Decagon Devices Pawkit Operator's Manual
Portable Water Activity
Measurement System
Operator’s Manual
Ve r. 1.3
Decagon Devices, Inc.
950 NE Nelson Court
Pullman , WA 99163
tel: (509) 332-5158
fax: (509) 332-5158
www.decagon.com/pawkit
pawkit@decagon.com
Copyright 2001
Decagon Devices, Inc.
All rights reserved
Pawkit
Table of Contents
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 1
About this Manual . . . . . . . . . . . . . . . . . . . . . 1
Customer Service. . . . . . . . . . . . . . . . . . . . . . 1
Warranty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Note to our Users . . . . . . . . . . . . . . . . . . . . . 2
Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . . 3
2. About the Pawkit . . . . . . . . . . . . . . . . . . 5
How Pawkit works. . . . . . . . . . . . . . . . . . . . . 5
Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Getting Started . . . . . . . . . . . . . . . . . . . . . . . 6
Components of your Pawkit system: . . . . 6
Preparing for Operation . . . . . . . . . . . . . . . 6
3. Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Sample Preparation and Insertion. . . . . . . . 9
Sample Preparation . . . . . . . . . . . . . . . . . . 9
Sample Insertion and
chamber movement . . . . . . . . . . . . . . . . . . 11
Taking Measurements . . . . . . . . . . . . . . . . . 15
Turning it off . . . . . . . . . . . . . . . . . . . . . . . . . 18
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Calibration Standards . . . . . . . . . . . . . . . 19
Steps in Calibration . . . . . . . . . . . . . . . . 20
Sampling Precautions. . . . . . . . . . . . . . . . . .23
Pawkit and Temperature . . . . . . . . . . . . . .24
4. Cleaning and Maintenance . . . . . . . . 26
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Pawkit
Table of Contents
Cleaning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .27
Disassembly . . . . . . . . . . . . . . . . . . . . . . 27
Sensor Filter Cleaning/Replacement. . . 30
Chamber Cleaning Instructions . . . . . . . . 31
Battery Replacement . . . . . . . . . . . . . . . 32
5. Repair Instructions . . . . . . . . . . . . . . . . 36
Shipping Directions . . . . . . . . . . . . . . . . . . .36
Repair Costs . . . . . . . . . . . . . . . . . . . . . . . . .37
6. Theory: Water Activity
in Products . . . . . . . . . . . . . . . . . . . . . . . .38
Water content . . . . . . . . . . . . . . . . . . . . . . . 38
Water activity . . . . . . . . . . . . . . . . . . . . . . .39
Effect of Temperature
on water activity . . . . . . . . . . . . . . . . . . . . . 41
Water Potential . . . . . . . . . . . . . . . . . . . . . .42
Factors in determining
Water Potential . . . . . . . . . . . . . . . . . . . . . .42
Sorption Isotherms—relating aw
to water content . . . . . . . . . . . . . . . . . . . . .44
7. Further Reading . . . . . . . . . . . . . . . . . . .47
Water Activity Theory
and Measurement . . . . . . . . . . . . . . . . . . . .47
Food Quality and Safety . . . . . . . . . . . . . .48
Water Activity and Microbiology . . . . . .49
Water Activity in Foods . . . . . . . . . . . . . . .50
Pharmaceuticals/Cosmetics . . . . . . . . . . . .54
Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . .55
Declaration of Conformity. . . . . . . . . . .57
ii
Pawkit
Introduction
1. Introduction
Welcome to the Pawkit water activity measurement
system. Pawkit allows you to make quick measurements of water activity to ensure the safety of your
product. We hope you find the contents of this manual useful in understanding your instrument and
maximizing its benefit to you.
About this Manual
This manual includes instructions on the operation,
calibration, and maintenance of your Pawkit water
activity system. Please read these instructions carefully to ensure that your samples are measured
accurately and that you can fully utilize the instrument’s potential.
Customer Service
If you ever need assistance with your Pawkit, or if
you just have questions, there are several ways to
contact us:
Phone:
Our toll-free telephone number is available to
those in the US and Canada, Monday through Fri-
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Pawkit
Introduction
day, between 8 a.m. and 5 p.m. Pacific time at
1-800-755-2751.
For those outside of the US and Canada, call
(509) 332-2756.
Fax:
Our fax number is (509) 332-5158. When you fax
us, please include your instrument’s serial number,
your name, address, phone and fax number along
with a description of your problem.
E-mail:
If you need technical support, or if you have application questions, you can send us messages via
email at pawkit@decagon.com. Again, please
include as part of your message your Pawkit’s serial
number, your name, address, phone, fax number,
and return e-mail address.
Warranty
The Pawkit has a 30-day satisfaction guarantee and
a one-year warranty on parts and labor.
Note to our Users
This manual is written to aid the end user in understanding the basic concepts of water activity,
enabling them to use our instrument with confidence. Every effort has been made to ensure that
2
Pawkit
Introduction
the content of this manual is correct and scientifically sound.
Seller’s Liability
Seller warrants new equipment of its own manufacture against defective workmanship and materials
for a period of one year from date of receipt of
equipment (the results of ordinary wear and tear,
neglect, misuse, accident and excessive deterioration due to corrosion from any cause are not to be
considered a defect); but Seller’s liability for defective parts shall in no event exceed the furnishing of
replacement parts F.O.B. the factory where originally manufactured. Material and equipment covered hereby which is not manufactured by Seller
shall be covered only by the warranty of its manufacturer. Seller shall not be liable to Buyer for loss,
damage or injuries to persons (including death), or
to property or things of whatsoever kind (including,
but not without limitation, loss of anticipated profits), occasioned by or arising out of the installation,
operation, use, misuse, nonuse, repair, or replacement of said material and equipment, or out of the
use of any method or process for which the same
may be employed. The use of this equipment constitutes Buyer’s acceptance of the terms set forth in
this warranty. There are no understandings, representations, or warranties of any kind, express,
implied, statutory or otherwise (including, but with-
3
Pawkit
Introduction
out limitation, the implied warranties of merchantability and fitness for a particular purpose), not
expressly set forth herein.
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Pawkit
About the Pawkit
2. About the Pawkit
The Pawkit is designed to be a simple, rapid and
portable system for measurement of water activity.
It is easy to use, durable, and requires little maintenance.
How Pawkit works
Pawkit uses a dielectric humidity sensor to measure
the aw of a sample. In an instrument that uses this
technique, a special porous polymer is placed
between two porous electrodes in the headspace of
a sealed chamber. The electrical properties of the
polymer change depending on the relative humidity
of the chamber. The electrodes give a signal based
upon the relative humidity in the closed chamber.
This signal is then translated by the software and
displayed as water activity on the instrument’s
screen. At equilibrium, the relative humidity of the
air in the chamber is the same as the water activity
of the sample.
Accuracy
The Pawkit is accurate to ±0.02 aw. For many applications, this accuracy is more than adequate. If you
5
Pawkit
About the Pawkit
require higher accuracy in your measurements, we
recommend you use Decagon’s AquaLab water
activity meter, which is a lab-grade, bench-top
instrument that has an accuracy of ±0.003 aw, and
measures based upon the chilled-mirror dewpoint
method. Contact Decagon for more details.
Getting Started
Components of your Pawkit system:
Your Pawkit should have been shipped to you with
the following items:
• Pawkit main unit
• Durable carrying case
• 110 disposable Sample cups
• 3 spare sensor filters
• 1 cleaning kit
• 1 reusable stainless steel cup
• 2 vials each of the following calibration solutions:
6.0 molal NaCl 0.760 aw
13.41 molal LiCl 0.250 a
w
6
Pawkit
About the Pawkit
Preparing for Operation
To ensure that your Pawkit operates correctly and
consistently, always place it on a level surface when
measuring. This reduces the chance that sample
material will spill inside the instrument. To avoid
inaccurate readings, place your Pawkit in a location
where the temperature remains fairly stable. This
location should be well away from air conditioner
and heater vents, open windows, outside doors,
refrigerator exhausts, or other items that may cause
rapid temperature fluctuation.
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Pawkit
Operation
3. Operation
Operation of the Pawkit is very simple. Once you
have ensured that you have a stable working environment, you are ready to begin sampling. Following is a description of the features and operation of
the instrument.
Features
Button I
Temp.
Sensor
Opening
Tab
LCD
Button II
Sample Chamber
Diagram of Pawkit features
8
Pawkit
Operation
Sample Preparation and Insertion
Your Pawkit system comes with 110 disposable
plastic sample cups and 1 stainless steel sample
cup. More cups are always available from Decagon
when you run out.
Sample Preparation
Special care should be taken in preparing the sample in order to get the best readings possible. Follow these guidelines when preparing samples.
• Make sure that the sample to be measured is
homogeneous. Multi-component 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 (crushing or grinding) to obtain a
representative sample.
• Completely cover the bottom of the cup with the
sample, if possible. Pawkit is able to accurately
measure a sample that leave 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.
9
Pawkit
Operation
• Fill the cup no more than half-full of the sample.
Pawkit does not require a large sample size to
make its reading. As long as the bottom of the
cup is covered by the sample and that the sample is representative of the product you wish to
measure, you should be able to make accurate
readings. If the sample cup is too full, you risk
contaminating the sensor, which will lead to
inaccurate readings.
• Make sure that the rim and outside of the sample cup are clean. Wipe any excess sample
material from the rim of the cup with a clean tissue. Material left on the rim or the outside of the
cup will be transferred to subsequent samples
and can affect the accuracy of your readings.
The rim of the cup forms a vapor seal with the
sensor. Therefore, any sample material left on
the cup rim may prevent this seal, and contaminate future samples.
• If a sample will be read at some other time, put
the sample cup’s disposable lid on the cup to
restrict water transfer. To seal the lid, place tape
or Parafilm completely around the cup/lid
junction. It is necessary to seal the cup if it will
be a long time before the measurement is made.
10
Pawkit
Operation
Sample Insertion and chamber movement
1. Open the Pawkit chamber by holding the case
near the LCD with one hand and pulling down
on the metal tab with the other hand. Flex the
base of the instrument until it snaps open, then
lay it on a flat surface.
2. Place your prepared sample cup inside the
instrument in the cup-holder as shown:
Insert cup into area shown
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Pawkit
Operation
Ensure cup is entirely within the chamber
3. Once the sample cup is properly inserted, press
down on the front end of the case to secure the
cup inside the instrument. You are now ready to
take readings (see “Taking Measurements” section below).
12
Operation
4. When you are finished with the sample, press
on the back of the instrument above the LCD as
illustrated to open the chamber and remove the
cup.
Pawkit
13
Pawkit
Operation
Do not press on the middle to close the instrument!
5. To close the instrument, turn it over, then press
on the two points shown with your thumbs until
it snaps closed.
14
Taking Measurements
1. Make sure the sample cup is inserted as
described in the previous section.
2. Press the left button (I) to turn on the instru-
ment. It will display the last reading taken. This
allows you to begin a measurement and leave
without having to attend the instrument throughout the measurement. If it is already on, proceed
to the next step.
Pawkit
Operation
15
Pawkit
Operation
3. Press button I to begin the water activity measurement. The LCD display will be reset to
0.00a
NOTE: Pressing button I any time during a measure-
ment will restart the water activity measurement.
4. Once the measurement process has been
started, it will begin to display water activity
measurements as well as temperature every 30
.
w
seconds. During this time you will be able to see
that it is measuring by looking at the “sunburst”
icon to the right of the water activity value. As it
measures, you will see the “beams” of the sunburst move from left to right:
24.6
C
0.89
a
W
16
Pawkit
Operation
5. After 5 minutes, the instrument will display the
final water activity and beep 5 times. The sunburst disappears when the water activity reading
is finished. At this point you can either restart
the measurement by pressing button I again, or
you can record the shown value and take the
sample cup out.
6. To remove the sample, press on the back of the
Pawkit to open the chamber, then remove the
cup as shown:
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