Executive Board
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The Netherlands
www.tue.nl
Tjeerd Spruijt and Rowan de Nijs
Calibration of a DHT11 sensor
Date
24 March 2015
Version
1
Calibration guide for Relative Humidity and Temperature
sensor DHT11.
Table of contents
Title
Calibration of a DHT11 sensor
1
1.1
1.2
1.3
Introduction
Products to be used
Time schedule
DHT11 Datasheet
3
3
3
3
2
2.1
2.2
Temperature Calibration
Freezing point test
Results
4
4
4
3
3.1
3.2
Relative Humidity test
Saturated Salt Solution test
Results
5
5
5
Technische Universiteit Eindhoven University of Technology
Introduction
1
This guide will treat the calibration of the DHT11 temperature and relative humidity sensor.
Both parts of this sensor need a separate calibration test, so two tests will be executed.
Please note that these calibration tests take some time to be performed!
1.1
Products to be used
1. Ice-cubes. Create them yourself using an ice-cube tray or bag which can be bought at
your local supermarket
2. A bowl. A bowl or jar is used for both tests, the same might be used. Make sure it has
a size of at least 10 cm in height and 10 cm in width
3. Towel and Hamer (or other stuff to crush your ice-cubes)
4. Water/Air tight bags
5. Table salt (quiet a lot!). For the relative humidity calibration, salt (Sodium Chloride) will
be used. The amount of salt is depending on the size of the jar or bowl you are using
6. Clingfilm/tape to seal the jar/bowl
7. And all the material to create a working DHT11 sensor
1.2
Time schedule
As mentioned in the introduction, the calibration tests take some time to be performed. Table
1 shows a global time planning for this assignment.
Table 1: Time schedule calibration tests
Day
Day 1
Day 2
Day 6
1.3
Task
Put the ice-cube tray/bag into the fridge
Perform temperature calibration test
Create Saturated Salt Solution test set up
Analyse Saturated Salt Solution test results
DHT11 Datasheet
Study the DHT11 datasheet, which can be found on the course sharepoint.
Some important calibration data you might find are the measurement ranges and accuracy.
3 Calibration of a DHT11 sensor / Version 1
Technische Universiteit Eindhoven University of Technology
Temperature Calibration
2
To test the temperature sensor, you will need a material with a known temperature. Like
boiling water which is certain to be 100°C, as it evaporates above this temperature. However,
in the datasheet mentioned before, it is said that this specific sensor can only measure up to
50°C, so boiling water is not an option. Instead of using boiling water, water at the freezing
point will be used. As you probably know, water which is almost freezing, is at zero degrees
Celsius.
Note that the more different calibration tests are done with a sensor, the more certain the
measurements outcomes are.
2.1
Freezing point test
Make sure that your sensor does not get wet during this test!
The steps below, can also be found in this link: http://www.grahamsensor.com/doc/Field%20Check.pdf
1. For this calibration test, the ice cubes which were placed into the fridge before, are
used. If you didn’t do this, you might want to create these now!
2. The next step is to crush the ice into small parts (<2mm) with a towel and hammer or
something else, and put it all in the jar/bowl
3. Add a small amount of water, just enough to remove any air between the ice. Make
sure that the jar/bowl is full enough to put your sensor under the ice without hitting the
bottom
4. Place the sensor in a watertight bag, and remove as much air as possible from the
bag.
5. Close the bag as tight as possible around the wires of the sensor, and close it with a
piece of tape
6. Stir the ice-water regularly
7. Put the sensor in the ice-bath and keep stirring once in a while
8. Read the data from the sensor
2.2
Results
If the test went correctly, the outcome should be around 0°C. The sensor however has an
accuracy of 2°C, so between -2 and 2°C might be possible outcomes. If it does not come to
these temperatures, you might need to wait and stir more, or you didn’t add enough ice to the
water.
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Technische Universiteit Eindhoven University of Technology
Relative Humidity test
3
To test the Relative Humidity sensor, there are different methods as well. One might be to
drop the sensor in a bowl of water and see if it reads 100%, this however requires a water
proof Relative Humidity sensor, SO DON’T TRY THIS WITH YOURS!
The calibration test that you will be doing is called a saturated salt solution test.
3.1
Saturated Salt Solution test
This test will take around 4 days to create a balanced situation.
The steps that are described below, can also be found on:
http://ambientweather.wikispaces.com/Humidity+Calibration,+Saturated+Salt+Test
1. First create a mixture of the salt with water until you have a completely saturated salt
solution.
2. Put about 2.5 cm of this solution in your jar/bowl, without standing water on top of the
salt solution.
3. Place the sensor above your solution in the jar/bowl, and seal it using the clingfilm,
airtight bag and/or tape. Make it airtight!
4. Leave the sensor in this measurement setup during 4 days. You can use a usb/power
supply to let the setup measure and store data on the SD-card
5. After 4 days the data can be read, and the relative humidity should be around 75%
3.2
Results
If the measurement was executed correctly, the sensor should measure around the 75%
relative humidity above the salt solution. Note that the sensor has a 5 % accuracy, so values
between 70 and 80% are possible.
If your sensor did not measure within this range, there might be a number of problems. Did
you use a salt solution with no standing water? Was the measurement setup airtight? Was the
measurement done on a relatively constant temperature? Is the sensor failing?
Probably a lot more can be going on, but these are things that are quiet easily to check/fix.
5 Calibration of a DHT11 sensor / Version 1