I have a orifice flow meter with the range 30 in wc to 100 in wc and accuracy is 0.5%. If I have a reading 50 in wc, what is the uncertainty and error?

1. 50 in wc x 0.5% = 0.25 in wc
2. 100 in wc x 0.5% = 0.50 in wc
3. 30 in wc x 0.5% = 0.15 in wc
4. (100-30) in wc x 0.5% = 0.35 in wc

If my reading is 25 in wc, what is uncertainty and error?

Shan

 

Without any other qualifiers, you have to assume +/- .50 in wc regardless of reading.

Regards

cww

 

Wow, pretty impressive uncertainty spec for DP flow.

DP transmitters are always accuracy spec'd in percent full scale. Your full scale is 100"wc, so the calculation would be

0.005 * 100" = ±0.5"w.c.
at the design temperature & pressure.

If your medium is a gas or steam, deviations from design temp & pressure values will produce errors in whole percentage numbers without all that much deviation from the design values.

I'll let the more emotional, old timer forum responders comment on the uncertainty at 25"w.c. Have at it, guys.

Carl

 

Shan,

DP-flow is quite complex to figure out the total accuracy with.
1) Accuracy from dp-transmitter
2) Accuracy from primary element (orifice)
3) Changing process conditions

The three main topics above can be split into more details, where of the transmitter is rather easy to calculate.
The accuracy of the orifice for example is depending on Beta ratio, straight runs of pipe etc.
But if everything is done by the book, and you are using a flow computer or a multivariable transmitter you might get slightly better than 1% accuracy.

/Ulf