Applies To:
  • CitectSCADA 6.xx, 7.xx
  • CitectHMI 6.xx, 7.xx
Summary:

Assuming we are trending a variable tag that have raw scale and engineering scale specified in order for the data to be scaled down accordingly and be presented in engineering value (and unit).

If the trend is being saved using the format of 2-byte scaled data, changing the scale in the variable tag (raw and engineering scale) will impact the trend history data to be displayed incorrectly in the trend page. All the trend history data will be scaled down (by the new scale) and displayed wrongly.

How can we retain the correct data and display it correctly after we change the scale of the variable tag? 

Solution:

2-byte scaled data will saved the data (value) that has been scaled down with the previous scale that is being used before the scale changed. Therefore, when the scale is changed, the scaled data will be further scaled down by the new scale introduced, resulting in "double scaling" and therefore, the display will be wrong (appears to be less than the actual value that is supposed to be displayed).

However, we can still make use of a tool named "Trend Converter" from Citect Toolbox (available in MyCitect -> Toolbox section) to convert the 2-byte scaled data to 8-byte trend data (actual value).

By having the 8-byte trend data which retains the actual value (raw), the trend history data can then be scaled down properly by the newly introduced scale.

There is one drawback however, which is after converting the 2-byte scaled data to 8-byte trend data, the trending must be saved under the format of 8-byte trend data from then onwards in order for the history data to be displayed correctly (if need be, or else the trend history data can be deleted anytime when it's not being used anymore).

NOTE: 8-byte trend data will takes up much more disk space. 
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