21 September 2011

Long-term drift: When is recalibration warranted?

I went to my dentist yesterday morning for a 6-month cleaning and check-up. Since I have had no special dental problems for several years, I quizzed him about the need for inspections every 180 days. He replied with an eloquent summary of the microbiological risks of letting small problems go exponential and turn into big problems. OK. So I will be seeing him again next spring.

Microbes don’t normally cause drift in environmental sensors, and drift rates rarely go exponential, but we could all use a wise old dentist to remind us to floss (a.k.a. keep our sensor optics clean). Even with conscientious preventative maintenance, sensors, like teeth, need periodic check-ups. Some sensors in some environments can benefit from recalibration every 6 months. Other sensors, in other environments, can be rock-solid stable for 6 years. Few manufacturers have taken the time to rigorously characterize drift rates as a function of the environment the sensor (or meter) was used in. Even when the recommended interval for recalibration is based on good statistical data, most users ignore it. How many users follow the recommended recalibration interval of two years for Campbell Scientific dataloggers?

Some manufacturers make a significant profit from their recalibration services and recommend frequent recalibration without the data to help users know when it is really warranted. Both Campbell Scientific and Apogee strive to help users understand when recalibration is warranted. We also offer low cost and fast turnaround for recalibration services.

Apogee is working to build a statistical database for the drift rate of our sensors in a wide range of environments. Many apparent drift problems can be solved by appropriate cleaning in the field (www.apogeeinstruments.com/infraredradiometer/cleaning.html). Our Clear Sky Calculator (www.clearskycalculator.com) was developed so that users of all types of radiation sensors could use the sun as a calibration lamp. We are pleased with the expanding use of this on-line calculator. It has helped many users fix problems without having to disconnect the sensor and call us for an RMA. This calculator has also been useful in identifying errors in software, such as an incorrect multiplier.

One effective way to test for drift is to put two sensors side by side and compare their output. If the sensors have the same output, either they are both still accurate, or they have both drifted down at the same rate. Alternatively, purchasing a new sensor and putting it next to an older sensor in situ is an effective way to determine the need for recalibration. The new Apogee hand-held infrared meter (www.apogeeinstruments.com/infraredradiometer/meter-pricing.html) provides a unique opportunity to check the accuracy of IR sensors in field installations.

We look forward to working with you in the quest to make better measurements. Keep flossing.



Bruce Bugbee


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