The Keys to Calibration
- Conduct calibration tests using glass spheres.
- Physically examine each sieve using microscopic projection.
However, there is another way to calibrate working test sieves that is both easy and inexpensive.
Use Certified Test SievesAt the outset, use certified test sieves. A certified test sieve is one that complies with a national or international standard. An American Society for Testing & Materials (ASTM) certification, for instance, assures that the sieve openings are within the plus or minus tolerance allowed in ASTM E-11-95.
A master set of sieves is needed as the standard against which working sieves are checked. The two types of certified sieves commonly used for master sets are certified and certified mid-point. Mid-point sieves are certified that their openings fall in the middle of the ASTM E-11-95 specification, and they provide a solid foundation for a dependable sieve calibration system.
The sieves from a master set should be used only to calibrate working sieves. They should never be used in regular sieving operations and then reused in a master set. If a sieve from a master set is used in an operational test, it should be marked as a working sieve, and a replacement should be purchased for the master set.
Great care should be taken when handling and cleaning the master set of sieves. The master set is the standard and is the most important part of the calibration process.
Keep Track of Your Working SievesIf you have more than one working stack of test sieves, it is important that the individual sieves in a stack be used together. Using one sieve from another stack will corrupt your calibration results.
One of the most effective ways to ensure stack trueness is to keep a record of the serial numbers of each sieve in each working stack. Another way is to mark each sieve with a stack identification number. These techniques will ensure that you have a valid calibration tracking procedure.
Use Representative Calibration SamplesGlass spheres or calibrated samples are available from industry associations or the National Institute of Standards and Technology (NIST). However, unless your customers or other product specifications call for these types of samples, the material used for calibrating should be from a representative sample of the product being tested in your operations. This provides a calibration basis that translates easily to operation procedures.
Follow the Right Calibration SetupThe results of processing a part of the calibration sample through the master stack should be compared to the results of processing a second part of the calibration sample through the working stack. This comparison will enable you to identify any working sieves that are out of tolerance because of wear or breakage, which will distort operating results.
To ensure that you obtain effective calibration data, follow these steps:
- Split the sample into four representative samples using either a manual or mechanical device. A hand sample divider that splits samples into quarters will work fine.
- Weigh and record the four representative samples.
- Place the master set of sieves on the sieve shaker. Use one quarter of the sample that was split and run a test. It is important that the same shaker and the same settings are used for all calibration tests, and it is also recommended that the same person run all of the calibrating tests.
- Record the weight retained in each sieve. It is easier to compare the results if they are calculated to the percent retained.
- Place a working set of sieves on your sieve shaker. Use one quarter of the sample, and run a test. Note: it is critical that the same shaker and settings are used as in the test that used the master set.
- Record the weight retained in each sieve and convert it to the percent retained.
- Compare the results from the master set of sieves with the results of the working set.
- Record the difference retained in each sieve of the working set versus the same sieve in the master set. This difference is the allowable curve. Keep these records on file for future reference. You will use the same curve every time you calibrate.
Evaluate Your Working SetOnce you have completed the calibration steps and finished the calculations, you should then evaluate the working set. Compare the difference of the allowable curve established with the difference in the calibration test. If your results are not within the tolerances you have specified, replace those sieves with new calibrated sieves.
Establish a Replacement ScheduleTo make this testing procedure produce traceable and practical product quality standards, a given frequency of performing calibration checks on working sieves should be established and implemented. Based upon the frequency of calibration checks, a replacement schedule should be established for the master sieves. The frequency of these tests and replacements should be based upon operational test schedules, sieve maintenance procedures, the characteristics of the material tested and the tolerances for acceptable quality standards.
With this procedure, most of the mystery and complexity can be removed from a sieve calibration program.