●●●●
The sensor for the
micro gust thermal
anemometer (MGTA)
is a LM399 voltage
reference.
The LM399 is a remarkably
precise temperature
stabilized zener diode
reference with a nominal
voltage of about 6.9V. The
LM399 zener diode structure
is contained within a tiny
temperature controlled
oven. While the initial zener
voltage can vary from about
6.6V to 7.3V, once biased to
between 1 and 10
milliamps, the zener voltage
can be very stable (to
microvolts) over temperature.
■ The assembled board.
M TIHCREO GRUMST AL
ANEMOMETER (MGTA)
At Geller Labs, we use LM399s
in our high performance, low
cost voltage standards for hobbyists, schools, and amateur scientists as the 01HS high stability
option. Even with its special thermally insulating cover, all of our
products using the LM399 are contained within an enclosure — such
as a plastic box — to further shield
the LM399 from room air currents.
One day it occurred to me to
stop fighting the insulation problem
and to investigate the possibility of
using an LM399 as a sensor to measure tiny air currents on the surface of
a lab table. An LM399 was removed
from its plastic thermally insulating
case and powered up on the bench. A
digital multimeter (DMM in the current mode) was connected in series
with the heater (pins 3, 4) power
source. As you will see, we
only need the oven section
of the LM399 to make a
measuring instrument. I
blew a puff of air towards the
LM399 from a couple of feet
away and was pleasantly surprised when the DMM clearly responded a second or so
later. As an avid electronics
hobbyist, I set to work to
design a simple scaling circuit and amplifier to make a
workable front end for a
thermal anemometer.
A thermal anemometer
is a wind measuring instrument based on a heated
element, such as a heated wire. The
underlying principle of operation is
that the resistance of the wire
changes as heat is removed by the
air flowing over the wire. While most
thermal anemometers end up as relatively complex circuits such as
Wheatstone bridges inside of analog
closed servo loops, our LM399 version turns out to be remarkably easy
to build. Since it responds to very
tiny air currents, I call it a Micro
Gust Thermal Anemometer (MGTA).
Procedure — Building
and Fabricating
The first step to build an MGTA
is to disassemble an LM399H.
Always wear safety glasses when
taking something apart! One
disassembly method is to gently
squeeze the LM399 in a vise or with
pliers. As the top begins to buckle
and release, gently pry it off with a
small screw driver. Try to minimize
damage to the thermally insulating
cover. You may even want to mount
the LM399 through its bottom
cover section, so the cover can be
slid back up over the exposed
metal can (TO- 46 package) to create a second less sensitive scale. A
third wind scale could be made by
reinstalling the cover section.
The next step is to build the
amplifier circuit. It can be assembled using any reasonable construction method ranging from a
hobbyist’s perf board to dead bug
40
February 2006
