What would the turndown ratio of dual dp cells on an orifice system with are calibrated 0-150 mbar and 0-500 mbar set up in parallel? How is this worked out?

Thanks

 

Presumably the 0-500mbar DP transmitter is used over the upper range of 150 - 500 mbar.

The 500 mbar transmitter DP turndown is 500:150 or 3.3:1.

That would correspond to a range of 100% max flow rate at 500mbar to 54.8% flow rate at 150mbar. (square root of 30% (150/500) DP is 0.548 or 54.8% flow rate)

So, the flow rate turndown is 100%:54.8% or 1.82:1

Turndown is not a fixed value, it's a self-defined value at the limit of acceptable accuracy. Measurements beyond turndown are available, but at accuracies below an acceptable limit.

The range of the 150mbar DP starts where the 500mbar DP 'stops'.

So what is the 150mbar DP turndown? What do you accept as a limit of accuracy?

If you use the same 3.33:1 DP turndown (150:50) for the 150mbar DP as for the 500mbar DP, then the same 54.8% of max flow rate for the 150mbar DP transmitter is at the "bottom" of the range of 500mbar DP.

So, the 150mbar's 54.8% of max flow rate of 500mbar's 54.8% flow rate (54.8*0.548 = 0.30) is 30% of the maximum flow rate of the 500mbar's transmitter. A low point of 30% of max flow is a 3.33:1 turndown.