Build and calibrate a thermometer

You are the engineering team

What temperature is 524?

The Arduino returns a number from 0 to 1023. It has no unit.

The 100 minutes

• 10 min · introduction
• 15 min · wiring
• 10 min · first reading
• 30 min · four calibration baths
• 10 min · test bath
• 25 min · report and discussion

Three rules

What each group needs

• Arduino Uno, USB cable, one laptop
• breadboard and five male-to-male jumpers
• the thermistor: a small black bead with two legs
• one 10 kΩ resistor: brown, black, orange, gold
• freezer bag, paper tape or rubber band, a piece of eraser or card
• a digital reference thermometer, shared
• tupperware, two cups, ice, tap water, spoon
• millimetre graph paper, pen, ruler

The circuit in one picture

The breadboard

Unplug, then extend the thermistor

The resistor

The thermistor wires

5V and A0

Ground

Check the rows before plugging in

Look at your own breadboard. How many things sit in row 20?

Put the thermistor in its bag

Upload the sketch

// raw reading on A0
void setup() {
  Serial.begin(9600);
}

void loop() {
  int raw = analogRead(A0);
  Serial.println(raw);
  delay(1000);
}

Open the Serial Monitor

• Magnifier icon at the top right, 9600 baud at the bottom right.
• In room air the number sits roughly halfway, around 500.
• Pinch the bead through the bag: the number should rise slowly.

If it does not work

The reading is stuck near 1023. Which side of the circuit is open?

1. the 5V side: a10, b10, the thermistor joints
2. the ground side: c25, a25
3. the A0 jumper

Four baths, cold to hot

• Bath 1 · ice cubes floating in cold water: 0 °C. Stir before measuring.
• Bath 2 · cold tap water.
• Bath 3 · room temperature.
• Bath 4 · hot water from the teacher, diluted until the reference reads 40 to 50 °C.

Each bath at least 5 °C from the one before.

In every bath

Does your thermometer work?

• Prepare one more bath, at least 3 °C from all four calibration baths.
• One person reads the reference and keeps the value secret.
• Record five readings of N in this bath, as in every other bath, and take the mean: this mean is N in the equation below.
• Quick equation from the ice bath and the hot bath, using the mean N and mean T already recorded for each:

Announce T. Then read out the reference. Difference = T calculated − T reference, with its sign.

The report, in order

1. Question
2. Prediction
3. Variables
4. Instruments
5. Method
6. Results
7. Graph
8. Gradient and equation
9. Test bath
10. Sensitivity and errors
11. Conclusion

What you change, what you measure

• Independent: the reference temperature. You choose the bath.
• Dependent: N, the sensor's response.
• Controlled: same sensor, bag, wiring, depth, stirring, waiting time, same reference thermometer.

What to record

• The resolution and reading uncertainty of each instrument — worked out on the vademecum slides just ahead.
• T before and T after, for every bath.
• The time N took to settle.
• Five readings of N, their mean and their half range.

Resolution

One reading, how sure are you?

Sensitivity

Random error and half range

Systematic error and anomalous results

• Systematic error: it moves every reading the same way, for example reading before the sensor has settled. Repeating the measurement does not remove it.
• Anomalous result: a point far from the trend. Check it, repeat it if you can, and say what you did. Do not delete it quietly.

By hand, on millimetre paper

• N on the vertical axis, T on the horizontal axis, labelled N / counts and T / °C.
• Scales use more than half the grid. Points are small crosses.
• One straight best-fit line, with about as many points above as below.
• Do not join the first and last points.
• Gradient from a big triangle with two points that are not data points. Intercept N₀ at 0 °C.

Want the computer to do the maths?

Install pyserial

• Click Start, type PowerShell, open it.
• Type the line below and press Enter.
• Wait for a line ending in "Successfully installed pyserial".

py -m pip install pyserial

Find the COM port

• Close the Arduino IDE's Serial Monitor first — only one program can read the port at a time.
• Open Device Manager (Start, type "Device Manager").
• Expand Ports (COM & LPT).
• Note the number next to "Arduino Uno", e.g. COM3.

Ten readings, mean and spread

import serial, statistics, time

PORT = "COM3"   # change to your port
ser = serial.Serial(PORT, 9600, timeout=2)
time.sleep(2)
ser.reset_input_buffer()

readings = []
while len(readings) < 10:
    line = ser.readline().decode().strip()
    if line.isdigit():
        readings.append(int(line))

print(readings)
print("mean =", statistics.mean(readings))
print("stdev =", statistics.stdev(readings))

A live graph

import serial, time
import matplotlib.pyplot as plt

ser = serial.Serial("COM3", 9600, timeout=2)
time.sleep(2)
xs, ys = [], []
plt.ion()
fig, ax = plt.subplots()
line, = ax.plot(xs, ys)
t0 = time.time()

while True:
    raw = ser.readline().decode().strip()
    if raw.isdigit():
        xs.append(time.time() - t0)
        ys.append(int(raw))
        line.set_data(xs, ys)
        ax.relim(); ax.autoscale_view()
        plt.pause(0.05)

What to hand in