1. calibration of conductivity sensors
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calibration of conductivity sensors

2. DISCLAIMER & USAGE living with the lab
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3. Calibration cal ∙ i ∙ brate [kal-uh-breyt]
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cal ∙ i ∙ brate [kal-uh-breyt]
-verb (used with object), -brat ∙ ed, -brat ∙ ing.
1. to determine, check, or rectify the graduation of (any
instrument giving quantitative measurements).
Calibration
Associate sensor output with salt concentration
Relate sensor output and salt concentration using an equation (linear regression)
The goal is to be able to compute the salt concentration based on sensor output

4. The Basic Idea living with the lab
Adding salt to the water will increase the availability of Cl- ions at the anode
More ions at the anode will increase the rate at which chemical reactions can occur
The “electrical resistance” of the salt water will decrease as more salt is added to the water
The analog voltage on the + side of the 10kΩ resistor will increase as more salt is added
Correlating this voltage with the salt concentration will allow us to “calibrate” the conductivity sensor e- 5V
10 kΩ
Cl- Cl
Cl2
OH-
H2O H
anode – oxidation (loss of electrons)
cathode – reduction (gain of electrons)
Na+
ion migration
Na+ is a spectator ion

5. The Circuit and Sketch 10 kΩ living with the lab
#include <SoftwareSerial.h>
SoftwareSerial mySerial(3,2);
void setup() {
mySerial.begin(9600); delay(500); // set data rate to 9600 baud; wait for bootup
mySerial.write(254); mySerial.write(1); // clear screen & move to top left position
mySerial.write(254); mySerial.write(129); // move cursor to row 0, position 1
mySerial.write("Conductivity Sensor"); // print a text string starting at (0,1)
mySerial.write(254); mySerial.write(196); // move cursor to row 1, position 4
mySerial.write("Calibration"); // print a text string starting at (1,4)
mySerial.write(254); mySerial.write(213); // move cursor to row 3, position 1
mySerial.write("analog input="); // print text string at (3,1)
mySerial.write(254); mySerial.write(12); // turn cursor off to keep screen clean
pinMode(4,OUTPUT); }
void loop() {
digitalWrite(4,HIGH); // apply 5V to the conductivity sensor
delay(100); // hold the voltage at pin 3 for 0.1s
int analogS=analogRead(0); // read voltage on + side of 10kohm resistor
digitalWrite(4,LOW); // turn off power to the conductivity sensor
mySerial.write(254); mySerial.write(226); // move cursor to row 3, position 14
mySerial.print(analogS); // print the analog input reading (0 to 1023)
mySerial.write(" "); // overwrite previously printed numbers
delay(1000); // delay 1 second between measurements
10 kΩ
pin 3 = 5V when HIGH (set high periodically to measure conductivity)
analog input 0 (measures voltage across 10kΩ resistor)

6. Salt Concentrations living with the lab
Each group of students should put about 1.5 inches of water in four bottles
The four bottles should contain . . .
DI water
0.05% weight NaCl
0.10% weight NaCl
0.15% weight NaCl
Please take ONLY the amount that you will need to use TODAY
Be sure to label your water bottles
Swish a small amount of DI water around in your bottle to wash out impurities before filling with calibration water

7. Steps salt concentration (%wt) output to LCD 0.00 0.05 0.10 0.15
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Steps
Configure your flow loop as required for homework
Implement the conductivity sensor circuit on your breadboard
Flush tank with DI water.
Pour enough DI water into your fishtank to fill the flow loop
Turn on the pump to run the flow loop for about a minute to “wash out” the impurities.
Prime the pump if necessary by switching the three-way valve to the drain
Turn the three-way valve toward the drain to flush the system
Repeat to completely clean the system
Fill the system with DI water
Collect calibration data for DI water
Fill the system with 0.05 wt% salt water. Flush once, and refill with 0.05 wt% salt water
Collect calibration data for 0.05wt% salt water
Repeat steps 6 and 7 for 0.10 wt% salt water and 0.15 wt% salt water
salt concentration
(%wt)
output to LCD
0.00
0.05
0.10
0.15
collect this data

8. Fit Output of Sensor to Salt Concentration
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Fit Output of Sensor to Salt Concentration

9. Determine Equation for Salinity Control
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Determine Equation for Salinity Control
Use algebra to invert calibration equation.
Calibration equation: Inverted equation:
𝑜𝑢𝑡𝑝𝑢𝑡=66.11 ln 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦=4.396× 10 −5 ∙ 𝑒 ∙𝑜𝑢𝑡𝑝𝑢𝑡
An equation like this will be used to predict %wt salt based on sensor output.
Try polynomial and power fits too to see how they compare.
We will discuss the best form of the equation to use later we write our Arduino program to control salinity.
the end 