calibration of conductivity sensors living with the lab calibration of conductivity sensors

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Calibration cal ∙ i ∙ brate [kal-uh-breyt] living with the lab 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

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

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)

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

Steps salt concentration (%wt) output to LCD 0.00 0.05 0.10 0.15 living with the lab 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

Fit Output of Sensor to Salt Concentration living with the lab Fit Output of Sensor to Salt Concentration

Determine Equation for Salinity Control living with the lab Determine Equation for Salinity Control Use algebra to invert calibration equation. Calibration equation: Inverted equation: 𝑜𝑢𝑡𝑝𝑢𝑡=66.11 ln 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦 +663.23 𝑠𝑎𝑙𝑖𝑛𝑖𝑡𝑦=4.396× 10 −5 ∙ 𝑒 0.01513∙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 