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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