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Objective
Our goal was to develop and test a wireless methanedetection system for monitoring of methane
.
in septic systems is a problem on Cape Cod because
the ma orit of sewa e is handled b se tic s stems.
We were interested in measuring methane levels
produced by septic systems to determine if sufficient
quantities were available for capture and reuse in thesystem to remove nitrogen.
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Kim, Han, Little, Johnson and Perov with Septic Test Facility in background
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ypes o ensors- Two heated catalytic
e ements are set in a oc ;one is sealed; the other is incontact with the sample.
Wheatstone Bridge circuit. If
the sample containsh drocarbons orcombustible inorganiccompounds, they will burnincreasing the temperature of
t e sensors an c anging t eresistance. The change inresistance is proportional to
.
PID Analyzers, LLC copyright 2003-2006
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Sensor
BatteryandSolarpanel
Sensor placed above the level of sewage in the septic tank
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ElectricalSchematicforSystem
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Methane Data Monitoring System
The CH4 sensor is connected to an amplifier board whichh n r n f r h i n l r i fr n m f r
transmission to the base computer in a nearby building.
There is a solar powered battery box that supplies poser to
the sensor and amplifier circuit. The layout is shown in the
agram.
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Sensor and AmplifierThe sensor head has a range of 0 to 5% methane and
constitutes half of a Wheatstone bridge circuit. The
.
The second art of the brid e is art of the am lifier
board which amplifies the differential signal by 10 or
100 times (gain controlled by a switch). The output
goes to an input of the analog to digital converterboard MDA300 which is attached to the Crossbow
mote .
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RF Crossbow Mote
433 MHz to a base computer located in an office about50m away.
The mote is programmed to operate in a low-power moden w c t s eeps most o t e t me ow power an awa es
every 5 minutes for 30 seconds to take measurements and
. .batteries.
The data packet from the mote contains the mote battery
voltage, humidity, and temperature inside the mote box.
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Mote Box and Power
weather-proof box (see yellow box in figure) sittingoutside the septic tank. The sensor head (inside the
tank) is at the end of a 1 to 2 m 3 wire cable.
e amp er toget er w t t e sensor ea , w en
activated, draw 110 mA from a +6 volt battery source
.provided by a solar powered battery box connected to
the mote box b a cable (see dia ram)
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Solar Panel Arrangement
the battery box to maintain the +6V and the -6Vneeded by the electronics. In full sun, each panel can
supply up to 70mA at 7V. The largest consumer was
the +6V battery source so this was monitored by usingon e o t e ana og nputs to t e oar
attached to the mote.
The graphs show that the solar panels are sufficient to
kee the batteries char ed even in harsh winter
conditions.
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6V batter and mote batter volta es
6
7
3
4
5
oltage
0
1
2V
6-Feb 11-Feb 16-Feb 21-Feb 26-Feb
Time
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Sewage Inputvs Time(perday)
250
300
200
e(gal)
100
Sewag
0
50
Time
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Feb14
1.2
1.3
ensor o tage vs me
0.9
1
1.1
0.6
0.7
0.8
adc0(v)
0.2
0.3
0.4
0.5
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1.3
adc0(v)vs wholeperiod
1
1.1
1.2
0.8
0.9
c0(v)
0.5
0.6
0.7a
0.3
0.4
0.2
Time
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Future plans
We will use a temperature controlled box with air (no
of temperature and time over a period of months. This willallow us to check the drift of the sensor and determine the
time intervals needed for recalibration.
We wi a so p ace a t ermocoup e at t e ocation o t e
methane sensor as an additional input to the mote to
.will help in understanding methane production inside the
se tic s stem in different environmental conditions.
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onc us on-
wireless methane data collection system. The data areconsistent with sewage flow conditions as stated by the
septic test facility. This device will allow the facility to
determine detailed behavior of different types of septicsystems an test t e v a ty o co ect on an reuse o
methane to eliminate nitrogen from septic systems.
Acknowledgements: We are very appreciative of
the assistance of ersonnel at the Barnstable
County Alternative Septic System Test Center