ASHRAE Journal - June 2020 - 64

COLUMN ENGINEER'S NOTEBOOK

DDC Systems: The Ultimate Data
Logger (Maybe)
For modern buildings, DDC
systems perform the data logging function. Given current
technology, they can be the
ultimate data logger. But that
only will become a reality if they
are properly designed, commissioned and maintained.
64

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TABLE Data logger features.

2016

2013

X
X

2017

2013

2008

2016

FASTEST TIME, S

CO 2

ACCELERATION

EXTERNAL

2000
2008

NOTES

RESOLUTION, BITS

A
B

LIGHT

HUMIDITY

TEMPERATURE

INTERNAL

SAMPLES
DATA SAMPLE
STORAGE CAPACITY

SENSORS
NUMBER OF
DATA CHANNELS

WIRELESS

BLUETOOTH

INFRARED

INTERFACE
USB

ITEM

VINTAGE (NOTE 4)

can imagine. Table 1 describes
some of the features and options
of these data loggers shown in
Figure 3.
I want to highlight that current technology loggers can
measure industry-standard
signals such as 4-20 mA and
1-5 Vdc in addition to using the
internal and external sensors
in each vendor's product line
(Figure 4). This opens the door to
logging just about anything; if
there's a parameter that needs
to be measured, you are just
about guaranteed that multiple
vendors make a sensor that
can measure it, and provide a
4-20 mA output..
Several vendors make webenabled loggers like the one
shown in Figure 5. While more
expensive than the loggers
illustrated in Figure 3, the added
hardware cost can quickly pay
for itself. Multichannel loggers sampling data at a high
frequency can quickly fill up
their memory. Web-enabled
loggers allow you to retrieve the
data without a trip to the field,
saving time and travel costs in
situations in which you need to
gather a lot of data over a long
period to establish a baseline or
verify an improvement.

7,944

0.50

1, 2, 8

X 12

43,000

1.00

2, 3., 8

21,800

0.01

5, 8

X 16

1,900,000

1.00

2, 6, 8

X 12

84,650

1.00

2, 8

5,000

60.00

7, 8

X 12

208,076

1.00

8, 9, 10
11

84,650

1.00

84,650

1.00

NOTES

1. Even though this is one of the older loggers, it had the fastest sampling rate up until the Item C loggers showed up in 2016.
2. Loggers with external sensor jacks can interface with a wide array of sensors, including thermistors, current transformers (CT)
and industry standard signals like 4-20 mA, 1-5 Vdc, and 2-10 Vdc.

3. The black device plugged into the external port is a 20 amp CT. The odd-looking extension cord is set up to break out the hot,
neutral and ground conductors so the logger can be used to log data from an appliance or a PC. If you clamped the CT around
the entire extension cord, the fields in the two current-carrying conductors would cancel each other out.

4. I estimated the vintage of the logger based on when I became aware of the product or procured my first one, which generally
was around the time they appeared on the market.

5. I am experimenting with these loggers to see if I can pick up things like damper blade rotation or the motion of the inlet guide
vane actuator on a centrifugal chiller.

6. The logger is shown with thermistor inputs for two of the four channels and a 600 amp CT on the third channel, a configuration
you might use to gather information regarding the power consumed and load on an air-cooled chiller. But the external sensors
could be any of the inputs listed in Note 2.

7. The logger I happen to have just has a temperature sensor. But the product line is capable of multiple inputs and can monitor any
of the signals listed in Note 2. The sampling time is much slower than the other loggers. I suspect this is due to limitations associated with transmitting the data wirelessly. But, it is competitive with low-end current technology and legacy DDC systems. For
a facility with no central monitoring system, even at once a minute, this logging solution would provide the user with useful data
that would pick up most of the common problems that might otherwise go unnoticed, including scheduling issues, simultaneous
heating and cooling and an unstable control loop.

8. For some of the loggers, the analog to digital conversion bit count is not specified, so I estimated it based on the temperature
sensor resolution called out in the specifications.

9. While thermocouples like the one this logger uses as an input are less accurate than thermistors generally speaking, they can
tolerate much higher temperatures. In addition, they are thin and flexible, which allows you to weave them through a preheat
coil to capture the temperature downstream of it in situations where the coil is bolted directly to the cooling coil with no access
in-between, allowing you to detect and document simultaneous heating and cooling.

10. You may notice that one of the thermocouple loggers looks a bit discolored. I was using it to monitor boiler flue temperature, and
the mounting system failed, causing it to be blown around, tethered to the flue by its little sensor lead, but exposed to the flue
gases, which discolored and deformed it. I included it to illustrate how tough these little guys are; when I plugged it in, it was
fully functional and had captured the data I was after. It now is retired, resting in a place of honor on a shelf in my office.

11. This is not a logger; it is a data shuttle. It allows you to pull data from loggers and relaunch them without having to connect them
to your computer. If you have ever balanced your laptop on top of a step ladder while trying to connect a USB cable to a logger
about 4 feet above you, you will appreciate this little gizmo. I have also used it as a way to retrieve data from a remote site by
sending it back and forth in the mail with the operators so they can pull the data for me.

12. The Bluetooth communication feature of this product line is very handy in situations where you want to log data in an occuppied
zone, perhaps the supply temperature coming out of a diffusser because once the logger is deployed, you can retrieve the data
without having to go up on a ladder, move tables, etc. I have retrieved data from a meeting room while a meeting was going on,
standing outside the door to the room.
J U N E 2020

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ASHRAE Journal - June 2020

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