# Choosing the right sensor

Hi

Me and my colleague are doing a project with a height measurement device for humans and we want to measure the height of persons of all ages (40cm - 210cm) with a sensor, either using a infared, laser, ultrasound or decoder.

The measuring will take place in an environement with dust, sun (lots of heat), wind and sometimes high humidity.

Our question is: Which sensor would be most suitable for this project?

Thanks!

/Philip

Is there a good reason not to use the tried-and-tested mechanical one?

My feeling is the ultrasonic will work quite well as long as you position it above the person being measured and point the sensor directly downwards towards their head. I don't think any of the environmental factors you mention should affect your results much.

Though I'm not sure what you mean with the "decoder" option.

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Atmospheric conditions will affect the speed of sound, so ultrasound will need compensation.

Fair enough...

Speed of sound at 20C is 37.17m/s, and at 40C (a really hot day) is 37.74m/s. This is first approximation based on square-root-increasing speed of sound, not considering humidity or frequency.

Assuming he has his sensor 1m above a person's head the travel time of sound (there and back, 2m) is 53.81ms at 20C and 52.99ms at 40C. If 20C is assumed but 40C is reality then the 52.99ms reading (assuming the sensor can read that accurately) will correspond to a distance of 1.970m, divided by 2 is 0.98m whereas the actual distance is 1.00m. So the reading would be 2cm off.

Compensating for 2cm of error in a span of 20C sounds like it would be easily doable using a thermistor to get a measure of the temperature.

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measure the height of persons of all ages

People of less than a year can't stand up. Have you thought of that?

Thanks for all your answers. Regarding the mechanical solution it restricts options and opportunities which a sensor Will give us.

Yes we have considered the fact that young children have to lay down.

We have also looked at the infrared sensor, but we are afraid that the precision is low at long range (above 1.5 m). Any advice or experience with measuring long distances with ir sensors? A deviation of 1 cm per 10 degrees is quite much and we mean to use it under many different circumstances.

We would use the decoder with a measuring tape to record the rotations when it is pulled. But it would be optimal to avoid the tape.