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Monday, July 6, 2015

From the testing lab: paper calibration standards FAQ


 Why are paper standards used instead of some other material for primary calibration?

There are two main reasons to use paper for the calibration media for brightness, color, fluorescence, whiteness and other paper measurement devices:

  1. When transferring calibration from a Master instrument to any other the best possible agreement is achieved using the same media as that which will be measured.
  2. It ensures that all the characteristics of the resulting measurement are taken into account during the calibration process e.g. translucency, fluorophores, pigments, directionality, uniformity, gloss.
How long are the paper calibration standards good for after they are opened?

Standards are only certified for 3 weeks after they are opened.  Depending on the way they are handled, they could still read correctly after that period.  It is the mills decision on how often to purchase the standards and how important is it for their instrument to be accurate as possible.

Is there any alternative calibration standard that would last longer?

The paper standards are the only thing we offer for the primary calibration according to the TAPPI Test Methods (e.g. T 452) and ISO Standards (2469).  Because the brightness of a sheet can change due to age of the standard, surface characteristics, temperature & humidity, cleanliness of the lenses in the instrument and how the standards are handled during calibration, the certification is short.  This doesn’t mean the standards won’t last longer, but it is related to the care of the standards.  Here are best practices for the care of the instrument.

  1. Keep the surface of the instrument as clean as possible. 
  2. Put something over the sample opening when not in use. 
  3. Protect the surface finish of the working standard. For TAPPI instruments, keep the opal in a small box or on a towel to protect the surface finish. 
  4. Clean the working standard before you use it (even when it looks clean and wiping it on your shirt is a bad practice). 
  5. Recalibrate to a clean working standard at least every 8 hours; more often could be needed depending on lab and equipment.
  6. Replace the lamp every 3 months per the manual.  Just because the lamp is working does not mean the spectral response is correct. 
  7. Primary Calibration to a new set of standards at least monthly.  This could be required more often depending on lab and equipment. 
  8. Semi-Annual Preventative Maintenance service.  We also offer Annual and Quarterly PM service.  
NOTE: The Color Touch instruments have an internal working standard which is protected from the elements and ensures better repeatability and less drift over time.  Also, the Color Touch instruments use a xenon flash lamp which has a lifetime of years not simply months.