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K-factor for a 8705/8732 mag meter

Hi,

I'm interested to modify online the K-factor for a 8705/8732 mag meter (operation that I made on other mag meters). This is an attempt to decrease the -1% error of a mag meter. The 8732 transmitter was replaced and the mag meter was tested with an authorized laboratory in 5 measurements points. The error is approximately constant : -1%.

I have only this text from the Rosemount manual: 

"Rosemount flow lab tests determine individual sensor output characteristics. The characteristics are identified by a 16-digit calibration number (...)The calibration number is more than a correction factor, or K- factor, for the sensor. The first five digits represent the low frequency gain. The ninth through thirteenth digits represent the high frequency gain. Both numbers are normalized from an ideal number of 10000. Standard configurations use the low frequency gain, but in noisy applications it may be worthwhile to switch to the higher frequency. An additional transmitter procedure, called Auto Zero, is recommended to perform at the higher coil drive frequency. The seventh and eighth digits represent the zero offset at both frequencies where the nominal value is 50."

I suppose a part of the 16-digit calibration number to be the K-factor, but is not documented. Can someone help me with the information ? 

leirbag

• 30 Apr 2015 12:29 PM
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2 Replies

• Mike Schaefer

  

  • 30 Apr 2015 1:58 PM

  We can never recommend that you adjust the calibration number in the field. By changing the calibration number you are accepting that you are losing all correlation to the NIST traceability of the factory calibration. If you are willing to do this and instead use the other authorized laboratory report as your calibration reference there are two ways to accomplish what you are trying to do.
  
  1) Use Special Units configuration. With this method you don't need to change the calibration number. Set up a Special Unit but label it the same as the unit you are already using. Use the Special Unit Conversion Factor to accomplish the flow reading adjustment.
  Example: Currently measuring in Gal/Min (gallons per minute). Set Special Volume Unit to "gal" (gallons). Set the Base Volume Unit to "gal" (gallons). Set the Conversion Factor to 0.99 to account for the meter reading 1% low. Set the Baset Time Unit to "Minute". Set the Special Rate Unit to "GPM" (only 4 characters are allowed).
  
  2) Adjust the first 5 digits of the calibration number. The first 5 digits of the calibration number are the gain value. If you're operating the coil drive frequency at 37 Hz instead of the default 5 Hz, then you will need to follow this same procedure with digits 9-13 of the 16 digit calibration number. To adjust the flow reading to account for the -1% error, multiply the gain number by 0.99 and adjust those digits in the configured calibration number accordingly.
  Example: First 5 digits of the calibration number are currently 09734. Multiply by the required correction - 9734 x 0.99 = 9637 (notice the leading zero is dropped for the calculation). Change the first 5 digits of the calibration to 09637 (the zero must be replaced when configuring the calibration number). By shifting the gain value 1% lower it will increase the flow reading by 1%. Again, by doing this you are losing NIST traceability to the factory calibration.
  
  If you have further questions or would like additional help with your specific calculations or configuration, please feel free to contact our Flow Technical Support at 800-522-6277 or email Flow.Support@emerson.com.

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• leirbag

  

  • 3 May 2015 3:03 PM

  In reply to Mike Schaefer:

  Thanks for your answer,
  
  I will use one of your suggested metodes. Also I'll try to use a certified mag meter in series with the test one. I'm not necessarily intend to modify the calibration number , is just a guess that after many years of operation of the 8705 tube, the calibration constant (K-factor) may have changed (a new transmitter don't solve this).

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