In the slow mode, the same algorithm applies; it’s just
that each channel is measured and averaged for a fixed
amount of time. The sketches for these different modes
are included with the downloads for this article.
An Experiment
Example: Temperature
Impact on Battery
Voltage
One of the first projects I used my data acquisition for
was measuring the open circuit voltage of batteries as
affected by ambient temperature. I built a simple insulated
box with a fan to circulate the air and a TMP36
temperature sensor to measure the ambient air
temperature (Figure 7).
Inside this box, I placed four fresh AA batteries and
brought their voltages out of the box to measure with the
four channels of the ADC. I measured the temperature
channel with one of the Arduino Due native ADC
channels. The TMP36 sensitivity is 10 mV/°C. With a
native resolution of 0.8 mV, the temperature resolution is
0.08°C. This is further reduced with averaging.
I naively expected the battery voltage to decrease
with lower temperature. In fact, I saw the opposite. The
battery voltage increased with lower temperature. Note,
this is with the batteries open circuited and no load
(Figure 8).
I put a bag of ice in the insulated box and watched the
temperature in the air cool down. As it cooled, I measured
the voltage of the four batteries and sampled the data
once every five seconds, averaging during this period.
After 45 minutes, the temperature only fell to about
10°C, and it looked like it wasn’t going much lower.
During this time, I saw a small increase in the battery
voltage of about 4 mV out of 1.6V. This is a 0.25%
increase. In this application, the large dynamic range of
the Digilent analog shield with averaging is essential.
I really wanted a lower temperature. I just happened
to have some dry ice lying around, so I opened the box
and threw in a bag of dry ice. The temperature dropped
to - 20°, stayed there for a while, and as the dry ice all
sublimated, the temperature began to creep back up. It’s
interesting to note that the temperature of dry ice is -78°C,
so if I had had more available, I could have gotten to a
much lower temperature.
As a special precaution, whenever using dry ice make
sure the room is well ventilated. The dry ice is really CO2.
As it evaporates, it will fill the room with CO2. Not safe.
The battery voltages mostly tracked the temperature,
increasing by as much as 18 mV in dropping the 40°C
from room temperature. It’s interesting to note that of the
four batteries, two behaved identically while the other two
had a slightly different behavior. All four came out of the
same box.
It took me about an hour to instrument this insulated
box and about 15 minutes to modify my standard sketch
to set up this experiment and start taking data. This is a
simple example of how I use this Swiss army
knife/laboratory grade data acquisition system as a
building block for all of my important measurements.
And now, you can use it too.
One of the nice features of both the Due and the
DAS is that they are relatively robust to ESD sensitivity.
This has been repeatedly tested by my lab assistant,
Maxwell (Figure 9). NV
■ FIGURE 8. The measured changes in the voltages of
the four AA batteries and the measured air
temperature inside the Styrofoam box.
■ FIGURE 9. An example of one of the many ESD
sensitivity experiments Maxwell has conducted,
illustrating just how robust this powerful data
acquisition system is to environmental stress.
June 2017 21
