Ask any metrologist, and you’ll likely get quite a diatribe about the challenge of making a good measurement of humidity. There are, however, some specific steps or best practices you can follow that will help improve your measurement of humidity through better accuracy, better repeatability, and fewer errors.
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Best practice No. 1: Choose the correct instrument
One of the more common problems with humidity measurement is choosing the wrong instrument for a specific application. For example, using an aluminum oxide sensor for very dry air (–100°C dew point) is the correct choice, but this type of sensor will yield inaccurate results when the air is both wet and dirty. There are many factors to consider when choosing the correct instrument, such as temperature of the environment and ambient pressure, external factors that will affect the sensor, and its range of measurement. Rely on the expertise of your hygrometer vendor to help make the proper choice. Or sign up for the free webinar, “How to Choose a Best-fit Hygrometer.” The next one is scheduled for Thurs., Dec. 15, 2011, at 10 a.m. PST.
Best practice No. 2: Follow the manufacturer’s instructions
Read the manual. A hygrometer’s user’s guide or owner’s manual usually includes a wealth of information about the installation, operation, and maintenance of your new humidity instrument. Use the vendor’s technical support, application engineers, and sales people as resources to assist in making a proper measurement.
Best Practice No. 3: Ensure a proper calibration
The most recent calibration must be within an appropriate time interval, traceable, documented, and in accordance with your standard operating procedures. For a comprehensive discussion about humidity calibration practices, download Vaisala’s complimentary calibration e-book.
Best practice No. 4: Keep records
Maintain a folder for each and every humidity instrument.
• Keep calibration certificates, records of repair, and other pertinent documents in a folder for easy access.
• Include records of adjustments and out-of-tolerance results if these are not included on the calibration certificate.
• Make a note of any damage or unusual events. For example, document when the forklift hit the transmitter and knocked it off the wall.
Best practice No. 5: Spot-check performance
Suppose your calibration interval is once per year. Is it possible that something happened within those 12 months that could cause the instrument to go out of tolerance? Usually the answer to this question is yes. Stuff happens. Set up a regular spot-check process as part of your instrument maintenance program. Depending on your environment, you may want to spot-check once a week, once a month, or once a quarter. A spot-check is not a complete calibration but usually entails a quick comparison of the instrument to a properly calibrated humidity standard. It’s also good practice to specify—in writing—certain actions that should be taken if the spot-check is outside specified tolerances.
Best practice No. 6: Be knowledgeable about the parameter
Understanding the theory behind this pesky and elusive parameter will help you make a better measurement. Make sure you know what the sensor is measuring. The sensor or instrument could be measuring relative humidity, dew point, or vapor pressure. Just because the output from the instrument is dew point, you cannot assume the sensor is measuring dew point; it may be that the instrument is measuring relative humidity and temperature, and then calculating dew point. By knowing that the sensor is measuring relative humidity, and understanding that relative humidity will vary as temperature changes, you can conclude that a better measurement is made by ensuring a stable temperature. If the output is calculated, it is good practice to know what formula is being used and where the formula originated.
Best practice No. 7: Be aware of potential external factors
External factors may negatively affect the measurement. As mentioned, if the sensor is measuring relative humidity, temperature variations will affect the measurement. External factors such as direct sunlight, breathing in the direction of the sensor, or being too close to the sensor will cause a high error with a relative humidity sensor. Differing pressures will affect a true dew point sensor; dew point under pressure can be quite a bit higher than dew point at atmospheric pressure.
By following these best practices, you will improve your humidity measurement and be able to rely on the results.
Comments
SOP
Hi Bruce, appreciate the practical info. And don't forget to write a comprehensive SOP (Standard Operating Procedure).
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