PAWKIT unit
Material to sample
Plastic sample cup
AQUALAB cleaning kit
WARNING: The PAWKIT is not intended for outdoor use.
Preparation
Select Clean, Level Location
Always place the PAWKIT on a level surface when measuring to ensure correct
and consistent operation.This also avoids the risk of sample material spilling
inside the instrument and causing contamination.
Place the PAWKIT in a location where the temperature remains fairly stable,
to avoid inaccurate readings (away from air conditioner and heater vents,
windows, etc.)
IMPORTANT: Temperature fluctuations or vibrations will strongly affect the accuracy of
measurement results.
Sample Preparation
Prepare the sample as described in Section2.1.1.

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PAWKIT
Table 1 Preparation (continued)
Installation Sample Placement
Place the PAWKIT over the sample cup as described
2.1.1 SAMPLE PREPARATION
Carefully prepare samples to get the best readings possible. Always use clean sampling
practices to ensure accurate and repeatable readings. Follow the steps below when
preparing samples.
1. Make sure the sample to be measured is homogenous.
Multicomponent samples (e.g., muffins with raisins) or samples that have outside
coatings (like deep-fried, breaded foods) can be measured, but may take longer to
equilibrate. Samples like these may require additional preparation (cutting, crushing, or
grinding) to obtain a representative sample.
2. Wash hands thoroughly or put gloves on prior to handling the PAWKIT and sample cup.
3. Fill the sample cup no more than half full.
Completely cover the bottom of the cup with the sample, if possible. The PAWKIT is able
to accurately measure a sample that leaves small spaces of the cup bottom exposed.
For example, raisins only need to be placed in the cup and not flattened to cover the
bottom. A larger sample surface area increases instrument efficiency by shortening
the time needed to reach vapor equilibrium. However, if the sample cup is too full,
contamination of the sensor becomes a risk, which can lead to inaccurate readings.
4. Clean the bottom, edges, and rim of the sample cup thoroughly with KIMWIPES®
strips (Section4.2).
Wipe any excess sample material from the rim of the cup with a clean KIMWIPES tissue.
Material left on the rim or the outside of the cup can be transferred to subsequent
samples and may affect the reading accuracy. The rim of the cup forms a vapor seal
with the sensor. Any sample material left on the cup rim may prevent this seal and
contaminate future samples.
If a sample reading will be taken at a later time, put the sample cup disposable lid on the cup
to restrict water transfer. To seal the lid, wrap tape or Parafilm®wrapper completely around
the cup/lid junction. It is necessary to seal the cup if it is going to be a long time before
measurements will be made again.
The PAWKIT makes its most accurate measurements when the temperature of the sample
and instrument are within 1 ˚C. If the sample is too warm, the thermometer icon on the left of
the screen appears (Figure1).

4
OPERATION
Thermometer
icon
Figure1 Thermometer icon on screen
The instrument beeps, when the sample temperature is too high, indicating there is a danger
of condensing water in the sample chamber and on the sensor. If this warning appears while
sampling, remove the PAWKIT, place the cup lid on the sample, and wait until the sample has
reached ambient temperature before attempting to read again.
If the sample is colder than the ambient temperature of the PAWKIT, the accuracy of the
reading after 5 min may be questionable. Wait until the sample temperature is similar to that
of the PAWKIT.
2.1.2 TAKING A MEASUREMENT
1. Open the PAWKIT by holding the case near the LCD with one hand and pulling down on
the plastic sensor cover tab with the other hand.
Snap cover
tab in place
Cover tab
Figure2 Open the PAWKIT
The sensor cover rotates and snaps into the open position.

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PAWKIT
2. Place the prepared sample cup onto a level surface.
Sample cup
Figure3 Sample cup placement
3. Place the opened PAWKIT onto the prepared sample cup.
The cup fits under the sensor into a recess in the bottom of the PAWKIT.
Figure4 PAWKIT placement on sample cup
A correctly positioned cup results in the PAWKIT being level on the bench when sitting on
the cup and the sensor cover legs. Ensure the rim of the cup is entirely within the recess
of the sample chamber (Figure4). Otherwise, the PAWKIT may not be level on the bench,
and the cup might not make a vapor seal with the sensor.

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OPERATION
4. Press the I button (located on the left) to turn on the instrument.
The instrument will display the last reading taken. This allows the PAWKIT to take a
measurement without anyone attending the instrument throughout the measurement. If
the PAWKIT is already on, proceed to the next step.
I
button
Figure5 I button
5. Press the I button a second time to begin the water activity measurement. The LCD
display resets to 0.00 aw.
NOTE: Pressing the I button any time during a measurement restarts the water activity measurement.
Once the measurement process has been started, the PAWKIT begins to display water
activity measurements as well as temperature after 5 s, updating the display every
second thereafter.
During this time the sunburst icon to the right of the water activity value indicates it is
measuring. As it measures, the beams of the sunburst move from left to right.
NOTE: The final water activity measurement is not displayed until the instrument beeps and the sunburst icon
disappears from the screen.
Sunburst and
beams icon
Figure6 Sunburst and beams icon

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PAWKIT
If an error code of 9.99 is received at any time during the process, it indicates that the
sensor has failed and that the instrument needs to be serviced. Refer to Section4.4 for
instructions on how to return the PAWKIT for repair.
NOTE: DO NOT lift or move the instrument during the measurement because it could contaminate the chamber,
break the chamber vapor seal, and invalidate the water activity measurement.
Figure7 DO NOT lift or move instrument during measurement
6. After 5 min, the instrument displays the final water activity and beeps 5 times. The
sunburst disappears when the water activity reading is finished.
NOTE: METER recommends recording the reading value before proceeding.
At this point, either restart the measurement by pressing I button again or end the
measurement procedure.
7. Remove the sample cup by lifting the PAWKIT
Lift the PAWKIT straight up as shown in Figure8 to avoid spilling the sample cup.
The sample may now be discarded or covered with a lid if it is to be remeasured at a
later time.

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OPERATION
Figure8 Remove PAWKIT when measurement complete
8. When finished taking readings, close the instrument.
With one hand holding the case near the LCD, pull down on the plastic sensor cover
tab with the other hand and rotate until it snaps into the closed position covering
the sensors.
Snap cover
tab in place
Figure9 Closing the PAWKIT
To turn off the PAWKIT, leave it idle for more than 5 min and it will shut off automatically. If
the PAWKIT has automatically shut itself off, pressing the I button wakes up the instrument
and displays the last water activity measurement.
NOTE: Remember to close the lid before storing the case.

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PAWKIT
3. SYSTEM
This section describes the specifications, components, and theory of the PAWKIT.
3.1 SPECIFICATIONS
MEASUREMENT SPECIFICATIONS
Water Activity
Range 0.00-1.00 aw
Resolution 0.01
Accuracy ±0.02
Sample Temperature
Resolution 0.1 °C
Accuracy ±0.2 °C
Read Time
5 min
PHYSICAL SPECIFICATIONS
Dimensions
Length 10.7 cm (4.21in)
Width 6.6 cm (2.60 in)
Height 2.0 cm (0.79 in)
Case Material
Stainless steel and valox 325 plastic
Sample Dish Capacity
7.5 mL, recommended (15 mL, full)
Weight
115 g (4 oz)
Display
64 ×128 graphical

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SYSTEM
ELECTRICAL AND TIMING CHARACTERISTICS
Operating Temperature
Minimum 4 °C
Typical NA
Maximum 50 °C
Operating Environment
0–90% relative humidity (noncondensing)
Power
2–3 V 16 mm coin cell batteries (life of approximately 3 years)
COMPLIANCE
Manufactured under ISO 9001:2015
EM ISO/IEC 17050:2010 (CE Mark)
3.2 PAWKIT COMPONENTS
Figure10 shows the features on the top and Figure11 shows hardware on the bottom of
the PAWKIT.
LCD
I
button
II
button
Sample
cup
Figure10 PAWKIT features

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PAWKIT
Thermopile
sensor
Sensor
chamber
Sensor
cover
Sensor
filter
Figure11 PAWKIT hardware
The PAWKIT uses a capacitance humidity sensor to measure the water activity of a sample.
The sensor is suspended behind the sensor filter and uses a special polymer material
sandwiched between two microporous electrodes to sense humidity changes. The sensor
measures a specific capacitance, which is translated by the software and displayed as
water activity on the instrument screen. At equilibrium, the relative humidity of the air in the
chamber is the same as the water activity of the sample.
The PAWKIT is accurate to ±0.02 aw. For many applications, this accuracy is more than
adequate. If a higher accuracy is required, METER recommends using the AQUALAB 4TE or
TDL Water Activity Meter. Contact Customer Support for more details.
The PAWKIT system comes with 60 disposable plastic sample cups and one stainless steel
sample cup. Additional quantities of cups can be purchased from METER.
3.3 THEORY
Water is a major component of foods, pharmaceuticals, and cosmetics. Water influences
the texture, appearance, taste and spoilage of these products. Quantifying these attributes
require two basic types of water analysis: moisture content and water activity (aw):
3.3.1 MOISTURE CONTENT
Moisture content implies a quantitative analysis to determine the total amount of water
present in a sample. There are two primary methods for determining moisture content: loss
on drying and Karl Fischer titration. Secondary methods, such as near infrared (NIR) and
nuclear magnetic resonance (NMR), may also be used. Moisture content determination
is essential in meeting product nutritional labeling regulations, specifying recipes, and
monitoring processes.

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SYSTEM
However, moisture content alone is not a reliable indicator for predicting microbial responses
and chemical reactions in materials. The limitations of moisture content measurement are
attributed to differences in the intensity that water associates with other components.
3.3.2 WATER ACTIVITY
Water activity (aw) is a measurement of the energy status of the water in a system. The
value indicates how tightly water is bound, structurally or chemically, within a substance.
The lower a sample’s water activity, the more tightly bound that water is within the sample.
The concept of water activity is of particular importance in determining product quality
and safety. It predicts safety and stability with respect to microbial growth, chemical and
biochemical reaction rates, and physical properties.
Water activity is a measure of the energy status of the water in a system and is a far better
indicator of product stability than moisture content. Figure12 shows how the relative
activity of microorganisms, lipids, and enzymes relate to water activity. While other factors,
such as nutrient availability and temperature, can affect the relationships, water activity is
the best single measure of how water affects these processes.
Ionic
Lipid oxidation
Browning reactions
Moisture sorption isotherm
Enzyme activity
Mold growth
Yeast
Bacteria
Covalent Solute &
Capillary
Water Activity
Water Activity—Stability Diagram
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1
Moisture Content
Relative Reaction Rate
Figure12 Water activity diagram adapted from Labuza (1976)
Researchers measure the water activity of a system by equilibrating the liquid phase
water in the sample with the vapor phase water in the headspace and measuring the
relative humidity of the headspace. In the PAWKIT, a sample in a sample cup is placed on
a level surface and the PAWKIT is seated on top of the cup, creating a seal. This chamber
contains an infrared thermometer and a capacitive humidity sensor. Changes in the
electrical capacitance of the polymide layer of the sensor occur as the relative humidity
of the chamber changes. The PAWKIT computes the relative humidity of the headspace

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PAWKIT
by monitoring the change in electrical capacitance. When the water activity of the sample
and the relative humidity of the air are in equilibrium, the measurement of the headspace
humidity gives the water activity of the sample.
In addition to equilibrium between the liquid phase water and the vapor phase, the internal
equilibrium of the sample is important. If a system is not at internal equilibrium, one might
measure a steady vapor pressure (over the period of measurement) which is not the true
water activity of the system. An example of this might be a baked good or a multicomponent
food. Initially out of the oven, a baked good is not at internal equilibrium; the outer surface is
at a lower water activity than the center of the baked good. One must wait a period of time in
order for the water to migrate and the system to come to internal equilibrium. It is important
to keep in mind that by this definition, water activity is an equilibrium measurement.
The water activity of a sample is influenced by factors that affect the binding of water. These
factors include temperature, osmotic, matrix, and pressure effects. Typically, water activity
is measured at atmospheric pressure, so only the osmotic, temperature, and matrix effects
are important.
TEMPERATURE EFFECTS
Temperature plays a critical role in water activity determination. Most critical is the
measurement of the difference between sample and the PAWKIT temperature. For water
activity measurements to be accurate to 0.02, temperature difference measurements need
to be accurate to 0.02 °C. The PAWKIT infrared thermometer measures the temperature of
the sample surface. The thermometer is carefully calibrated to minimize temperature errors;
however, achieving 0.02 °C accuracy is difficult when temperature differences are large. Most
accurate measurements are obtained when the sample is near instrument temperature.
Another effect of temperature on water activity occurs when samples are near saturation.
A sample that is close to 1.0 awand is warmer than the PAWKIT will cause water to
condense within the chamber. This causes errors in the measurement and in subsequent
measurements until the condensation disappears. For example, a sample at 0.75 awonly
needs to be 4 °C above the sample chamber temperature to cause condensation to form. For
high water activity samples, the operator needs to be aware that condensation can occur if a
sample is warmer than the PAWKIT.
The AQUALAB 4TE and TDL have several advantages in having a temperature-controlled
chamber. A few major reasons are detailed here.
1. Research purposes.
Researchers can use temperature control to study the effects of temperature on the
water activity of a sample, make a comparison of the water activity of different samples
independent of temperature, and conduct accelerated shelf-life studies or other
water activity studies where temperature control is critical. There are many shelf-life,
packaging, and isotherm studies in which temperature control would be very beneficial.

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SYSTEM
2. Compliance with government or internal regulations for specific products.
Though the water activity of most products varies by less than ±0.02 per degree Celsius,
some regulations require measurement at a specific temperature. The most common
specification is 25 °C, though 20 °C is sometimes indicated.
3. Minimization of extreme ambient temperature fluctuations.
If the environmental and PAWKIT temperatures fluctuate by as much as ±5 °C daily,
water activity readings may vary by ±0.01 aw. Temperature control eliminates variations
caused by changes in ambient conditions.
The AQUALAB 4TE and TDL models have thermoelectric components installed to allow the
instrument to maintain a set chamber temperature.
The water activity of the water in a system is influenced by factors that affect the binding
of water. They include osmotic, matrix, and pressure effects. Typically water activity is
measured at atmospheric pressure, so only the osmotic and matrix effects are important.
OSMOTIC EFFECTS
Osmotic effects are well known from biology and physical chemistry. Water is diluted when
a solute is added. If this diluted water is separated from pure water by a semipermeable
membrane, water tends to move from the pure water side through the membrane to the side
with the added solute. If sufficient pressure is applied to the solute-water mixture to just
stop the flow, this pressure is a measure of the osmotic potential of the solution. Addition
of 1 mol of an ideal solute to 1 kg of water produces an osmotic pressure of 22.4 atm. This
lowers the water activity of the solution from 1.0 to 0.98 aw. For a given amount of solute,
increasing the water content of the systems dilutes the solute, decreasing the osmotic
pressure, and increasing the water activity. Since microbial cells are high concentrations
of solute surrounded by semipermeable membranes, the osmotic effect on the free
energy of the water is important for determining microbial water relations and therefore
microbe activity.
MATRIX EFFECTS
The sample matrix affects water activity by physically binding water within its structure
through adhesive and cohesive forces that hold water in pores and capillaries and to particle
surfaces. If cellulose or protein were added to water, the energy status of the water would be
reduced. Work would need to be done to extract the water from this matrix. This reduction in
energy status of the water is not osmotic, because the cellulose or protein concentrations
are far too low to produce any significant dilution of water. The reduction in energy is the
result of direct physical binding of water to the cellulose or protein matrix by hydrogen
bonding and van der Waal forces. At higher water activity levels, capillary forces and surface
tension can also play a role.

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PAWKIT
3.3.3 WATER POTENTIAL
Water activity is closely related to a thermodynamic property called the water potential, or
chemical potential (µ) of water, which is the change in Gibbs free energy (∆G) when water
concentration changes. Equilibrium occurs in a system when µis the same everywhere in
the system. Equilibrium between the liquid and the vapor phases implies that µis the same
in both phases. This means the measured water potential of the vapor phase and can be
used to determine the water potential of the liquid phase. Gradients in µare driving forces
for moisture movement. In an isothermal system, water tends to move from regions of high
water potential (high aw) to regions of low water potential (low aw). Moisture content is not a
driving force for water movement, except in homogeneous materials.
3.3.4 SORPTION ISOTHERMS
Changes in water content affect both the osmotic and matrix binding of water in a product.
Therefore a relationship exists between the water activity and moisture content. This
relationship is called the sorption isotherm and is unique for each product. Besides being
unique to each product, the isotherm changes depending on whether it was obtained by
drying or wetting the sample. Typically, large safety margins are built into water content
specifications to allow for these uncertainties.
While the sorption isotherm is often used to infer water activity from moisture content, one
could easily go the other direction and infer the moisture content from water activity. This
is particularly attractive because water activity can be measured much more quickly than
water content. This method gives particularly good precision in the center of the isotherm. In
order to infer moisture content from water activity, an isotherm for the particular product is
needed. METER can run the isotherm for a fee. Visit metergroup.com/food/services/water-
activity-and-isotherm-testing for more information.

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SERVICE
4. SERVICE
This section describes the calibration and maintenance of the PAWKIT. Troubleshooting
solutions and customer service information are also provided.
4.1 VERIFICATION AND CALIBRATION
The PAWKIT takes water activity measurements by measuring the change in electrical
properties of a special polymer held between two electrodes. Because of the nature of the
capacitance humidity sensor, it is important to verify the PAWKIT water activity calibration
against known standards to guarantee optimal performance and accuracy. METER
recommends verification daily, once per shift, or before each use. METER also recommends
annual factory calibration to maintain optimal performance.
4.1.1 VERIFICATION STANDARDS
The PAWKIT uses three calibration standards:
• 6.00 mol/kg NaCl (0.760 aw)
• 13.41 mol/kg LiCl (0.250 aw)
• 2.33 mol/kg NaCl (0.920 aw)
Verification standards are specially prepared, unsaturated salt solutions having a specific
molality and water activity value that are accurately measurable. The verification standards
that were sent with the initial shipment are very accurate and readily available from METER.
The verification standards have been produced under a strict quality assurance regime, and
their accuracy is verified by an independent third party instrument. They are very accurate,
easy to use, and readily available from METER. Using verification standards to verify
accuracy can greatly reduce preparation errors. For these reasons, METER recommends
using these standards for the most accurate PAWKIT calibration. The verification standards
are shelf-stable for 1 year.
To use a verification standard, remove the twist top and pour the contents into an AQUALAB
sample cup. Information about the standard value and molality can be found printed on
the outside of the plastic vial. If for some reason METER verification standards are not
available, contact Customer Support for instructions on how to create a verification standard
salt solution.
The capacitance sensor can exhibit hysteresis. When a high water activity standard or
sample is measured, this may influence future readings. After measuring a high water
activity sample, allow 1 h for the sensor to dry. Additionally, desiccant or activated carbon
can be used to accelerate the drying process.
NOTE: To avoid inaccurate water activity readings, verification standards should be used once immediately after
opening and not stored in sample cups for repeated use.