Campbell KH20 User manual

KH20 Krypton Hygrometer
2/10
Copyright © 2010
Campbell Scientific, Inc.

Warranty and Assistance
The KH20 KRYPTON HYGROMETER is warranted by CAMPBELL
SCIENTIFIC, INC. to be free from defects in materials and workmanship
under normal use and service for twelve (12) months from date of shipment
unless specified otherwise. Batteries have no warranty. CAMPBELL
SCIENTIFIC, INC.'s obligation under this warranty is limited to repairing or
replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products.
The customer shall assume all costs of removing, reinstalling, and shipping
defective products to CAMPBELL SCIENTIFIC, INC. CAMPBELL
SCIENTIFIC, INC. will return such products by surface carrier prepaid. This
warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products
which have been subjected to modification, misuse, neglect, accidents of
nature, or shipping damage. This warranty is in lieu of all other warranties,
expressed or implied, including warranties of merchantability or fitness for a
particular purpose. CAMPBELL SCIENTIFIC, INC. is not liable for special,
indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country.
To obtain a Returned Materials Authorization (RMA), contact CAMPBELL
SCIENTIFIC, INC., phone [phone]. After an applications engineer
determines the nature of the problem, an RMA number will be issued. Please
write this number clearly on the outside of the shipping container.
CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC, INC.
RMA#_____
815 West 1800 North
Logan, Utah 84321-1784
For all returns, the customer must fill out a “Declaration of Hazardous Material
and Decontamination” form and comply with the requirements specified in it.
The form is available from our website at www.campbellsci.com/repair. A
completed form must be either emailed to [e-mail] or faxed to
435-750-9579. Campbell Scientific will not process any returns until we
receive this form. If the form is not received within three days of product
receipt or is incomplete, the product will be returned to the customer at the
customer’s expense. Campbell Scientific reserves the right to refuse service on
products that were exposed to contaminants that may cause health or safety
concerns for our employees.

KH20 Table of 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............................................................3
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 Vapor Fluctuations......7
6. Maintenance and Calibration......................................9
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..............................................................10
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. Vapor Density....................................................... A-2
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KH20 Table of Contents
ii
Tables
1. Datalogger Connections for Differential Measurement............................. 5
2. KH20 Calibration Ranges.......................................................................... 6
A-1. Linear Regression Results for KH20 ln(mV) vs. Vapor Density ......A-2
A-2. Final Calibration Values for KH20....................................................A-3

KH20 Krypton Hygrometer
1. General Description
The KH20 is a krypton hygrometer for measuring water vapor 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 vapor, and a small amount of the minor line is absorbed by
oxygen. The KH20 is not suitable for absolute water vapor 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
(CSI p/n 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
(CSI p/n 14113) to protect the electronics box and the connectors from
moisture, a 20 inch 3/4 IPS threaded aluminium pipe (CSI p/s 3874), a 3/4” x
3/4” Nu-Rail (CSI p/n 1017), and a 5/32 inch Allen wrench (CSI p/n 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
(p/n KH20CBL-L) and specify the desired lead length after -L.
Discussion on the principles and theory of measurement is
included in Appendix A.
NOTE
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
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KH20 Krypton Hygrometer
2.3 Physical
Dimensions
Sensor Head: 11.5” x 9” x 1.25” (29 cm x 23 cm x 3 cm)
Electronics Box: 7.5” x 5” x 2” (19 cm x 13 cm x 5 cm)
Rain Shield with Mount: 11.5” x 7” x 2.5” (29 cm x 18 cm x 6.5 cm)
Mounting Pipe: 20” (50 cm)
Carrying Case: 25” x 15” x 7” (64 cm x 38 cm x 18 cm)
Weight
Sensor Head: 3.55 lbs (1.61 kg)
Electronics Box: 1.4 lbs (6.4 kg)
Rain Shield with Mount: 4.75 lbs (2.2 kg)
Mounting Pipe with Nu-rail: 1.0 lbs (0.45 kg)
Carrying Case: 9.45 lbs (4.3 kg)
Shipping: 20.15 lbs (9.2 kg)
Package Includes:
KH20 sensor head with 5’ of cables
Power/Signal cable with 25’ lead length (CSI p/n KH20CBL-L)
Electronics Box
Rain Shield (CSI p/n 14113)
Horizontal Mounting Boom (20 inch 3/4” IPS threaded aluminium pipe)
(CSI p/n 3874)
Nu-rail (3/4” x 3/4”) (CSI p/n 1017)
Allen Wrench (5/32”) (CSI p/n 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 Aluminum Pipe, 12 inches long (CSI p/n 18048)
4. 3/4” x 1” Nu-rail (CSI p/n 1049)
5. Small Phillips and flat-head screw-drivers
6. 1/2 inch wrench
2

KH20 Krypton Hygrometer
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.
2. Mount a crossarm onto a tripod or tower.
3. Mount the 12 inch long pipe (CSI p/n 18048) to a crossarm via 1” x 3/4”
Nu-rail (CSI p/n 1049).
4. Mount the KH20 onto the 12” long pipe using a 3/4” x 3/4” Nu-rail (CSI
p/n 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.
NOTE
3

KH20 Krypton Hygrometer
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.
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.
NOTE
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.
4

KH20 Krypton Hygrometer
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 Color 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
5

KH20 Krypton Hygrometer
5. Datalogger Programming
The KH20 sensor outputs 0 to 5 Vdc analog 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 vapor range of approximately 2 to 19 g/m3.
The calibration is performed twice under the following two conditions:
window clean, and scaled. The water vapor absorption coefficient for three
different vapor ranges are calculated from the collected calibration data: full
range, dry range, and wet range. Table 2 shows a sample of the KH20 vapor
ranges over which three different water vapor absorption coefficients are
calculated. See Appendix A for more information on KH20 calibration.
TABLE 2. KH20 Calibration Ranges
Ranges Vapor Density (g/m3)
Full Vapor Range 2 – 19
Dry Vapor Range 2 – 9.5
Wet Vapor Range 8.25 – 19
Before the water vapor 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 vapor 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 vapor absorption coefficient for either the dry
or the wet vapor range will produce more accurate measurements than using
that for the full range. If the vapor range of the site is unknown, or if the vapor
range is on the border line between the dry and the wet vapor ranges, the full
range should be used.
6

KH20 Krypton Hygrometer
5.2 Example 1, CR3000 Program to Measure Water Vapor
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 vapor
concentration.
NOTE
'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 vapor absorption coefficient [m^3 / (g cm)].
Const xkw = x*kw 'Pathlength times water vaporabsorptioncoefficient [m^3 / g].
7

KH20 Krypton Hygrometer
'*** Variables ***
Public panel_temp
Public batt_volt
Public kh(2)
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
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KH20 Krypton Hygrometer
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-mold 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.
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.
NOTE
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
9

KH20 Krypton Hygrometer
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 vapor,
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. However, the scaling over a typical flux averaging period is small.
Thus, water vapor 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 vapor absorption coefficient for scaled
window from the calibration sheet if the window will be allowed
to scale during measurements.
NOTE
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.
10

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 vapor present in the light path. This
relationship can be rewritten as follows, where kwis the absorption coefficient
for water vapor, xis the path length for the KH20 sensor, and ρwis the water
vapor density.
wwxk
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.
wwxk
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 vapor 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
vapor, 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 vapor density, ρw. Figure A-1 shows the plot of the equation A-4
after we ran a KH20 over a full calibration vapor range.
A-1

Appendix A. Calibrating KH20
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
Vapor Density (g/m
3
)
KH2O Output (mV)
FIGURE A-1. KH20 ln(mV) vs. Vapor Density
We can perform the linear regression on the plot above to obtain the slope for
the relationship between the ln(mV) and the vapor 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 vapor, kw, and the KH20 path length, x.
TABLE A-1. Linear Regression Results for KH20 ln(mV)
vs. Vapor 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 vapor 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 vapor 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 vapor 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 vapor range for the site is unknown,
or if the vapor 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
A-2

Appendix A. Calibrating KH20
the KH20 calibration certificate contains. The data shown in Table A-2 is from
an actual KH20.
TABLE A-2. Final Calibration Values for KH20
Vapor Range
(g/m3) Slope (xkw) Y Intercept ln (V0) Coefficient (kw)
Full Range 1.74 ~ 19.25 -0.205 3087 -0.144
Dry Range 1.74 ~ 9.20 -0.216 3259 -0.151
Wet Range 7.95 ~ 19.25 -0.201 2899 -0.141
A-3

Appendix A. Calibrating KH20
A-4

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