ASHRAE Journal - June 2020 - 68

COLUMN ENGINEER'S NOTEBOOK

As a part of a retrocommissioning project, we deployed
an array of data loggers across the ﬁlter bank in the unit,
which was close coupled to the economizer section. We
complemented this with a second array downstream of
the cooling coil, the entering face of which was about
6 in. (152 mm) after the ﬁlter bank.
The image in Figure 7 is from a spreadsheet we built
that allows you to select a day and time from the data set
to see what the constantly varying temperature stratiﬁcation pattern looked like. The sensors were calibrated
relative to each other, so temperature differences indicated are true temperature differences, not differences
that are the result of sensor error.
At the time, the outdoor temperature was 52.2°F
(11.2°C), and the single point, mixed air sensor (located
where the yellow dot is) "thought" the temperature
was about the same. But the numerical average mixed
air temperature was actually 57.6°F (14.2°C), and the
numerical average temperature leaving the inactive
cooling coil was 60.2°F (15.7°C).
Two reasons exist for the difference between the

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average mixed air temperature and the average cooling coil discharge temperature. One is that the thermal
mass of the water-ﬁlled cooling coil places a thermal lag
between the two temperatures.
The other is that there was signiﬁcant velocity stratiﬁcation in the plenum. Think of it this way: if you mix
1 gallon (4 L) of 50°F (10°C) water with 99 gallons (375 L)
of 100°F (37°C) water, you don't end up with 100 gallons
of 75°F (24°C) water, even though the numerical average
of the temperatures is 75°F (24°C).
Bottom line: even if you have an averaging sensor, the temperature reported by it assumes that the
velocity proﬁle the sensor sees is uniform across its
length. In a mixed air plenum,with stratiﬁcation patterns varying as dampers modulate, this is often not
the case.

The Original Data Logger Still Has a Place
Signiﬁcant advances have been made in terms of our
ability to dialog with our buildings over the past 40
years. It may even seem there is no longer a place for
the original data logger, the operator with a clipboard. I
think the original data logging strategy will always have a
crucial role, even with current technology doing most of
the actual logging.
For one thing, it places the person observing the data
in the immediate proximity of the data source. As a
result, the observer, properly trained, can assess the
integrity of the sensing system, catching issues like those
described above.
The "properly trained" part is important, especially
the ability to perceive information in the context of the
system it is coming from, interpret it in real time and
identify things that don't add up in terms of the physics
of the system. These are all critical skills and help ensure
the integrity of the data we use to manage our facilities.
That data is key to achieving the resource conservation
and carbon reduction goals we must achieve moving
forward.
So, when the opportunity presents itself, get out in
the ﬁeld and practice being a data logger. Spending
time mingling with the building systems
and the people operating them will
make you a better designer; I'm sure
you will learn something that will further your career. And I bet you'll have https://bit.ly/2YUre1l
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fun doing it.

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ASHRAE Journal - June 2020

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