ASHRAE Journal - June 2020 - 66

COLUMN ENGINEER'S NOTEBOOK

FIGURE 4 A current technology logger set up to log conventional 0-5 Volt dc pressure transducers; a similar arrangement can be used for 4-20 ma signal.

FIGURE 3 Part of my collection of electronic data loggers.

D
E

Four-Channel
Logger
0 - 5 Vdc
Input Cable

0 - 100 psi
Gauge Pressure
Transmitter.
0 - 5 Vdc Output

C
B
H
A

It may be beneﬁcial to think of your DDC system as a
direct digital control and data logging (a DDCDL, if you
will) system when you design it. That's because to be
truly useful for commissioning processes, the control
system will need to have more data points than those
required for control.
For example, it is possible to control the discharge
temperature for an air-handling system using the
discharge temperature alone, by using sequencing to
manage the interactions of various processes such as
the economizer, cooling and heating coils. But having
sensors that report the mixed air temperature and the
temperatures leaving each of the coils will allow you to
detect and correct something like a leaking warm-up
coil valve that might otherwise be masked by the cooling
process downstream if all you had to go on was discharge
temperature.

The Need for Speed
For many systems, network architecture is a crucial
factor in determining communication speed. While it
may be technically possible for a controller to sample
data at once a minute or faster, the trafﬁc this creates on
the network could crash it if it has not been designed to
support it.
One solution is to simply not sample the data very
quickly; a lot of systems default to sampling rates of
once every 15 minutes or more for that reason. But if you
accept that, you are giving up a lot of the utility you otherwise would have procured with your system.
For example, when you are tuning control loops, a
graphic with dynamic trends painting the response
of the process when you upset it in real time is a huge
66

ASHRAE JOURNAL

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J U N E 2020

Junction Box
Containing Splices

120 vac to 12 Vdc
400 mA Power Supply

FIGURE 5 A web enabled logging solution.

beneﬁt. But for that to be useful, the data needs to be
refreshed frequently: at least once a minute and for
many processes ﬁve or 10 times a minute would be even
better.
For commissioning work, if you really want to pick up a
control process that has gone unstable, you probably will
need to sample it at least once a minute to avoid being
aliased. More than once, after ﬂagging a control process
as being unstable and requiring tuning by the control
vendor, I have discovered that the "tuning solution" had
been to set the trend sampling time to once every 30
minutes with the averaging option enabled. Generally,
that will make anything look ﬂat and smooth, despite
the chaos going on behind the scenes.
The bottom line is that "sampling time" and "too
fast" are mutually exclusive phrases until you know

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ASHRAE Journal - June 2020

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