# Transmitter Accuracy

B

hello

can you explain to me what is mean (%span) for transmitter accuracy?

example
for Rosemount pressure transmitter the accuracy is +_0.075 of span. this is for span more than 10:1.

is the accuracy of transmitter at 1:1 turn down is less than at 2:1
because the span at 1:1 is greater. so 0.075%* span is larger or this is wrong? the accuracy for all spans that has turndown less than 10:1 is same?

#### David_2

Interesting question because I actually did some arithmetic and the results don't match the spec, but hey, it's gamesmanship.

As far as I know, span is URL minus LRL. Turndown is URL/(URV-LRV) or URL/span.

For 0-2500mbar, the span is 2500. For 0-100", the span is 100. So
0.075% of span for 2500mbar is Â±1.875mbar

The currently posted Rosemount product data sheet with uncertainty specs is 00813-0100-4001, Rev SA

The performance specs start on page 45

I'll pick the the 3051CD (coplanar DP), with range 2, -250" to +250"wc, commonly used for orifice plates.
The range 2 URL is 250" (table on page 49 lists URL and LRL for all models)

The reference accuracy for range 2 is stated as Â± 0.04% of span.

There's a formula for spans with less than 10:1 turndown:
for spans less than 10:1, accuracy = Â± [0.015 + 0.005 (URL/Span)] % of Span

I'm not sure where the general number for reference accuracy, Â±0.04% of span, comes from because according to the formula, I find that the uncertainty at 5:1 turndown is 0.04%, but the inaccuracy increases at higher turndowns between 5:1 and 10:1.

Here's my arithmetic: <pre>
reference accuracy = Â± [0.015 + 0.005 (URL/Span)] % of Span

For a span of 0-250" (a 1:1 turndown):
Â±0.015 + 0.005(250/250)
Â±0.015 + 0.005
Â±0.020 % of Span = Â±0.0002 * 250 = Â±0.05"wc

For a span of 0-125" (a 2:1 turndown):
Â±0.015 + 0.005(250/125)
Â±0.015 + 0.010
Â±0.025 % of Span = Â±0.00025 * 125 = Â±0.031"wc

For a span of 0-100" (a 2.5:1 turndown):
Â±0.015 + 0.005(250/100)
Â±0.015 + 0.0125
Â±0.0275 % of Span = Â±0.000275 * 100 = Â±0.0275"wc

For a span of 0-50" (a 5:1 turndown):
Â±0.015 + 0.005(250/50)
Â±0.015 + 0.025
Â±0.040 % of Span = Â±0.0004 * 50 = Â±0.02"wc

For a span of 0-25" (10:1 turndown):
Â±0.015 + 0.005(250/25)
Â±0.015 + 0.050
Â±0.065% of span = Â±0.00065 * 25 = Â±0.01625"wc</pre>

>is the accuracy of transmitter at 1:1 turn down is less than at 2:1?

The arithmetic shows reference accuracies of
Â±0.020% span at 1:1 turndown, or Â±0.05"wc
Â±0.025% span at 2:1 turndown, or Â±0.031"wc

Is the accuracy less?
The % span value, Â±0.020, at 1:1 is less than the % span value, Â±0.031 at 2:1.
A lower reference accuracy % span value is considered 'more accurate'.

In the end, remember, we're talking reference accuracy here (lab conditions). Put that transmitter on a hot process and the temperature error becomes a factor.

B

Mr david_2
Thank you so much

#### David_2

How embarrassing, another correction:

As far as I know, span is URL minus LRL.

should be:
As far as I know, span is URV minus LRV.