Measuring Outdoor Conditions; Not So Easy as You Might Think

As you may recall from several
posts back
, one of the discoveries that came out of the damper
test I was discussing was that the minimum outdoor air flow rate
for the unit under test was probably excessive under most operating
conditions. When the students in the PEC RCx class who performed the
test began to dig into the implications of excessive minimum
outdoor air flows, they realized that they would need to measure
and document the ambient conditions in the local environment. This
realization lead to a lab exercise in which we used a number of
different instruments, ranging from a sling
psychrometer
to an
electronic thermometer and hygrometer
 to assess the
ambient conditions and conditions inside the unit. The sling had
the advantage of being a fairly fundamental instrument that has
been around for a long time requiring no batteries or periodic
recalibration. But, readings taken with it are time consuming and
highly subject to technique. The electronic equipment had the
advantage of ease of use and accuracy, but represented some
sophisticated technology that relied on more than a simple
fundamental principle. The results of the lab session are
illlustrated in the figure below, which is an the enlarged portion
of the inset psych chart with various the lab data plotted on
it.

The data scatter is, in my experience,more normal than not,
especially when it comes to measuring outdoor air temperature.
Reading accuracy can be influenced by a number of factors including
the accuracy of the equipment, measurement technique, and
measurement location. The actual values measured are highly
variable and influenced by factors like the sun, breezes, adjacent
hot or cold surfaces, adjacent bodies of water (including puddles
and water features, not just nearby oceans, lakes, rivers and
streams), conditioned air leakage from nearby equipment,
unconditioned air leaking into the conditioned air stream that is
being measured, and the impact of the person taking the
measurement.

Complicating it all is the difficulty associated with measuring
atmospheric moisture content in the first place, especially with a
sling psychrometer, the instrument which was used for a significant
number of the measurements. Specifically, technique is critical to
obtaining a good reading with a sling psychrometer: While the sling
psychrometer has an advantage in terms of being fundamentally
simple and based on a fundamental principle, the information
obtained from it is highly susceptible to the technique used when
taking the reading. Factors that may have lead to erroneous
readings in the class data include:

The water that the class used to wet the wick was filtered
but not distilled and
was sitting in a cup in the sun and
thus, not necessarily pure nor at the ambient wet bulb.

The thermometers in the psychrometer tended to rotate in
their housings, making it difficult to take a quick accurate
reading as soon as the operator stopped slinging the instrument.
Taking a reading immediately is very important; wet bulb
temperatures can quickly head back up with no air
motion.

The scale graduations on the thermometers were small making
them difficult to read.

There is tendency to support the bulb end of the instrument
with your hand when you stop slinging it in order to take a
reading. This exposes the thermometer bulbs to conditions that do
not reflect the ambient environment. 

It’s difficult to maintain the required slinging rate
of 2-3 revolutions per second for 1-1/2 minutes, but this is the
rate required to ensure a reliable wet bulb reading. If your wrist
and fore-arm is tired from the slinging effort, then the idea of
stopping, taking a reading and then slinging some more to make sure
the readings have stabilized is not particularly attractive. But,
it’s an important step for ensuring accuracy.

Taking 2 or 3 concurrent readings will identify an
“out faller”. But, this takes more time and seems like
a lot of effort when your wrist and arm feel tired.

Slinging a psychrometer inside a confined area is
not easy; I’ve personally broken more than one thermometer
when I accidentally swung the psychrometer against something hard
in the immediate vicinity.

Taking readings inside of equipment introduces some potential
problems irrespective of the technology applied. If access is
difficult, it’s tempting to simply insert the probe through an
access opening to obtain a reading. But, air flow through the
access opening may influence the reading if air from outside of the
system is drawn in through it. If the condition you are measuring
represents a mixture of two air streams such as the mixed air
temperature in an air handling system, then the measurement needs
to be taken at a point in the system where complete mixing has
occurred. Frequently, this will not be in the mixed air plenum.
And, moving to a point where complete mixing may have occurred
places you down stream of other elements like coils, fans, motors,
and leakage sources, all of which can influence your
measurement.  If you decide to take a number of readings and
average them, then you need to be aware of the fact that the true
mixed air temperature is mass flow dependent;  1 cfm of air at
0°F mixing with 99 cfm of air at 100°F does not yield 100
cfm of air at 50°F.  This means that if the velocity
profile is not uniform at the point where you are measuring the
temperatures you intend to average, then the mass flow rate is not
identical at each point and simply averaging the temperatures will
not yield the average temperatuer of the fully mixed air
stream.

The bottom line is that taking measurements in the field can be
trickier than you might first imagine for a number of reasons. Many
of the issues mentioned above are worthy of consideration for both
temporary, portable instrumentation as well as the permanently
installed equipment used by the monitoring and control system. As a
result, it is wise to take data from an unfamiliar system or source
with a “grain of salt” until you have some confidence in it.
Multiple readings that converge or add up are better than one
reading. In the next post, we’ll try to decipher which of the
outdoor readings taken by the class actually reflected the true
ambient conditions at the time.

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