1. Conductivity Sensor

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3. Calibrating the Conductivity Sensor
The salinity sensor will be calibrated using water with various salt concentrations
0.00% weight NaCl – We will use deionized water (DI water)
0.05% weight NaCl
0.10% weight NaCl
0.15% weight NaCl
Why not just use tap water?
Standard tap water contains dissolved ions such as sodium, calcium, iron, copper and bromide
The electrical resistivity of tap water can vary widely (15kΩ-cm is typical)
Ions have been removed from DI water resulting in a much higher resistivity (18MΩ-cm is typical)
Did you notice the strange units on resistivity (Ω-cm)? The electrical resistance R of a body (measured in Ω) is related to the electrical resistivity ρ (measured in Ω-cm) as 𝑳 𝑨
𝑅= 𝜌∙𝐿 𝐴
A = cross sectional area
L = length over which resistance is measured
ρ = resistivity which is a material property

4. How the Calibration Works
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. Salt Concentrations
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

6. Circuit and Sketch void setup() { Serial.begin(9600);
pinMode(8, OUTPUT); }
void loop() {
digitalWrite(8,HIGH); // apply 5V to the conductivity sensor
delay(100); // hold the voltage at pin 8 for 0.1s
int analogS=analogRead(1); // read voltage on + side of 10kohm resistor
digitalWrite(8,LOW); // turn off power to the conductivity sensor
Serial.println(analogS); // print the analog input reading (0 to 1023)
delay(1000); // delay 1 second between measurements
10 kΩ
digital output = 5V when HIGH
(use a digital pin) (set high periodically to measure conductivity)
analog input (measures voltage across 10kΩ resistor)

7. Steps for Calibration salt concentration (%wt)
10 kΩ
digital output = 5V when HIGH (set high periodically to measure conductivity)
analog input (measures voltage across 10kΩ resistor)
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
Make sure the analog value is near 0. This ensure you are starting with as few impurities in your system as possible.
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 4 and 5 for 0.10 wt% salt water and 0.15 wt% salt water
salt concentration (%wt)
Output to Serial Monitor
0.05
0.10
0.15
collect this data

8. Plot Calibration Data and Perform Linear Regression
Salt concentration
Arduino
(fractional)
output
0.0005
465
0.0010
511.5
0.0015
537.5
Perform linear regression to determine the expected output of the conductivity circuit as a function of salinity
Linear
Polynomial
Power

9. Examine Fits Over Possible Salinity Ranges
how your fit behaves beyond 0.15 wt% salt since your salinity may increase well beyond 0.15% when salty water is added
Do you see any potential problems?
Which fit seems to be the best? Why?

10. Equation Needed for Salinity Control
use four or five digits for these fitting constants
Notice that this plot and calibration equation is based on the fractional form of salinity
Invert the calibration equation to obtain
Salt concentration vs output
𝑜𝑢𝑡𝑝𝑢𝑡=1274.1∙ 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦= −24 ∙ 𝑜𝑢𝑡𝑝𝑢𝑡
equation to compute setpoints for control
equation to compute salinity from sensor output
Inversion Methods: Swap axes on your graph or use algebra (next slide)

11. Inverting Calibration Equation Using Algebra
output vs salt concentration
𝑜𝑢𝑡𝑝𝑢𝑡=1274.1∙ 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Divide both sides by
𝑜𝑢𝑡𝑝𝑢𝑡 = 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Raise both sides to the same power
𝑜𝑢𝑡𝑝𝑢𝑡 = 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Simplify and expand
∙𝑜𝑢𝑡𝑝𝑢𝑡 =𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Rule: 𝒙𝒚 𝒛 = 𝒙 𝒛 𝒚 𝒛
∙ 𝑜𝑢𝑡𝑝𝑢𝑡 =𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Simplify
−24 ∙ 𝑜𝑢𝑡𝑝𝑢𝑡 =𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦
Salt concentration vs output

12. Writing Equation with Exponents in a Sketch
𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦= −24 ∙ 𝑜𝑢𝑡𝑝𝑢𝑡
pow(base, exponent)