I was recently determining the indication reference point for a flare purge line and am trying to settle a disagreement with a colleague.

We have a rotameter (Variable Area Flow Meter) that has a range of 0.5 m³/hr to 7 m³/hr. (This is the uncorrected pressure, temperature and SG values but they are sufficient for this conversation ie) Air @ ATP).
It corresponds with a indication scale of 10% to 100%. I'm trying to determine what % display to achieve 3.35 m³/hr setpoint.

Employee #1) x = (MV - (LRV)/Span)*100% as:
x = ((3.35 - (0.5)/(7-0.5))*100% = 43.8% (45%scale)

Employee #2) x = (MV - (LRV)/Span)*100% as:
x = ((3.35 - (0)/7))*100% = 47.8% (50%scale)

We can't resolve better than 2.5% indication. So values should be corrected as shown.

Which employee is correct, and why?

 

"Which employee is correct, and why?"

Employee #1, assuming scale is linear.

The setpoint is 2.85m³/hr greater than 0.5m³/hr, the LRV and 0% scale, and the span is 6.5m³/hr, not 7.