September 25, 2014

Crafting an electronic psychrometer

In Cigar Journal 2/2014 and 3/2014 Marc André reports in detail about different types of hygrometers and their calibration.

As I reported earlier, I use in my humidor a homemade, microcontroller-driven humidification system, which determines the moisture over three capacitive humidity sensors. The initial setup was calibrated by conventional hair hygrometer, which were calibrated using the salt method. Because this calibration has some difficulties, I always treated the determined humidity values ​​with a certain skepticism.

To avoid these difficulties, I now have crafted an electronic psychrometer.

This determines the relative humidity very accurately from the difference between the ambient temperature and the so-called wet-bulb temperature. For this purpose, one thermometer is covered with a cotton stocking, moistened with distilled water. This thermometer will measure the "evaporative cooling". This is lower, the drier the surrounding air. Who doesn't want to do the calculations, can find the relative humidity on a psychometer table.

It is more accurate to calculate the exact air humidity from the two temperature values​​, the air pressure, and a psychrometric constant. The corresponding physics has already been described in countless articles, so I beg to refer to Wikipedia/relative humidity and Wikipedia/psychrometer.

Fundamentally is the psychrometer formula, which describes the relationship between the vapor pressure e, the psychrometric difference (t-t'), the saturation vapor pressure at the temperature of the wet thermometer E', the air pressure p and a constant specific psychrometer coefficient (psychrometric constant) C:

e = E' - [ C x p x ( t - t' )]

The relative humidity f is the ratio of the actual water vapor pressure e to the saturation vapor pressure E' at the prevailing air temperature t:

E' = 6,1078 x exp [( 17,08085 x t ) / ( 234,175 + t )]

f = e / E'

That is the theory. Now the practice.

Used hardware:
Temperature sensors: Dallas DS18B20
Microcontroller: Arduino UNO

I chose the DS18B20 because he works very well with the Arduino microcontroller, is easy to program and has a high accuracy at an acceptable tolerance. The tolerance of the thermometers are +/- 0.5 ° C and within the calculations it results an inaccuracy in the relative humidity of +/- 0.6%. This is an outstanding value for a hygrometer and sufficient for me. Even if it is a bit bigger.

The wiring diagram itself is very simple and needs no further explanation.

And the required software to calculate the relative humidity is then just a finger exercise, especially since I omitted a separate display shield and show the measured values on the laptop via USB.

As in the case of a centrifugal psychometer, an air flow of approximately 2 m/s is also required here. This is the only way to ensure the perfect function. In my humidor the psychrometer is always positioned in the airflow of the circulating fan. For this purpose I switch the fan to continuous operation.