FOX Tech Tip
2 October 2015
FOX Model FT1 Minimum Full Scale Explained
The FT1 has a minimum full scale range of 15 to 1,000 SFPM (standard feet per minute) velocity. What is the purpose of this specification? To answer this question you first need to understand the FT1 accuracy specification:
For air it is: 1%R (1% of reading) plus 0.5%FS (0.5% of full scale).
If you have a 2″ pipe (ID of 2.067″) and maximum flow rate of 25 SCFM, the full scale velocity would be 1,073 SFPM. This is an acceptable full scale because it is above 1,000 SFPM. The accuracy of the reading at 15 SCFM, for example, would be ±1% of 15 (±0.15 SCFM) plus ±0.5% of 25 (±0.125 SCFM). Therefore, the accuracy specification at 15 SCFM would be ±0.275 SCFM (0.15 + 0.125). The error as a percent of reading would be ±0.275 divided by 15 or ±1.83%.
We set a minimum full scale of 1,000 SFPM as part of our accuracy specification so that the customer cannot select an unreasonably low full scale resulting in an accuracy specification that the meter cannot meet.