Calibrate your sensor

Instructions

In this lesson you gathered soil moisture sensor readings, measured as values from 0-1023. To convert these into actual soil moisture readings, you need to calibrate your sensor. You can do this by taking readings from soil samples, then calculating the gravimetric soil moisture content from these samples.

You will need to repeat these steps multiple times to get the readings needed, with different wetness of soil each time.

1. Take a soil moisture reading using the soil moisture sensor. Write down this reading.

2. Take a sample of the soil, and weigh it. Write down this weight.

3. Dry the soil - a warm oven at 110°C (230°F) for a few hours is the best way, you can do this in sunlight, or place it in a warm, dry place until the soil is completely dry. It should be powdery and loose.

   💁 In a lab for the most accurate results you would dry in an oven for 48-72 hours. If you have drying ovens at your school, see if you can use these to dry for longer. The longer, the more dry the sample and the more accurate the results.

4. Weigh the soil again.

   🔥 If you dried it in an oven make sure it has cooled down first!

The gravimetric soil moisture is calculated as:

• W_wet - the weight of the wet soil
• W_dry - the weight of the dry soil

For example, say you have a soil sample that weights 212g wet, and 197g dry.

• W_wet = 212g
• W_dry = 197g
• 212 - 197 = 15
• 15 / 197 = 0.076
• 0.076 * 100 = 7.6%

In this example, the soil has a gravimetric soil moisture of 7.6%.

Once you have the readings for at least 3 samples, plot a graph of soil moisture % to soil moisture sensor reading and add line to best fit the points. You can then use this to calculate the gravimetric soil moisture content for a given sensor reading by reading the value from the line.

Rubric

Criteria	Exemplary	Adequate	Needs Improvement
Gather calibration data	Capture at least 3 calibration samples	Capture at least 2 calibration samples	Capture at least 1 calibration sample
Make a calibrated reading	Successfully plot the calibration graph and make a reading from the sensor, and convert it to gravimetric soil moisture content	Successfully plot the calibration graph	Not able to plot the graph