conductivity sensor implementation

living with the lab

© 2011 LWTL faculty team

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living with the lab

conductivity measurement circuit• we use the same voltage divider circuit used earlier for the photoresistor circuit

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10 kΩ

digital output = 5V when HIGH(set high periodically to measure conductivity)

analog input(measures voltage across 10kΩ resistor)

∆𝑉 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑠𝑒𝑛𝑠𝑜𝑟+∆𝑉 10𝑘Ω𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟=5𝑣𝑜𝑙𝑡𝑠

• what happens to the electrical resistance of the water as it becomes more salty?

• if the resistance of the salt water decreases, then what happens to the voltage drop across the conductivity sensor?

• if the voltage drop across the conductivity sensor decreases, then how does this influence the voltage drop across the 10kΩ resistor?

• so, increasing the salinity of the water causes the analog input read by the Arduino to (increase or decrease)?

it decreases

it decreases

it increases

0 to 1023

what happens when salt is added to water????• The ionically bonded NaCl molecules dissociate into Na+ and Cl- ions and become mobile

• they are surrounded by polar water molecules (they are hydrated)

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electrodes(no voltage applied)

Na+

O

H H

Na+ O

H

H

OH

H

O

H H

O

H H

OH H

Cl-

Cl-

Cl-

Cl-

Na+

Na+

Na+

Na+

Cl-

Cl-

Cl-

Na+

Na+

Na+Na+

Na+

Cl-O

H

H

O

H H

OH H

OH H

O H H

O

H H

Cl-

Cl-

Cl-

Cl-

Cl-

Cl- Na+

Na+

Na+

Na+

Na+ Na+

Cl-

Cl-

Cl-

Cl-

Na+

Na+

water

applying voltage to induce electron flow

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e-

5V

e-

10 kΩ

e-

Cl-

Cl

Cl

Cl2

Cl-

e-

Cl-

e -

e-

OH-

H2O

H2OOH-

H

He-

e-

anode – oxidation(loss of electrons)

cathode – reduction(gain of electrons)

Cl-

Cl-

Cl-

Cl-

Na+

Na+

Na+

Na+

Na+

Na+

Cl-

Cl-

Cl-

Cl-

Na+

Na+

Na+

Na+

Na+

Cl-

ion migration

Na+

Na+ is a spectator ion

2𝐶𝑙− (𝑎𝑞 )→𝐶𝑙2 (𝑔 )+2𝑒−

oxidation occurs at the positively charged anode:

2𝐻2𝑂 (𝑙)+2𝑒−→𝐻2 (𝑔 )+2𝑂𝐻−(𝑎𝑞)

reduction occurs at the negatively charged cathode:

electrons are really not directly conducting through the water from one electrode to the other (like when electrons move through a copper wire)

It seems like an Na+ would accept an electron and be reduced . . . why not???

why is H2O reduced and not Na+ ???

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reactions further down in the table are

less likely to occur

• reactions with positive voltages will occur spontaneously

• you must apply external voltage across the electrodes to make a reaction with a negative potential occur

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the net reaction of conductivity system

the net reaction occurring in the system is . . .

• we must apply at least 2.19 V to the conductivity circuit to drive the reaction

• applying 5 V is sufficient, and higher voltages will increase the rate of oxidation & reduction reactions

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living with the lab

system wiring

10 kΩ

digital output = 5V when HIGH(set high periodically to measure conductivity)

analog input(measures voltage across 10kΩ resistor)