Electronic Torricelli Barometer
Although it does not have the same charm as real mercury barometers
with long glass tubes on pieces of carved and polished wood, the
Torricelli barometer discussed here is a functional equivalent and
electronic replica of the Torricelli barometer. Actually, rather than
displaying the atmospheric pressure on the traditional digital displays,
we preferred to reproduce the general look of this respected
predecessor of electronic barometers.
The mercury tube is, of course, replaced by a simple LED scale which,
if not as beautiful, is still less toxic for the environment in case of
breakage. As indicated on the drawing, the pressure sensor utilized is a
Motorola MPX2200AP. This circuit is adapted for measuring absolute
pressure and has a range well suited for atmospheric pressure. Without
entering too deep into the technical details, such sensors deliver an
output of voltage proportional not only to the measured pressure but,
unfortunately, to their supply voltage as well.
Hence they must be powered from a stable voltage which is ensured
here by the use of IC1. Since the output of the MPX2200 is differential
and at a very low level, we had to resort to the use of four operational
amplifiers IC4.A to IC4.D, contained in one LM324, to obtain levels
that can be processed easily. As long as potentiometer P1 is adjusted
correctly, this group of operational amplifiers delivers a voltage of 1
volt per atmospheric pressure of 1,000 hPa to the LM3914.
Since the atmospheric pressure will be within the range 950 to 1040
hPa at sea level, we need to make an expanded-scale voltmeter with this
LM3914 in order to better exploit the 10 LEDs that it can control. That
is the role of resistors R7 and R8 which artificially raise the minimum
voltage value the chip is capable of measuring. Consequently, we can
‘calibrate’ our LED scale with one LED per 10 hPa and thus benefit from a
measurement range which extends from 950 hPa to 1040 hPa. In principle,
you should not have a need to go beyond that in either direction.
The circuit may be conveniently powered from a 9-volt battery but
only if used very occasionally. Since this is usually not the case for a
barometer, we advise you to use a mains adaptor instead supplying
approximately 9 volts. Calibration basically entails adjusting the
potentiometer P1 to light the LED corresponding to the atmospheric
pressure of your location at the time. Compare with an existing
barometer or, even better, telephone the closest weather station. They
will be happy to give you the information. After Evangelista Torricelli,
1608-1647, Italian physician who proved the existence of atmospheric
pressure and invented the mercury barometer.