ASHRAE Journal - June 2020 - 63

COLUMN ENGINEER'S NOTEBOOK

provided chilled water for
FIGURE 1 Parallax in action. Viewed straight-on, the gauge reads 40 psi (left). But, if you don't take the time to bend
down and read it that way, you may think the reading is more along the lines of 41 to 42 psi (right).
cooling the community center
during the summer. In addition to monitoring construction
and starting up the systems, I
returned four times a year for
the ﬁrst several years to perform
ongoing commissioning. (Back
then we called it keeping the
system working properly.)
Because of the dynamics of
the systems, we needed a way to
track multiple events at once and
FIGURE 2 My electromechanical data loggers; single channel on the right and two
create a record for our reports.
channel plus on-off event on the left (left photo) and a close-up of the recording
Thus, I was introduced to my ﬁrst electromechanical
mechanism (right photo).
data logger. I still have a couple that still work (Figure 2).
These were a real blessing compared to the clipboard
and pencil approach. You could capture an event even
if you were not standing there when it happened. And,
they provided a continuous record of events during a test
or troubleshooting sequence. Models that sampled once
every ﬁve seconds made loop tuning a bit easier compared to trying to document the rapidly changing process
variable during your tuning effort.
Being electromechanical, they were not quite as rugtrends that were based on sample times of one, ﬁve, or 10
ged as my clipboard and pencil. If I dropped the latter,
minutes or more. (See the sidebar "Aliasing" for more.)
worst case, I might have to clip the paper back in and
advance the lead in my mechanical pencil. If I dropped
Modern Electronic Data Logger
the former, I probably needed to recalibrate it or send it
By 1990, there were small, electronic, single-channel
in for a service call.
loggers that could be purchased for $100 or less, probBut at the time they were a useful tool. And with the
ably 20% to 30% the cost of one of the electromechanical
ability to record every ﬁve seconds, they were able to
loggers. This took the industry to a whole new level with
complement early direct digital control (DDC) systems'
a huge array of options for logging any parameter you
capabilities if you thought you were being aliased by

Aliasing
Harry Nyquist, an electronics engineer specializing
in feedback control and stability, demonstrated that if
you want to paint an accurate picture of a process, you
need to sample it at least twice as quickly as the fastest
disturbance that is going on in it. Consider a condensate pump where you are sampling motor current once
every 15 minutes to prove the pump is running.
If the pump cycles on for one minute out of every
four minutes at peak load, then to truly capture the

operating cycle, you would need to sample the process
once every 30 seconds (twice during the shortest disturbance time of one minute).
Fifteen minute samples would not reﬂect what was
going on at all; the pump could have cycled multiple
times between samples or shut down right after the
ﬁrst sample and started right before the next, creating
the illusion that the pump had run for 15 minutes.
For more on aliasing, visit https://tinyurl.com/
y99dmp3f ("Aliasing and Other Factors Affecting the
Accuracy of Field Data").

J U N E 2020

ashrae.org

ASHRAE JOURNAL

https://av8rdas.wordpress.com/2016/03/16/aliasing-and-other-factors-affecting-the-accuracy-of-field-data/ https://av8rdas.wordpress.com/2016/03/16/aliasing-and-other-factors-affecting-the-accuracy-of-field-data/ https://www.ashrae.org/

ASHRAE Journal - June 2020

Table of Contents for the Digital Edition of ASHRAE Journal - June 2020