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Participant

Join Date: May 2016
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# Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 1:33 PM

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?

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Guru

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#1

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 2:08 PM

There's no way to tell for sure without understanding the dynamics of the airflow.....

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#6
In reply to #1

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 8:34 PM

#2 would be correct in theory though...

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Guru

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#2

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 2:19 PM

#1. Their equation is longer, so they must be smarter.

If you need more accuracy, get a better gauge.

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#3

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 3:02 PM

Call the factory and have them sort out their contradictory data. There is no way anyone here can do it.

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Participant

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#4

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 5:24 PM

This problem has little to do with Air flow or vendor equipment, it has to do with calculating calibration span and zero offset. Employee #1 believes the span is from 0.5-7.0 (Delta6.5), employee #2 believes span should be 0-7.0 with no offset.

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#5
In reply to #4

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/17/2016 5:34 PM

Read the calibration instructions.

You MAY have to search out the web site for the latest information.

You found this site, you should be able to find theirs.

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#7

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 3:29 AM

The easiest way to settle this particular disagreement is to phone the equipment manufacturer over a speaker-phone with both parties present and discuss the matter directly. CR4 does not offer training in the use of the telephone.

<unsubscribes>

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#8

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 7:08 AM

Neither is correct IMO.

Presumably the meter has % indications, and you have to do a calculation or use a graph to find the flowrate.

Assuming flow and reading are linearly related, it should be

(3.35 - 0.5)/(7 - 0.5)*(100 - 10) + 10 = 49.46%

In general, for readings R % and flows Q,

(R - Rmin)/(Rmax - Rmin) = (Q - Qmax)/(Qmax - Qmin) so rearranging and putting your figures in

R = (90*Q + 20)/6.5 = 49.46%

I don't understand your 43.8% (45% scale) and 47.8% (50% scale). Presumably you just look at where the float is on the 0 - 100% scale, not worry about what that is as a fraction of the 90% range.

I believe Rotameters are linear with flow, but to confirm, just check that the readings are equally spaced.

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#9
In reply to #8

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 12:57 PM

Thanks for the response Codemaster, all the others who didn't understand the concept of the debate preferred to preach hate and throw garbage posts like learn to use a phone. Real value added guys. If your solution to every problem is to ask someone else, then you are no different than myself for posting where "guru's" can chat... except in my experience, I know far more than any vendor selling equipment and figured I could talk to likewise people here.

Codemaster: The reference to scale reading is based on indication resolution. The indicator dashes are set equally every 5%, so we can only resolve half that increment for proper meter reading. I can't tell an operator to set the flow rate through a needle valve to 49.4%, but I can tell him to set it to 50%. Thanks again for your response.

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#10
In reply to #9

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 1:44 PM

OK

I should have said (R - Rmin)/(Rmax - Rmin) = (Q - Qmaxmin )/(Qmax - Qmin) but you probably spotted that.

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#13
In reply to #9

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 4:53 PM

I think you do not understand the concept of good manners.

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#11

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 3:12 PM

You should base on the function principle of a rotameter: the moving part has a constant weight so that the pressure difference is always the same in ALL positions. This means that the flow is proportional to the annular area between "floating" part and wall.

For the lowest flow the outer diameter is D(0), the flow area will be A(0)=π/4*(D(0)^2-d^2). If the new position is at "h" from the lower the area will be A(h)=π/4*((D(0)+k*h)^2-d^2). The flow for this position will be Q(h)=Q(0)*A(h)/A(0). If you scale for "h" is linear then the flow relation to the scale is not.

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#12

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

05/18/2016 4:42 PM

Rotameters readings are linear to flow and normally has a range of 10 to 100%.

100% is equal to 7 m3/hr then the lower marking should be 0.7 M3/hr .Why is it 0.5 M3/hr

That is where I guess the confusion arises.

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#14

### Re: Rotameter (Variable Area Flow Meter) Percent Indication Debate

07/27/2016 1:44 PM

Out of curiosity, why does this purge flow measurement application require such precision? Will a 2.5% error actually matter?

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