ACR Stick-on data loggers

Walter:

    Belated response to your comments on my posting about ACR loggers.

    1. Real-time readout: I probably oversimplified. When you hook up
the logger to a PC, there is in fact a real-time command (as distinct
from downloading the accumulated data). But this does presuppose hooking
up to a PC. There is also a cable that can be used for a continuous
hookup; I'm a bit vague about the details of this (and I don't have the
manual here), but I think this can be either hard-wired or possibly used
through a modem.

    2. Calibrating: Yes, these are supposed to be recalibrated by
suspending over saturated salt solutions. This is apparently true of
*any* electronic sensor. A less sure way to recalibrate is to compare
the readings with some other instrument whose readings you trust, which
could lead to almost metaphysical arguments about what instrument do you
trust. Someone has finally admitted that sling psychrometers are
unreliable, and we know that care and fussy technique are required to
get reliable readings from even aspirating psychrometers. If one were
starting a program with a number of loggers, one might try to have one
electronic humidity meter that was kept calibrated (with salts,
presumably) to use (carefully!) as a standard for resetting the loggers
if they prove to need it. With any electronic sensor the drift out of
calibration should be slow, I think, unless they are exposed to a dose
of contamination.

    3. Software: Except for certain specific (and probably short-term)
diagnostic problems with mechanical equipment, the most important kind
of information needed from monitoring (it seems to me) (& unless one is
simply checking on conditions that are close to standard) is *trends*:
trends by season, by air-handling unit, and long-term trends (that would
reflect such things as the deterioration of HVAC equipment). The
problem, then, with loggers (as exactly also with conventional
hygrothermographs) is that one has for many purposes too much data;
trends are difficult or impossible to grasp. I have a slide of an 18"
high pile of religiously-accumulated hygrothermograph charts; I ask my
students "what do these tell us?"

    I think one needs to be able to do things like plot each day's high
and low T & RH (rather than each 30 minutes' worth) in order to be able
to see things like seasonal trends. With ACR's native software, you can
only plot all the data points from each download. The most serious
limitation, though, is that you can't add data from end to end in order
to graph it; in other words, if you download one month's data, you can't
add the next month's data to that to get a graph for the two months.
With a spreadsheet, of course, you can manipulate (and graph) the data
in any way that you want to try to understand what is really
happening--with, of course, time & effort.

    In the example you cite, you couldn't directly compare (i.e.,
superimpose) graphs of the data from the controlled and uncontrolled
areas with the ACR software, whereas you can of course with a
spreadsheet.

    As I may have said in my original posting, getting both reliable and
useful information from these new gadgets is no easier than with
conventional hygrothermographs; they just require different kinds of
fussing. However, I do think that on balance they will provide better
info than the old-fashioned kind normally do.

    Paul

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                  Conservation DistList Instance 4:57
                   Distributed: Saturday, May 4, 1991
                        Message Id: cdl-4-57-002
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