October 2006

“Cavity” Equipment Assures Accuracy of Solar Radiation Measurements

When the Florida Legislature created FSEC more than 30 years ago as a research and education institute, they also charged the new center with testing and certifying the solar equipment sold in the state.

Over time, FSEC’s testing became so widely recognized and respected that many other states required FSEC certification on their systems, and the center was even selected to perform technical and administrative operations for the national Solar Rating & Certification Corporation.

Photo of Mary setting up equipment for testing.
Mary Watkins setting up for calibration testing.

Earlier this month, an FSEC staffer completed an important annual ritual that assures that FSEC’s test equipment is properly calibrated so that the results of the testing and the research is accurate.

“It’s something we’ve been doing for the past five years or so,” noted Jim Huggins, acting director of the Testing and Operations Division, “bringing our Absolute Cavity Pyrheliometer to Golden, Colorado, for a week or two of carefully monitored testing and calibration.”

The imposing name of this small piece of equipment doesn’t accurately convey its importance to all the work done by Huggins’ division.  “Think about it this way,” he explained, “there really is no absolute reference for measuring solar radiation, yet everything we do deals with solar radiation and our need to accurately know how much of it we are dealing with.  You can look into reference books and find the physical reference used to measure distance.  You look up the way that temperature is accurately measured.  But you won’t find any kind of absolute reference for solar radiation, and we really need to know what it is.”

Photo of equipment set up for calibration.
Many research organizations bring their equipment for calibration.

That’s why FSEC and a number of other testing and research organizations around the world use a pyrheliometer.  “It’s a very precise instrument that is finely tuned to measure the exact amount of solar radiation.  Keep in mind that we have more than 50 pyranometers that must be calibrated so that they accurately measure solar radiation for all of our experiments,” he added.  “We make sure that the ‘cavity’ is precise, and then we use that instrument to calibrate all of the other solar radiation equipment that we have.  Since our mission is working with the sun, we need to know how much sunshine we’re working with, and we need to know with great accuracy.”

FSEC staff calibrate all their instruments on a regular basis so that temperature, flow and other variables are correct when used.  The importance of solar radiation in FSEC’s work, along with the difficulty is setting the standards, makes this one aspect of the program a little more difficult.

“Here’s how we handle this situation,” Huggins said.  “There are actually five of these cavity instruments around the world that have been selected by the World Meteorological Organization (WMO) as world radiation references.  Every five years, these instruments are brought to WMO in Davos, Switzerland, pointed at the sun and solar radiation is measured.  Exhaustive testing pinpoints the accuracy of their readings and the five cavities are calibrated to be precisely the same.  The U.S. cavity is kept at NOAA in Boulder, Colorado, so when they get back to the U.S., they invite organizations with their own cavity equipment to come together to fine-tune the readings for consistency.”

Photo of overview of all the set-ups of equipment for calibration.
The annual inter-comparison helps assure accuracy in testing solar equipment.

FSEC’s Mary Watkins took our pyrheliometer out to Colorado a couple of weeks ago.  More than 15 other groups involved with solar radiation measurements also brought their equipment.  This “inter-comparison” gives the participants close to two weeks to assure good weather for the testing and time for putting on seminars and discussions on the equipment and the test procedures.

Whether you’re a manufacturer who wants to know exactly how your solar equipment is performing, a consumer who wants the confidence that the equipment you buy will perform as advertised, or a government official who wants to be sure that the rebate or incentive you’re giving solar equipment buyers is truly for the savings they will get, then you can appreciate the importance of this small piece of equipment that assures the accuracy of FSEC’s testing.

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