We intend to measure very low flow (about 2% of the original flow)using a smart flow transmitter (dp type) across flow orifice having accuracy specified as 0.1% of span. We can tolerate degraded accuracy a bit, say upto 1% of span. Suppose if we use the same transmitter for 0 to 10% of the flow range instead of 0-100% of the range and re-range that transmitter for 0-10%, whether the accuracy would remain 0.1% of the original span (i.e.100% range) or new span(i.e. 10%range)? We do not intend to change the primary sensor -orifice which has rangeability of 1:4. Whether we would get poorer accuracy at 2% flow? What best accuracy we can expect by re-ranging alone?

 

At 10% flow it remains only 1/100 of dp at max flow, at 2% it only remains 1/2500 of dp at max. If you got 100inwc/2500 mmwc at max flow, it only remains 0.04inwc/1 mmwc. Expected accuracy at 2% flow? If you're lucky you might be able to detect a dp at all...

You need to replace orifice plate, or maybe change measuring method: magnetic or Coriolis depending on application.

/Ulf

 

I'd be concerned about signal to noise at 2% flow.

Have you considered a bypass line sized for the 2% flow rate?

I assume your medium is a liquid, not a gas where any deviation from design conditions of temperature and pressure throw any volumetric flow readings off by large percentages.

 

Furthermore since the DP from an orifice plate is a square root relationship at the bottom end the errors must be awful.

Do as a previous poster said and reduce the size of your measuring element.

Regards
Roy

 

The following is an indication of the mass flow uncertainty associated with a DP cell (±0.1% span) using ISO5167/AGA 3. Included are low uncertainties for temperature, pressure and density. This will give you an idea of how the uncertainty shifts over turndown/range.
<pre>
Turndown HIGH DP Mass Flow Unc
1.00 : 1 500.00 mbar 0.89 %
3.02 : 1 165.54 mbar 1.19 %
5.04 : 1 99.19 mbar 1.64 %
7.06 : 1 70.81 mbar 2.16 %
9.08 : 1 55.06 mbar 2.69 %
11.10 : 1 45.04 mbar 3.24 %
13.12 : 1 38.10 mbar 3.80 %
15.14 : 1 33.02 mbar 4.36 %
17.16 : 1 29.13 mbar 4.92 %
19.18 : 1 26.06 mbar 5.48 %
21.20 : 1 23.58 mbar 6.05 %
23.22 : 1 21.53 mbar 6.61 %
25.24 : 1 19.81 mbar 7.18 %
27.27 : 1 18.34 mbar 7.75 %
29.29 : 1 17.07 mbar 8.32 %
31.31 : 1 15.97 mbar 8.88 %
33.33 : 1 15.00 mbar 9.45 %
35.35 : 1 14.15 mbar 10.02 %
37.37 : 1 13.38 mbar 10.59 %
39.39 : 1 12.69 mbar 11.16 %
41.41 : 1 12.07 mbar 11.73 %
43.43 : 1 11.51 mbar 12.30 %
45.45 : 1 11.00 mbar 12.87 %
47.47 : 1 10.53 mbar 13.44 %
49.49 : 1 10.10 mbar 14.01 %
51.51 : 1 9.71 mbar 14.58 %
53.53 : 1 9.34 mbar 15.15 %
55.55 : 1 9.00 mbar 15.72 %
57.57 : 1 8.68 mbar 16.29 %
59.59 : 1 8.39 mbar 16.86 %
61.61 : 1 8.12 mbar 17.43 %
63.63 : 1 7.86 mbar 18.00 %
65.65 : 1 7.62 mbar 18.57 %
67.67 : 1 7.39 mbar 19.14 %
69.69 : 1 7.17 mbar 19.71 %
71.71 : 1 6.97 mbar 20.28 %
73.73 : 1 6.78 mbar 20.85 %
75.76 : 1 6.60 mbar 21.42 %
77.78 : 1 6.43 mbar 21.99 %
79.80 : 1 6.27 mbar 22.56 %
81.82 : 1 6.11 mbar 23.13 %
83.84 : 1 5.96 mbar 23.70 %
85.86 : 1 5.82 mbar 24.27 %
87.88 : 1 5.69 mbar 24.85 %
89.90 : 1 5.56 mbar 25.42 %
91.92 : 1 5.44 mbar 25.99 %
93.94 : 1 5.32 mbar 26.56 %
95.96 : 1 5.21 mbar 27.13 %
97.98 : 1 5.10 mbar 27.70 %
100.00 : 1 5.00 mbar 28.27 %
</pre>