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Flow Meters Accuracy and Terminology

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Flow meters are very popular devices that help to determine the quantity of fluids through pipes at any given time. The majority of flow meters are applied in custody transfer applications where a seller of a given fluid –gasoline, natural gas etc needs to make sure that the actual quantity of fluid is being sold to a buyer and of course the buyer needs to ensure that he is getting value for his money by ensuring that he gets the actual quantity of fluid being paid for. Also in many industrial control applications, flow metering is critical to deliver control objectives.
So whatever the area of application of your flow meter, you need to understand the terminology used in flow meter specifications and also the accuracy level of the device. Now when it comes to flow meter accuracy, it can get a little tricky and even misleading if you don’t have a thorough understanding of the types of accuracy metrics flow meters manufacturers employ to define their products. A clear understanding of flow meter accuracy is therefore necessary if you must get your flow meter specifications right.

Flow meter Terminology
 

Repeatability
This describes the ability of a flow meter to indicate the same value for an identical flow rate on more than one occasion. It should never be confused with accuracy. Good repeatability does not guarantee accuracy.


Turn Down
Turn Down or Turn down Ration describes the range of flow rates between maximum and minimum flow over which a flow meter will work satisfactorily within the accuracy limits and repeatability of tolerances specified by the manufacturer. Effective range and Rangeability are alternative terms used to describe meter turn down.

Flow meter Turn Down = Maximum Flow/Minimum Flow

It is very important to select a flow meter with sufficient range for the application. Failure to do this will introduce considerable errors especially at low flow rates. Selecting the right flow range requires a diligent and holistic study of the maximum, normal and minimum flow that is required for a given application. The turn down ratios of some common flow meters in use are given below:


 Flowmeter Type    Turn Down or Operating Range   Minimum Flow
Orifice Plate             4:1  25% of Maximum flow 
Turbine             10:1  10% of Maximum flow
Coriolis             80:1  1.25% of Maximum flow 


As shown in the table above, if we are using these flow meters to measure a given liquid in an application where the expected maximum flow rate is 2,400gpm then:
For the Orifice plate meter, minimum flow = 25% of maximum flow = 600gpm.
Since the turn down of the Orifice meter is 4:1, this means that at flow rates between 600gpm and 2,400gpm the flow meter can still meet its claimed or stated accuracy. However, at flow rates lower than 600gpm this flow meter cannot meet its stated specification, so large flow errors occur. At best, the recorded flows below 600gpm are inaccurate - at worst they are not recorded at all, and are lost  

This problem of considerable errors at low flow rates due to insufficient turn down is particularly worst for differential pressure flow meters where flow is proportional to the square of pressure.

For the Turbine meter, minimum flow = 10% of maximum flow = 240gpm
For the Coriolis meter, minimum flow = 1.25% of maximum flow = 30gpm
At flow rates below these for the turbine and Coriolis flow meters, the meters can no longer give accurate measurements.

So it is important to do proper flow study for your application to enable you select a flow meter technology with the sufficient turn down to meet the range of flow rates expected for your application.


Flow meter Accuracy
This is a measure of a flow meter's performance. It is the amount of error that can occur when measurements are taken with the flow meter. Flow meter accuracy determines how precise or correct flow measurements are to the actual value or flow rate and is used to determine the suitability of the flow meter for a particular application.
Flow meter accuracy statements can be expressed as a percentage of either:
(a) Flow Rate (%R)
(b) Full Scale (%FS)
(c) Calibrated Span (%CS)
(d) Upper Range Limit (%URL)

Percent of Flow Rate(%R):
The error in the measurement of flow rate when the accuracy of the meter is stated as a percent of flow rate can be determined by:

Error = % flow rate * Measurement

Suppose we have a flow meter whose accuracy is stated as ±1% of flow rate and is being used to measure flow in the range 0 – 100gpm then we can determine the errors at 100%, 50%, 25% and 10% of the flow range as:
At 100% of flow, Error = ±0.01 *100 = ±1gpm =      1% Rate
      50% of flow, Error = ±0.01*50 =   ±0.5gpm =   1% Rate
      25% of flow, Error = ±0.01*25 =   ±0.25gpm = 1% Rate
      10% of flow, Error = ±0.01*10 =   ±0.1gpm =   1% Rate

In terms of %R, we obtain 1% at the various levels of flow. A plot of % R error against flow gives the graph below:


As revealed by the graph, the % Rate Error between the range of low flows and high flows remains constant.

Percent of Full Scale (%FS):
The error in the measurement of flow rate when the accuracy of the meter is stated as a percent of full scale can be determined by:

Error = % FS * Full Scale Flow

Consider a flowmeter with a stated accuracy of ±1%FS. If the meter is being used to measure liquid volume in the range 0 – 100gpm then we can determine the errors at 100%, 50%, 25% and 10% of the flow range as:
At 100% of flow, Error = ±0.01 *100 = ±1gpm  = 1% Rate
      50% of flow, Error = ±0.01*100 =   ±1gpm = 2% Rate
      25% of flow, Error = ±0.01*100 =   ±1gpm = 4% Rate
      10% of flow, Error = ±0.01*100 =   ±1gpm = 10% Rate

Note that  % R = Error /Measurement

As shown from this example, while the error is constant at any given flow, the error expressed as a percent of flow rate increases with a decrease in flow rate. This is not desirable because at low flow, the errors become considerable.
A plot of % R error against flow gives the graph below:

Percent of Calibrated Span (% CS):
The error in the measurement of flow rate when the accuracy of the meter is stated as a percent of calibrated span can be determined by:

Error = % CS * Calibrated Span

Consider a flowmeter with a stated accuracy of ±1%CS. If the meter is being used to measure liquid volume in the range 0 – 100gpm but with a calibrated span of 200gpm and an upper range limit(URL) of 400gpm then we can determine the errors at 100%, 50%, 25% and 10% of the flow range as:
At 100% of flow, Error = ±0.01 *200 =  ±2gpm  = 2% Rate
      50% of flow, Error = ±0.01*200 =   ±2gpm = 4% Rate
      25% of flow, Error = ±0.01*200 =   ±2gpm = 8% Rate
      10% of flow, Error = ±0.01*200 =   ±2gpm = 20% Rate
A plot of % R error against flow gives the graph below:


Percent of Upper Range Limit (%URL):
The error in the measurement of flow rate when the accuracy of the meter is stated as a percent of upper range limit can be determined by:

 Error = %URL* Upper Range Limit

Again consider a flowmeter with a stated accuracy of ±1%URL. If the meter is being used to measure liquid volume in the range 0 – 100gpm but with an upper range limit (URL) of 400gpm then we can determine the errors at 100%, 50%, 25% and 10% of the flow range as:
At 100% of flow, Error = ±0.01*400 =  ±4gpm  = 4% Rate
      50% of flow, Error = ±0.01*400 =   ±4gpm = 8% Rate
      25% of flow, Error = ±0.01*400 =   ±4gpm = 16% Rate
      10% of flow, Error = ±0.01*400 =   ±4gpm = 40% Rate
A plot of % R error against flow gives the graph below:

A combination of these graphs gives the graph below:




A careful look at this single graph depicting all the various flowmeter accuracy statements shows that:
(a) Flowmeter accuracy expressed as a percent of calibrated span (%CS) is similar to full scale (%FS) and %URL accuracy statements where the absolute error is a percentage of the calibrated span
(b) A calibrated span statement becomes a full scale statement when the instrument is calibrated to full scale.
(c) A calibrated span statement becomes a %URL statement when the instrument is calibrated at URL

From the discussions so far, it should be noted that flowmeter Performance specified as a percent of rate (%R), percent of full scale (%FS), percent of URL (%URL), and percent of calibrated span (%CS) are different and must not be confused with each other.

In preparing flowmeter accuracy specifications therefore, all accuracy statements should be converted into uniform % actual flow rate and these % actual flow requirements should be specified separately for minimum, normal, and maximum flows. In this way, one will truly appreciate the magnitude of the errors at the various flow levels – minimum, normal and maximum flows- and therefore determine which flowmeter is best for their application.