Campbell Scientific CS506 User Manual

CS506 Fuel Moisture Sensor

Revision: 3/12

Copyright © 1998-2012

Campbell Scientific, Inc.

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CS506 Table of Contents

PDF viewers: These page numbers refer to the printed version of this document. Use the
PDF reader bookmarks tab for links to specific sections.

1. General Description..................................................... 1

2. Specifications ..............................................................1

3. Installation.................................................................... 2

4. Wiring............................................................................ 4

5. Datalogger Programming............................................ 5

5.1 CR1000 Programming..............................................................................6

5.2 CR10X Programming ...............................................................................7

6. Maintenance ................................................................. 8

Figures

Tables

3-1. Back view of the 26817 fuel moisture/temperature mounting

stake; mount ~12-in. over duff layer....................................................3

3-2. Front view of the 26817 fuel moisture/temperature mounting stake.......4

4-1. CS506 Wiring..........................................................................................4

5-1. Wiring for Example Programs ................................................................5

i

CS506 Fuel Moisture Sensor

1. General Description

The CS506 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 small­diameter (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 and provides an
indication of moisture changes in forest fuels.

The CS506 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 CS506 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%.

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 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 waterproof 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

1

CS506 Fuel Moisture Sensor

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 CS506 output is essentially a square wave with an amplitude
of ±0.7 VDC. The frequency of the square wave output ranges from
approximately 31 to 58 kHz. Its output period ranges from 17 to 32 μs.

*Fuel moisture accuracy:
(with a new stick)
90% of all

measurements rms error

range

0 to 10% ±1.25% ±0.74%
10 to 20% ±2.00% ±0.90%
20 to 30% ±3.40% ±1.94%
30 to 50% ±4.11% ±2.27%

Range: 0-50%

Power Supply: 5 VDC minimum to

18 VDC maximum

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

on at 5 V (>4 VDC maximum 18 VDC)

3. Installation

Current usage: 65 mA active/ 45 uA quiescent

Output signal: ±0.7 V square wave with an output frequency of

approximately 31 to 58 kHz.

Weight: <0.5 kg (<1 lb)

Dimensions
Dowel: 1.3 cm (1/2 in) diameter, 50.8 cm (20 in) long
Electronics: 10.2 x 6.4 x 1.9 cm (4 x 2.5 x 0.75 in)

*The above accuracy is a static accuracy derived at slow changing conditions
with experimental data for the CS505. See Appendix A for explanation of
accuracy.

As shown in Figure 3-1 and Figure 3-2, both the CS506 and CS205 install on
the 26817 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 forest-floor duff layer. The stake is carefully
hammered into the ground so that it is vertical. The mounting bracket is
positioned so that the sensor will be approximately 12 in. (30 cm) above the
ground, and then secured by tightening the nuts on the U-bolts. Once the
bracket is installed, insert the CS506 electronics into the two spring clips. The
fuel moisture sensor (part number 26601) installs on the CS506 electronics
with the supplied Phillips-head screws. The CS205 fuel temperature stick is
inserted into the mounting stake’s compression fitting. The 107 temperature

2

CS506 Fuel Moisture Sensor

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 three ultraviolet light resistant
cable ties. One cable tie passes through the two slots where the fuel moisture
stick attaches to the CS506 sensor in order to loosely secure the cable of the
CS205/107. The other two cable ties are used to secure both cables to the
mounting stake.

CS506
Sensor

26601 Ponderosa

Pine Dowel

CS205

Stick

107 Temperature

Probe

FIGURE 3-1. Back view of the 26817 fuel moisture/temperature

mounting stake; mount ~12-in. over duff layer

3

CS506 Fuel Moisture Sensor

107 Temperature

Probe

CS205 10-Hour Fuel

Temperature Stick

CS506 Fuel

Moisture

Sensor

4. Wiring

26601 Ponderosa

Pine Dowel

FIGURE 3-2. Front view of the 26817 fuel moisture/temperature

mounting stake

Connections to Campbell Scientific dataloggers are given in Table 4-1. When
Short Cut for Windows software is used to create the datalogger program, the
sensor should be wired to the channels shown on the wiring diagram created
by Short Cut.

TABLE 4-1. CS506 Wiring

Color Description

Red Power 12 V 12 V 12 V

Black Ground G G G

Green Signal Analog Channel Analog Channel Analog Channel

Orange Enable Control Port Control Port Control Port

CR800/CR1000/
CR3000

CR10(X)/CR500/
CR510

CR23X

4

Clear Ground

G

5. Datalogger Programming

This section is for users who write their own datalogger programs. A
datalogger program to measure this sensor can be created using Campbell
Scientific’s Short Cut Program Builder software. You do not need to read this
section to use Short Cut.

The CS506 has a built in enable circuit. When voltage on the enable lead is
less than 1 VDC, the sensor is off. When a voltage greater than 4 VDC,
commonly 5 VDC, is applied to the enable lead, the sensor is on. The output
signal is a ±0.7 volt square wave. The CRBasic instruction PeriodAvg is used
with the CR800-series, CR1000, and CR3000 dataloggers to measure the
period of the output signal in microseconds on a single-ended analog input
channel. For Edlog dataloggers, instruction P27 Period Average may be used
with the CR500, CR510, CR10, CR10X, and CR23X dataloggers. The CS506
is not compatible with 21X, CR7, or CR200-series dataloggers.

Since fuel moisture does not change very rapidly, the sensor is typically
measured only once per hour.

After the period of the output signal in microseconds is measured it is
converted to percent water content using one of two equations:

CS506 Fuel Moisture Sensor

θ(τ ≤17.7) = 7.6298τ -130.0904

2

θ(τ > 17.7) = 0.0406τ

where θ is the percent of water by weight in the fuel moisture stick and τ is
period average in microseconds.

There is a slight disconnect in the two equations at 17.7 μs where the linear
equation gives a water content of 4.98% while the quadratic equation gives

5.68%. A sudden small increase or decrease in the measured water content
near 5% is to be expected as the datalogger changes from one equation to the
other.

TABLE 5-1. Wiring for Example Programs

Color CR1000 CR10X

Red 12 V 12 V

Black G G

Green SE1 SE1

Orange C1 C1

Clear

+ 3.7685τ - 73.7974

G

5

CS506 Fuel Moisture Sensor

5.1 CR1000 Programming

'CR1000

'CR1000 Program for CS506

'Declare Variables and Units

Public FuelM
Public PA_uS

Units FuelM=%
Units PA_uS=uSec

'Define Data Tables

DataTable(Table1,True,-1)
DataInterval(0,60,Min,10)
Sample(1,FuelM,FP2)
Sample(1,PA_uS,FP2)
EndTable

'Main Program

BeginProg
Scan(10,Sec,1,0)

'CS506 Fuel Moisture Sensor measurement FuelM and PA_uS:

If IfTime(0,1,Hr) Then
PortSet (1,1)
PeriodAvg(PA_uS,1,mV250,1,0,0,100,10,1,0)
PortSet (1,0)
If PA_uS <= 17.7 Then
FuelM = 7.6298* PA_uS - 130.0904
Else
FuelM= 0.0406* PA_uS ^2 + 3.7685 * PA_uS -73.7974
EndIf
EndIf

'Call Data Tables and Store Data

CallTable(Table1)
NextScan
EndProg

The following program measures a CS506 fuel moisture sensor using the
CRBasic PeriodAvg() instruction. See Table 5-1 for wiring details that match
this program.

6

5.2 CR10X Programming

The following program measures a CS506 fuel moisture sensor using Edlog’s
Instruction 27 (Period Averaging). See Table 5-1 for wiring details that match
this program.

;{CR10X}

;CR10X Program for CS506

*Table 1 Program
01: 10.0000 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: 4 200 kHz Max Freq @ 2 V Peak to Peak, Period Output
3: 1 SE Channel
4: 100 No. of Cycles
5: 1 Timeout (0.01 sec units)
6: 1 Loc [ PA_uS ]
7: 1 Multiplier
8: 0 Offset

4: Do (P86)
1: 51 Set Port 1 Low

5: If (X<=>F) (P89)
1: 1 X Loc [ PA_uS ]
2: 4 <
3: 17.701 F
4: 30 Then Do

6: Polynomial (P55)
1: 1 Reps
2: 1 X Loc [ PA_uS ]
3: 2 F(X) Loc [ FuelM ]
4: -130.09 C0
5: 7.6298 C1
6: 0.0 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

7: Else (P94)

CS506 Fuel Moisture Sensor

7

CS506 Fuel Moisture Sensor

8: Polynomial (P55)
1: 1 Reps
2: 1 X Loc [ PA_uS ]
3: 2 F(X) Loc [ FuelM ]
4: -73.797 C0
5: 3.7685 C1
6: 0.0406 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

9: End (P95)

10: End (P95)

11: If time is (P92)
1: 0 Minutes (Seconds --) into a
2: 60 Interval (same units as above)
3: 10 Set Output Flag High (Flag 0)

12: Set Active Storage Area (P80)^18555
1: 1 Final Storage Area 1
2: 60 Array ID

13: Real Time (P77)^12334
1: 1220 Year,Day,Hour/Minute (midnight = 2400)

14: Sample (P70)^15015
1: 2 Reps
2: 1 Loc [ PA_uS ]

6. Maintenance

8

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

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

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