Campbell CS505 Instruction Manual

CS505 FUEL MOISTURE SENSOR

INSTRUCTION MANUAL

6/98

COPYRIGHT (c) 1998 CAMPBELL SCIENTIFIC, INC.

WARRANTY AND ASSISTANCE

The

CS505 FUEL MOISTURE SENSOR

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. 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
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CAMPBELL SCIENTIFIC, INC.

RMA#_____
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Logan, Utah 84321-1784

CAMPBELL SCIENTIFIC, INC. does not accept collect calls.
Non-warranty products returned for repair should be accompanied by a purchase order to cover the repair.

815 W. 1800 N.
Logan, UT 84321-1784
USA
Phone [phone]
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www.campbellsci.com

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CS505 FUEL MOISTURE SENSOR

1. GENERAL DESCRIPTION

The CS505 Fuel Moisture Sensor provides an
automated measurement of the moisture
content of a standard 10-hour fuel moisture
dowel. The moisture content of the 10-hour fuel
sensor represents the moisture content of
small-diameter (10-hour time lag) forest fuels.

Traditionally, the standard fuel moisture stick
consists of a rack of four 1/2 inch diameter
ponderosa pine dowels. The resulting rack is
about 20 inches long with an oven-dry weight of
100 grams. The characteristic time constant of
the rack is 10 hours. The rack is mounted 12
inches or about 30 centimeters above the forest
floor. The rack is left outside continually
exposed to the same conditions as forest fuels.
The rack absorbs and desorbs moisture from
its surroundings. As the rack transfers
moisture, its weight changes. Periodic weighing
of the rack determines changes in moisture
content provides an indication of moisture
changes in forest fuels.

The CS505 sensor incorporates the same
carefully selected USFS standard ponderosa
dowels as the traditional weighing fuel moisture
racks. No artificial materials (e.g., epoxy
sealant) are added to the dowel that would
adversely influence the natural behavior
characteristics of the dowel. Because the
complete dowel surface is accessible for
moisture exchange, the response of the CS505
is similar to that of the traditional weighing
racks. To optimize probe-to-probe repeatability
and to allow probe interchangeability without
individual calibration, two additional sorts are
performed on the dowels before they are
selected to be used as a sensor. First, the
dowels are sorted dry by density to improve
accuracy in the dry range of 0 to 15%. Second,
the dowels are sorted after a 50-minute soak by
weight to reduce probe-to-probe time response
variation and minimize variability in the wet
range of 20 to 50%.

resin content. These variables change over
time and after repeated wetting and drying
cycles. Only a small amount of overall
measurement error is due to the electronic
circuitry.

2. SPECIFICATIONS

The fuel moisture sensor consists of two
stainless steel strips pressed into groves in a
standard 1/2 inch ponderosa pine dowel and
secured with nylon tie wraps. The probe
connects to the electronics with two Phillips
head screws. A shielded four-conductor cable
is connected to the circuit board to supply
power, enable the electronics, and monitor the
signal output. The printed circuit board is
encapsulated in a water proof epoxy housing.

High speed electronic components on the
circuit board are configured to oscillate when
power is applied. The output of the circuit is
connected to the fuel moisture probe which acts
as a wave guide. The oscillation frequency and
therefore output signal of the circuit is
dependent on the dielectric constant of the
media surrounding the stainless steel strips.
The dielectric constant is predominantly
dependent on the water content of the wood.
Digital circuitry scales the oscillation frequency
to an appropriate range for measurement with
a datalogger. The CS505 output is essentially a
square wave with an amplitude of ±2.5 VDC.
The frequency of the square wave output
ranges from approximately 600 to 1500 Hz.

*Fuel moisture accuracy:

(with a new stick)

90% of all

range measurements

0 to 10% ±2% ±1.0%
10 to 20% ±3% ±1.5%
20 to 30% ±5% ±2.2%
30 to 50% ±6% ±2.4%

Range: 0-50%

rms error

Even after careful selection and sorting is
performed to choose the most representative
dowels, the majority of measurement error is
due to the variability of wood. Wood’s ability to
transfer moisture is dependent on many
variables, primarily cell structure and wood

Power Supply: 9 VDC minimum to

18 VDC maximum

Enable voltage: off at 0 V (<1 VDC)

on at 5 V (>1.5 VDC maximum
12 VDC)

1

CS505 FUEL MOISTURE SENSOR

Current usage: 70 mA active/ 10 uA

quiescent

Output signal: ±2.5 V square wave with an

output frequency of
approximately 600 to 1500
Hz.

Dimensions:

sensor: 1/2 inch dowel, 20 inch long
electronics: 4 x 2.5 x 0.75 inches

*The above accuracy is a static accuracy
derived at slow changing conditions with
experimental data.

3. INSTALLATION

As shown in Figure 1 and Figure 2, both the
CS505 and CS205 install on the 10974
mounting stake. The probes install horizontally
and should point south in the northern
hemisphere and north in the southern. The
rack is mounted above a representative forest­floor duff layer. The stake is carefully

hammered into the ground so that it is vertical.
Don’t hammer on the spot-welded clips that
hold the CS505 electronics. Once the stake is
installed, insert the CS505 electronics into the
two spring clips. The fuel moisture sensor (part
number 10824) installs on the CS505
electronics with the supplied Phillips-head
screws. The CS205 fuel temperature stick is
inserted into the mounting stake’s compression
fitting. The 107 temperature probe is then
inserted into the CS205 stick. Tighten the
compression fitting so that it compresses the
split wood and snugly holds the 107 probe.

The mounting stake ships with a package of 12
ultraviolet light resistant cable ties. The
mounting stake has five holes punched along
the edge of the shaft. There are two pairs of
holes higher and one single hole lower on the
shaft. When the stake is inserted into the
ground to a depth level with the lower hole, the
probes are at 12 inches or about 30 centimeters
above the ground surface. The two upper pairs
of holes are used to attach two tie wraps per
cable—one pair for both the CS505 and
CS205/107.

FIGURE 1. Exploded View of CS505 and CS205 Mounting

FIGURE 2. CS505 Installation Options Over Forest Floor, 30cm Over Duff Layer

2

4. WIRING

CS505

CS505

Red - 12V Power

Orange - Enable

Black - Ground

Clear - Shield

Green - Signal

CS505 FUEL MOISTURE SENSOR

FIGURE 3. CS505 Wire Color and Function

color function CR10(X),CR500 21X/CR7 CR23X

red power 12 V 12 V 12 V
black ground G G
green signal Analog Channel Pulse Channel Analog Channel
orange enable Control Port or

Switched Excitation

Switched Excitation Control Port or

Switched Excitation

clear ground G

5. PROGRAMMING

Instruction P27 measures the period of the
output signal in microseconds. To prepare for

The CS505 has a built in enable circuit. When
voltage on the enable lead is less than 1.3
VDC, the sensor is off. When a voltage greater
than 1.3 VDC, commonly 5 VDC, is applied to

the polynomial, the output is scaled by 0.001. A
second-order polynomial is used to convert
signal period to percent moisture content with
the following constants:

the enable lead, the sensor is on. The output
signal is a ±2.5 volt square wave. Instruction
P27 Period Average in the CR500, CR10X, and
CR23X dataloggers measures the frequency of
the square wave with an analog channel.

Since fuel moisture doesn’t change very rapidly,
the sensor is typically measured only once an

C0 = -220.14
C1 = 365.89
C2 = -114.96

or, expressed as a second order polynomial
y=-220.14+365.89x-114.96x

hour.

2

3

CS505 FUEL MOISTURE SENSOR

5.1 CR10X EXAMPLE PROGRAM

A program to measure a CS505 fuel moisture sensor using the Period Averaging instruction P27 of a
CR10(X), CR500, or CR23X datalogger. P27 is the preferred measurement technique with a Campbell
Scientific, Inc. datalogger. P27 is not an option in the 21X datalogger.

;{CR10X}

color function

red power 12V
black ground G
orange control C1
green signal 1H (single-ended 1)
clear shield G

*Table 1 Program

01: 5 Execution Interval (seconds)

1: If time is (P92)

1: 0 Minutes (Seconds --) into a
2: 60 Interval (same units as above)
3: 30 Then Do

2: Do (P86)

1: 41 Set Port 1 High

3: Period Average (SE) (P27)

1: 1 Reps
2: 4 2 V Peak to Peak/200 kHz Max. Freq., Output Period
3: 1 SE Channel
4: 10 No. of Cycles
5: 5 Timeout (units = 0.01 seconds)
6: 1 Loc [ FuelMoist ]
7: 0.001 Mult
8: 0.0 Offset

example CR10X Program wiring connections

Measure the CS505 once an hour.

Turn on sensor by applying 5V to the enable lead.

Measure the sensor’s output period.

Prescaling for the polynomial.

4: Polynomial (P55)

1: 1 Reps
2: 1 X Loc [ FuelMoist ]
3: 1 F(X) Loc [ FuelMoist ]
4: -220.14 C0
5: 365.89 C1
6: -114.96 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

5: Do (P86)

1: 51 Set Port 1 Low

6: End (P95)

-Input Locations­1 FuelMoist 5 1 2

4

Convert sensor period to fuel moisture.

Turn off the sensor.

CS505 FUEL MOISTURE SENSOR

5.2 21X EXAMPLE PROGRAM

A program to measure a CS505 fuel moisture sensor using the Pulse Count Instruction
P3 of a 21X, CR10(X), CR500, or CR23X. The fuel moisture sensor is measured when
flag 1 is set high. The measurement is made then flag 1 is set low.

;{21X}

color function

red power 12V
black ground
orange enable control port 5
green signal pulse channel 1
clear shield

*Table 1 Program

01: 1 Execution Interval (seconds)

1: If time is (P92

1: 0 Minutes into a
2: 60 Minute Interval
3: 11 Set Flag 1 High

2: If Flag/Port (P91)

1: 11 Do if Flag 1 is High
2: 30 Then Do

3: Do (P86)

1: 45 Set Port 5 High

4: Beginning of Loop (P87)

1: 1 Delay
2: 2 Loop Count

example 21X Program wiring connections

Set flag 1 high hourly, to measure CS505.

If flag 1 is high, initiate the reading.

Turn on the CS505 by applying 5 volts to the enable lead.

Delay for complete count interval.

5: End (P95)
6: Pulse (P3)

1: 1 Reps
2: 1 Pulse Input Channel
3: 21 Low Level AC, Output Hz
4: 1 Loc [ CS505kHz ]
5: 0.001 Mult
6: 0.0 Offset

7: Do (P86)

1: 55 Set Port 5 Low

8: Z=1/X (P42)

1: 1 X Loc [ CS505kHz ]
2: 2 Z Loc [ CS505mSec ]

End of loop to delay.
Determine the CS505 output frequency.

Convert to kHz

Turn off the CS505

Convert frequency in kHz to period in milliseconds.

5

CS505 FUEL MOISTURE SENSOR

9: Polynomial (P55)

1: 1 Reps
2: 2 X Loc [ CS505mSec ]
3: 3 F(X) Loc [ fuelmoist ]
4: -220.14 C0
5: 365.89 C1
6: -114.96 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

10: Do (P86)

1: 21 Set Flag 1 Low
11: End (P95)
*Table 2 Program

02: 0.0000 Execution Interval (seconds)
*Table 3 Subroutines
End Program

-Input Locations­1 CS505kHz 1 1 1
2 CS505mSec 1 1 1
3 fuelmoist 1 0 1

Convert period to percent moisture content.

Set flag 1 low

End

6. MAINTENANCE

The sensor element should be changed at least
once a year with a new element in the spring.
Since the characteristics of wood change so
rapidly, more frequent replacements may be
desirable.

To change the sensor element, loosen the
Phillips head screws and replace with the new
element. Tighten the screws after replacing the
element.

6

APPENDIX A. EXPLANATION OF EXPERIMENTAL DATA

EXPERIMENTAL DATA ERROR DISTRIBUTION

6

Error % WATER

CONTENT
(by weight)

5

4

RMS ERROR 0.98%

3

2

1

0

-1

-2

-3

-4

-5

-6
0 1020304050

RMS ERROR 1.43%

RMS ERROR 2.21%

RMS ERROR 2.37%

% WATER CONTENT

(by weight)

FIGURE A-1. Experimental Data Error-Distribution

Experiments were conducted to characterize
measurement error and to determine the
calibration polynomial coefficients used to
convert the output signal to percent moisture
content.

Populated circuit boards (before epoxy) oscillate
with a period in the range of 0.6761 and 0.6780
milliseconds. In these experiments, three
worst-case units were chosen. Only fuel
moisture sticks that pass the standard rigorous
selection criteria and then additional sorting for
both oven-dry density and sorption qualities are
chosen for production. For this experiment, 36
fuel moisture sticks were randomly selected
from 300 production units. Electronic
measurements were compared to actual stick
weights. The sticks were soaked for 45
minutes, then after a five-minute dry, measured
electrically and weighed every 10 to 60 minutes
until they reached 15%. Then the sticks were
placed in a 103°C oven for further drying; more
measurements were taken. At each weigh

point the sticks where rotated and measured by
each of the three electronic units. Over one
thousand data points were measured for this
calibration experiment.

The full scale range is broken into four sub
ranges: 0-10, 10-20, 20-30, and 30-50 percent.
The computed rms errors are shown in Table 1.
For each range band an error band was
determined in which more than 90 percent of all
measurements fall (also shown in Table A-1
and Figure A-1).

A-1

APPENDIX A. EXPLANATION OF EXPERIMENTAL DATA

TABLE A-1. Accuracies at

Measurement Ranges

90% of all

range measurements

0 to 10% ±2% ±1%
10 to 20% ±3% ±1.5%
20 to 30% ±5% ±2.2%
30 to 50% ±6% ±3%

*At least ninety percent of all experimental data
fall within these error bands at the noted range
divisions.

The above accuracy is a static accuracy derived
at slow changing conditions with experimental
data. In rapidly changing conditions (e.g., in the
beginning of a rain event) the rate of response
for each wood dowel differs. After sufficient
time to equilibrate, the dowels will come to the
same water content. This difference in rate of
response is inherent in the traditional weighing
fuel stick racks. The effects are lessened by
the additional sorting of the dowels performed at
the factory. Readings above 50% are caused
by surface water on the probe or other external
factors. Readings are mathematically limited to
70%.

rms error

The sensor error is the sum of three influences:

1. Error due to the variability and non­reproducibility of the response
characteristics of the wood: density,
sorption characteristics, resin content, etc.

2. Error due to the aging of the wood
(cracking, discoloration, mass loss as resin
evaporates). This effect is not a factor for
new sticks.

3. Offset error due to the electronic circuitry
interchangeability, this error is negligible
over 10% fuel moisture.

A-2