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Measuring Outdoor Conditions; Not So Easy as You Might Think
July 31, 2007
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.
Posted by David Sellers on July 31, 2007 | Comments (0)
