Campbell KH20 User manual

KH20
Krypton
Hygrometer
User Manual
Issued 23.3.10
Copyright ©2010 Campbell Scientific Inc.
Printed under Licence by Campbell Scientific Ltd.
.
CSL 876

Guarantee
This equipment is guaranteed against defects in materials and
workmanship. This guarantee applies for twelve months from date of
delivery. We will repair or replace products which prove to be defective
during the guarantee period provided they are returned to us prepaid. The
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•Equipment which has been modified or altered in any way without the
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•Batteries
•Any product which has been subjected to misuse, neglect, acts of God
or damage in transit.
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particular purpose. Campbell Scientific is not liable for consequential
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Please inform us before returning equipment and obtain a Repair
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charges are greater than the cost of the repair.
Campbell Scientific Ltd,
Campbell Park, 80 Hathern Road,
Shepshed, Loughborough, LE12 9GX, UK
Tel: [phone]
Fax: [phone]
Email: [email protected]
www.campbellsci.co.uk

PLEASE READ FIRST
About this manual
Please note that this manual was originally produced by Campbell Scientific Inc. primarily for the
North American market. Some spellings, weights and measures may reflect this origin.
Some useful conversion factors:
Area: 1 in2(square inch) = 645 mm2
Length: 1 in. (inch) = 25.4 mm
1 ft (foot) = 304.8 mm
1 yard = 0.914 m
1 mile = 1.609 km
Mass: 1 oz. (ounce) = 28.35 g
1 lb (pound weight) = 0.454 kg
Pressure: 1 psi (lb/in2) = 68.95 mb
Volume: 1 UK pint = 568.3 ml
1 UK gallon = 4.546 litres
1 US gallon = 3.785 litres
In addition, while most of the information in the manual is correct for all countries, certain information
is specific to the North American market and so may not be applicable to European users.
Differences include the U.S standard external power supply details where some information (for
example the AC transformer input voltage) will not be applicable for British/European use. Please
note, however, that when a power supply adapter is ordered it will be suitable for use in your country.
Reference to some radio transmitters, digital cell phones and aerials may also not be applicable
according to your locality.
Some brackets, shields and enclosure options, including wiring, are not sold as standard items in the
European market; in some cases alternatives are offered. Details of the alternatives will be covered in
separate manuals.
Part numbers prefixed with a “#” symbol are special order parts for use with non-EU variants or for
special installations. Please quote the full part number with the # when ordering.
Recycling information
At the end of this product’s life it should not be put in commercial or domestic refuse
but sent for recycling. Any batteries contained within the product or used during the
products life should be removed from the product and also be sent to an appropriate
recycling facility.
Campbell Scientific Ltd can advise on the recycling of the equipment and in some cases
arrange collection and the correct disposal of it, although charges may apply for some
items or territories.
For further advice or support, please contact Campbell Scientific Ltd, or your local agent.
Campbell Scientific Ltd, Campbell Park, 80 Hathern Road, Shepshed, Loughborough, LE12 9GX, UK
Tel: [phone] Fax: [phone]
Email: [email protected]
www.campbellsci.co.uk

i
Contents
PDF viewers note: These page numbers refer to the printed version of this document.
Use the Adobe Acrobat® bookmarks tab for links to specific sections.
1. General Description.....................................................1
2. Specifications ..............................................................1
2.1 Measurements ..........................................................................................1
2.2 Electrical ..................................................................................................1
2.3 Physical....................................................................................................2
3. Installation....................................................................2
3.1 Siting........................................................................................................2
3.2 Mounting..................................................................................................2
3.2.1 Parts and Tools Needed for Mounting ...........................................2
3.2.2 Mounting the KH20 Sensor............................................................2
3.2.3 Mounting the Electronics Box........................................................3
4. Wiring ...........................................................................5
5. Datalogger Programming............................................6
5.1 KH20 Calibration.....................................................................................6
5.2 Example 1, CR3000 Program to Measure Water Vapour Fluctuations ...7
6. Maintenance and Calibration......................................8
6.1 Visual Inspection......................................................................................9
6.2 Testing the Source Tube ..........................................................................9
6.3 Testing the Detector Tube........................................................................9
6.4 Managing the Scaling of KH20................................................................9
6.5 Older KH20............................................................................................10
6.6 Calibration..............................................................................................10
Appendix
A. Calibrating KH20..................................................... A-1
A.1 Basic Measurement Theory.................................................................A-1
A.2 Calibration of KH20............................................................................A-1
Figures
1. Mounting KH20 onto a tripod....................................................................3
2. Proper mounting position of the electronics box .......................................4
3. Attaching cables to the electronics box......................................................5
A-1. KH20 ln(mV) vs. Vapour Density.....................................................A-2

ii
Tables
1. Datalogger Connections for Differential Measurement .............................5
2. KH20 Calibration Ranges ..........................................................................6
A-1. Linear Regression Results for KH20 ln(mV) vs. Vapour Density.....A-2
A-2. Final Calibration Values for KH20....................................................A-3

1
KH20 Krypton Hygrometer
1. General Description
The KH20 is a krypton hygrometer for measuring water vapour fluctuations in the
air. The name KH20 (KH-twenty) was derived from KH2O (K-H2O), and the
sensor has been known with this name since 1985. It is typically used with the
CSAT3 3-D sonic anemometer for measuring latent heat flux (LE), using Eddy
Covariance technique. Please refer to the Open Path Eddy Covariance System
operator’s manual for details on flux measurements using the KH20 sensor.
The KH20 sensor uses a krypton lamp that emits two absorption lines: major line
at 123.58 nm and minor line at 116.49 nm. Both of these lines are absorbed by
water vapour, and a small amount of the minor line is absorbed by oxygen. The
KH20 is not suitable for absolute water vapour concentration measurements due
to its signal offset drift.
The KH20 heads are sealed and will not suffer damage should they get wet. In
addition, the electronics box and the connectors are housed inside a rain shield
(#14113) that protects them from moisture. The KH20 is suitable for long-term
continuous outdoor applications.
The KH20 sensor is comprised of two main parts: the sensor head and the
electronics box. The sensor head comes with cables with 6 ft of lead length that
connect the sensor to the electronics box. It is shipped with a rain shield (#14113)
to protect the electronics box and the connectors from moisture, a 20 inch 3/4 IPS
threaded aluminium pipe (#3874), a 3/4” x 3/4” Nu-Rail (#1017), and a 5/32 inch
Allen wrench (#4697) for mounting. The sensor comes with a signal/power cable
which has 25 ft lead length. If a longer cable is desired, the customer can order a
replacement cable (# KH20CBL-L) and specify the desired lead length after -L.
Discussion on the principles and theory of measurement is included
in Appendix A.
2. Specifications
2.1 Measurements
Calibration Range: 1.7 to 19.5 g/m3(nominal)
Frequency Response: 100 Hz
Operating Temperature Range: -30 to +50°C
2.2 Electrical
Supply Voltage: 10 V to 16 Vdc
Current Consumption: 20 mA max at 12 Vdc
Power Consumption: 0.24 Watts
Output Signal Range: 0 to 5Vdc
NOTE

KH20 Krypton Hygrometer
2
2.3 Physical
Dimensions
Sensor Head: 29 cm x 23 cm x 3 cm (11.5” x 9” x 1.25”)
Electronics Box: 19 cm x 13 cm x 5 cm (7.5” x 5” x 2”)
Rain Shield with Mount: 29 cm x 18 cm x 6.5 cm (11.5” x 7” x 2.5”)
Mounting Pipe: 50 cm (20”)
Carrying Case: 64 cm x 38 cm x 18 cm (25” x 15” x 7”)
Weight
Sensor Head: 31.61 kg (.55 lbs)
Electronics Box: 6.4 kg (1.4 lbs)
Rain Shield with Mount: 2.2 kg (4.75 lbs)
Mounting Pipe with Nu-rail: 0.45 kg) (1.0 lbs)
Carrying Case: 4.3 kg (9.45 lbs)
Shipping: 9.2 kg (20.15 lbs)
Package Includes:
KH20 sensor head with 5’ of cables
Power/Signal cable with 25’ lead length (#KH20CBL-L)
Electronics Box
Rain Shield (#14113)
Horizontal Mounting Boom (20 inch 3/4” IPS threaded aluminium pipe)
(#3874)
Nu-rail (3/4” x 3/4”) (#1017)
Allen Wrench (5/32”) (#4697)
3. Installation
3.1 Siting
When installing the KH20 sensor for latent heat flux measurement in an eddy
covariance application, proper siting, sensor height, sensor orientation and fetch
are important. Please refer to Section 2 Installation and Mounting of the Open
Path Eddy Covariance System manual for more discussion on siting.
3.2 Mounting
3.2.1 Parts and Tools Needed for Mounting
You will need the following hardware to mount the KH20 sensor:
1. Tripod (CSI model CM115 standard) or tower
2. CM20x series crossarm (CSI model CM204 standard)
3. 3/4 inch IPS Aluminium Pipe, 12 inches long (#18048)
4. 3/4” x 1” Nu-rail (#1049)
5. Small Phillips and flat-head screw-drivers
6. 1/2 inch wrench
3.2.2 Mounting the KH20 Sensor
Mount the KH20 sensor head as follows:
1. Attach the 20 inch long mounting boom to the KH20.

User Manual
3
2. Mount a crossarm onto a tripod or tower.
3. Mount the 12 inch long pipe (#18048) to a crossarm via 1” x 3/4” Nu-rail
(#1049).
4. Mount the KH20 onto the 12” long pipe using a 3/4” x 3/4” Nu-rail (#1017).
Mount the KH20 such that the source tube, the longer of the two tubes, is
positioned on top, as shown in Figure 1. Gather any loose cables and tie them
up, using cable ties, onto the tripod or tower mast.
Figure 1. Mounting KH20 onto a tripod.
3.2.3 Mounting the Electronics Box
Mount the electronics box as follows:
1. First, remove the front cover of the rain shield by loosening the two pan-head
screws on the bottom front of the rain shield, and then pushing the cover all
the way up, and sliding it out.
It will be difficult to mount the rain shield to a mast with the front
cover on, since the 1/2 inch nut holding the bottom U-bolt is located
inside the rain shield.
2. Before mounting the rain shield onto a tripod, first mount the electronics box
inside the rain shield. Remove the four pan-head screws from the back panel
of the rain shield. Align the electronics box, and use the four pan-head
screws to secure the electronics box onto the back panel. Make sure the
electronics box is pushed all the way up, and the screws are positioned at the
bottom of the mounting slot on the electronics box (see Figure 2). This will
provide enough room to attach the connectors to the bottom of the electronics
box later on.
NOTE

KH20 Krypton Hygrometer
4
Figure 2. Proper mounting position of the electronics box.
If the electronics box is not pushed all the way up during mounting,
you will not have enough room to attach the connectors to the
bottom of the electronics box, as the U-bolt for the rain shield will
block the position of the connectors.
3. Mount the rain shield onto the tripod or tower mast using the U-bolt provided.
Make sure that the distance between the KH20 sensor head and the rain shield
is within 5 feet so that the cables from the sensor head will be within reach of
the electronics box. Also make sure that the rain shield is mounted vertically
with an opening pointing downward so that the rain will effectively run down
the rain shield and not penetrate inside.
NOTE

User Manual
5
4. Connect the three cables to the bottom of the electronics box around the U-
bolt on the rain shield (see Figure 3). If there is not enough room for the
connectors around the U-bolt, make sure the electronics box is mounted at a
highest possible position (see step 2).
Figure 3. Attaching cables to the electronics box.
5. Place the front cover back on the rain shield and tighten the two pan-head
screws to secure it in place.
6. Gather any loose cables and tie them up, using cable ties, onto the tripod or
tower mast.
4. Wiring
The KH20 sensor is shipped with a power/signal cable with a 25 ft standard lead
length. Table 1 shows the connections for the KH20 to the CR1000, CR3000, and
CR5000.
Table 1. Datalogger Connections for Differential Measurement
Function Wire Colour CR1000/CR3000/CR5000
KH20 Signal + White Differential Input (H)
KH20 Signal - *Black Differential Input (L)
KH20 Power + Red 12V
KH20 Power - Black (from Red/Black set) G
Shield Clear
*Jumper to with user supplied wire

KH20 Krypton Hygrometer
6
5. Datalogger Programming
The KH20 sensor outputs 0 to 5 Vdc analogue signal. These signals can be
measured using the VoltDiff instruction on the CRBasic dataloggers or the Volt
(Diff) (P2) instruction on the traditional Edlog dataloggers.
5.1 KH20 Calibration
Each KH20 is calibrated over a vapour range of approximately 2 to 19 g/m3. The
calibration is performed twice under the following two conditions: window clean,
and scaled. The water vapour absorption coefficient for three different vapour
ranges are calculated from the collected calibration data: full range, dry range, and
wet range. Table 2 shows a sample of the KH20 vapour ranges over which three
different water vapour absorption coefficients are calculated. See Appendix A for
more information on KH20 calibration.
Table 2. KH20 Calibration Ranges
Ranges Vapour Density (g/m3)
Full Vapour Range 2 – 19
Dry Vapour Range 2 – 9.5
Wet Vapour Range 8.25 – 19
Before the water vapour absorption coefficient, kw, is entered into the datalogger
program for the KH20, the following decisions must be made:
•Will the windows be allowed to scale?
•What vapour range is appropriate for the site?
Once the decision is made, the appropriate kwcan be chosen from the calibrations
sheet. The calibration sheet also contains the path length, x, for a specific KH20.
Using the water vapour absorption coefficient for either the dry or the wet vapour
range will produce more accurate measurements than using that for the full range.
If the vapour range of the site is unknown, or if the vapour range is on the border
line between the dry and the wet vapour ranges, the full range should be used.

User Manual
7
5.2 Example 1, CR3000 Program to Measure Water Vapour
Fluctuations
The following example program measures the KH20 at 10Hz, and stores the
average values into a data table called ‘stats’, as well as the raw data into a data
table called ‘ts_data’.
The KH20 cannot be used for measuring absolute water vapour
concentration.
'CR3000 Series Datalogger
'This datalogger program measures KH20 Krypton Hygrometer.
'The station operator must enter the constant and the calibration value for the KH20.
'Search for the text string "unique" to find the locations of these constants
'and enter the appropriate values found from the calibration sheet of the KH20.
'*** Unit Definitions ***
'Units Units
'ln_mV ln(mV) (natural log of the KH20 millivolts)
'mV millivolts
‘rho_w g/m^3
'*** Wiring ***
'ANALOG INPUT
'1H KH20 signal+ (white)
'1L KH20 signal- (black)
'gnd KH20 shield (clear)
'EXTERNAL POWER SUPPLY
'POS KH20 power+ (red)
' datalogger POWER IN 12 (red)
'NEG KH20 power- (black)
' KH20 power shield (clear)
' datalogger POWER IN G (black)
PipeLineMode
'*** Constants ***
'Measurement Rate '10 Hz
Const SCAN_INTERVAL = 100 '100 mSec
'Output period
Const OUTPUT_INTERVAL = 30 'Online flux data output interval in minutes.
Const x = 1 'Unique path length of the KH20 [cm].
Const kw = -0.150 'Unique water vapour absorption coefficient [m^3 / (g cm)].
Const xkw = x*kw 'Pathlengthtimeswatervapourabsorption coefficient[m^3/g].
'*** Variables ***
Public panel_temp
Public batt_volt
Public kh(2)
NOTE

KH20 Krypton Hygrometer
8
Public rho_w
Alias kh(1) = kh_mV
Alias kh (2) = ln_kh
Units panel_temp = deg_C
Units batt_volt = volts
Units kh_mV = mV
Units ln_kh = ln_mV
Units rho_w = g/m^3
'*** Data Output Tables ***
'Processed data
DataTable (stats,True,-1)
DataInterval (0,OUTPUT_INTERVAL,Min,10)
Minimum (1,batt_volt,FP2,False,False)
Average (1,panel_temp,FP2,False)
Average (2,kh(1),IEEE4,False)
EndTable
'Raw time-series data.
DataTable (ts_data,True,-1)
DataInterval (0,SCAN_INTERVAL,mSec,100)
Sample (1,kh_mV,IEEE4)
EndTable
'*** Program ***
BeginProg
Scan (SCAN_INTERVAL,mSec,3,0)
'datalogger panel temperature.
PanelTemp (panel_temp,250)
'Measure battery voltage.
Battery (batt_volt)
'Measure KH20.
VoltDiff (kh_mV,1,mV5000,1,TRUE,200,250,1,0)
ln_kh = LOG(kh_mV)
rho_w = ln_kh/xkw
CallTable stats
CallTable ts_data
NextScan
EndProg
6. Maintenance and Calibration
The KH20 sensor is designed for continuous field application and requires little
maintenance. The tube ends for the KH20 have been sealed with silicone
elastomer using an injection-mould method. Therefore, the tubes are protected
from water damage, and the KH20 continues to make measurements under rainy
or wet conditions. If the water tends to pool up on the tube window and blocks
the signal, you can turn the sensor head at an angle so as to shed the water off the
tube window. The rain shield protects the electronics box and the connectors
from moisture.

User Manual
9
6.1 Visual Inspection
•Make sure the optical windows are clean.
•Inspect the cables and connectors for any damage or corrosion. If you see a
discoloration on the white co-axial cable, you may suspect that the cable has
water damage.
6.2 Testing the Source Tube
The source tube is the longer of the two tubes. You can check to see if the source
tube is working properly by performing the test explained below.
First, make sure the UV light is emitted from the source tube. To do this, you
may look into the source tube (the longer of the two tubes), and you should see a
bright blue light emitted from it.
Avoid looking into the source tube for an extended period of time
when the KH20 is powered on to minimize the prolonged exposure
to the UV light.
If you see a faint or flickering blue light, perform the following test.
Check the current drain on the KH20
Typical current drain for the KH20 during normal operation should be
15 ~ 20 mA. The current drain of around 5 mA or less indicates the problem
on the source tube. Obtain an RMA from Campbell Scientific, Inc. and send
the unit in for repair.
Check the voltage signal output from the KH20
If the voltage output reading is below 50 mV, you may have problems with
either the source tube or the detector tube. For the detector tube testing, see
Section 6.3 below.
6.3 Testing the Detector Tube
If the source tube tests fine but the output from KH20 is still in question, perform
the following test. Prepare a piece of paper and insert it between the source tube
and the detector tube to completely block the optical path. You should see an
immediate decrease in the voltage reading, and it should go close to zero. No
noticeable change in the voltage output, when the optical path is completely
blocked, indicates a problem in the detector tube. If the decrease in the voltage
reading takes place but the reading remains below 50 mV, when the paper is
removed from the optical path, the source tube may be at fault. Obtain an RMA
from Campbell Scientific, Inc. and send the unit in for repair.
6.4 Managing the Scaling of KH20
The KH20 cannot be used to measure an absolute concentration of water vapour,
because of scaling on the source tube windows caused by disassociation of
atmospheric continuants by the ultra violet photons (Campbell and Tanner, 1985
and Buck, 1976). The rate of scaling is a function of the atmospheric humidity.
In a high humidity environment, scaling can occur within a few hours. That
scaling attenuates the signal and can cause shifts in the calibration curve.
NOTE

KH20 Krypton Hygrometer
10
However, the scaling over a typical flux averaging period is small. Thus, water
vapour fluctuation measurements can still be made with the hygrometer.
To see if the source tube window has been scaled, get a clean, dry cotton swab
and slide it across the source tube window. The scale is not visible to the naked
eye, but if the window is scaled, you will feel a slight but noticeable resistance
while you slide the swab across the window. There will be little resistance if the
window is not scaled. If you determine the window is scaled, you can clean it
with a wet cotton swab.
Use distilled water and a clean cotton swab to clean the scaled window. After
cleaning the window, slide a clean, dry swab across the window to confirm the
scale has been removed.
You can use the water vapour absorption coefficient for scaled
window from the calibration sheet if the window will be allowed to
scale during measurements.
6.5 Older KH20
The older version of the KH20 used to suffer permanent damage when exposed to
water, due to corrosion and loss of vacuum within the tubes. The new version of
the KH20 has employed a new sealing around the window to prevent this type of
damage.
6.6 Calibration
For quality assurance of the measured data, Campbell Scientific, Inc. recommends
the KH20 be recalibrated every two years. Please contact Campbell Scientific,
Inc. to obtain an RMA number for recalibration.
For more information on the calibration process, refer to Appendix A of this
manual.
NOTE

A-1
Appendix A. Calibrating KH20
A.1 Basic Measurement Theory
The KH20 uses an empirical relationship between the absorption of the light and
the material through which the light travels. This relationship is known as the
Beer’s law, the Beer-Lambert law, or the Lambert-Beer law. According to the
Beer’s law, the log of the transmissivity is anti-proportional to the product of the
absorption coefficient of the material, k, the distance the light travels, x, and the
density of the absorbing material, ρ. The KH20 sensor uses the UV light emitted
by the krypton lamp: major line at 123.58 nm and the minor line at 116.49. As the
light travels through the air, both the major line and the minor line are absorbed by
the water vapour present in the light path. This relationship can be rewritten as
follows, where kwis the absorption coefficient for water vapour, xis the path
length for the KH20 sensor, and ρwis the water vapour density.
ww xk
eT
ρ −
=A-1
If we express the transmissivity, T, in terms of the light intensity before and after
passing through the material as measured by the KH20 sensor, Vand V0,
respectively, we obtain the following equation.
ww xk
e
V
V
ρ −
=
0A-2
Taking the natural log of both sides, and solving for the density, ρw, yields the
following equation.
)ln(ln
10
VV
xkw
w−
−
= ρ
A-3
If the path length, x, and the absorption coefficient for water, kware known, it
becomes possible to measure the water vapour density ρw, by measuring the signal
output, V, from KH20.
A.2 Calibration of KH20
The KH20 calibration process is to find the absorption coefficient of water vapour,
kw. To do this, we rewrite the equation A-3, and solve for ln(V).
0
lnln VxkV ww
+
−
=
ρ
A-4
It now becomes obvious from the equation A-4 that there is a linear relationship
between the natural log of the KH20 measurement output, lnV, and the water
vapour density, ρw. Figure A-1 shows the plot of the equation A-4 after we ran a
KH20 over a full calibration vapour range.

KH20 Krypton Hygrometer
A-2
4
4.5
5
5.5
6
6.5
7
7.5
8
1.74 3.02 4.17 5.44 6.71 7.95 9.2 10.47 11.69 12.9 14.22 15.46 16.78 18.04 19.25
KH2O Output (mV)
Vapour Density (g/m
3
)
Figure A-1. KH20 ln(mV) vs. Vapour Density
We can perform the linear regression on the plot above to obtain the slope for the
relationship between the ln(mV) and the vapour density. The slope for the graph
above will be the coefficient, kwxwhich we are after. Table A-1 below shows the
result of linear regression analysis. Again the slope is the product of the
absorption coefficient of water vapour, kw, and the KH20 path length, x.
Table A-1. Linear Regression Results for KH20 ln(mV)
vs. Vapour Density
Description Values
Slope (xkw) -0.205
Y Intercept (ln(V0) 8.033
If we substitute these values, along with the measured lnV into equation A-3, we
can obtain the water vapour density, ρw. Campbell Scientific, Inc. performs the
calibration twice for each KH20: once with the window cleaned, and again with
the window scaled. We then break up the vapour density range into dry and wet
ranges, and compute the kwvalues for each sub range, as well as for the full range.
If you know the vapour density range for your site, it is recommended that you
select the coefficient, kw, that is appropriate for your site, the dry range or the wet
range. If the vapour range for the site is unknown, or if the vapour range is on the
border line between the dry and the wet ranges, use the value for the full range.
Table A-2 shows the final calibration values the KH20 calibration certificate
contains. The data shown in Table A-2 is from an actual KH20.