The installation of instruments in general, and flow meters in particular, can be carried out at different levels. This section defines the most important aspects.
Precision
A very common term in the flow measurement is accuracy.Accuracy is defined as "the proximity to the absolute agreement between the measured value and the actual value of what is measured." The accuracy is therefore a qualitative term, and not quantitative. Nor can we talk about accuracy in absolute terms, but always "on a measurement accuracy" as a calibration standard expression verifiable. Sometimes he uses the term "accuracy" in place of "accuracy", but it is a measure of repeatability and should not be used in this sense.
The "accuracy" can be specified in terms of percentage or proportion for reading (% v.1.), Or in terms of percentage of the full scale value (% span). The following figure illustrates this difference.
Shows the measurement results of the two counters of the same type, one with an accuracy of 0.5% over the full scale (manufacturer A) and the second with an accuracy of 1% of reading (manufacturer B). As the flow rate is reduced, the error in the first counter is incremented, while the second counter (% v.1.) The error remains. In operational terms, if the accuracy of the counter is expressed in terms of full scale value, we must take into account the field of work values. This is common in older technologies, such as flowmeters with disc diaphragm or venturi flow meters.
Figure: Accuracy of measurement, flow meters
Repetibilldad
The repeatability is defined as " the quantity which characterizes the ability of an accountant to give identical indications or responses to repeat an application with the same values as measured at the working conditions set ".
In other words, if an accountant has a repeatability of 0.1% over the value of reading, the variations in responses to repeat the application will not differ by more than 0.1% if the flow rate is constant ¬ man. You should not confuse a good repeatability with good accuracy. Figure shows four cases. In case (a) the accuracy is perfect and all readings are within the range specified (io therefore also the repeatability is excellent).
Fig: Accuracy and repeatability
In case (b), a sporadic reading is outside the limits (a more real), but again the accuracy is generally good. In case (c) the repeatability is excellent, but all readings are biased about the correct value, and in case (d) repeatability accuracy is poor and unacceptable. In short, a good accuracy guarantee good repeatability ¬ chalk, but a good repeatability alone does not guarantee a good accuracy.
Linearity
Flowmeters are often characterized by a linearity of 0.5 or 1%. This means that the deviation of the responses of the flow with respect to a best-fit linear function relating the actual flow output values indicated by the flow is less than 1%.
In the following figure, the graph represents the linearity of a flow expressed as a percentage of full scale value. In the measurement of flow is defined a coefficient ¬ patient (GA) that represents the output value as an average "ideal" for the entire working range of values (Rt). Similarly, we define a range of limit values (eg ± 1% v.1., Dotted line), in which the values can fluctuate. This is the measure of the linearity of the flowmeter. Note, however, that changing the range of values of work, the output values may be biased.
The recommended ratio (GL) for the range of work values (Ro) is slightly below the average coefficient obtained over the entire range of values specified (GA). A specified value field, sometimes called turndown, and is the ratio of maximum and minimum flows.
Linearity of a flowmeter Fig.
Uncertainty
The uncertainty is defined as the range of values between which lies the true value with a given probability (see Figure below). The flow measurement is not possible to measure anything with absolute precision (ie with an error of zero) because the flow is never stable. Small perturbations in pressure and temperature affect the response of the instrument, which is never perfect, plus a host of other external effects and electronic type.
A stable value of reading time and the concept of uncertainty together constitute a way to identify and combine all these factors so that the variable being measured is well defined. Note that the quantitative value of the "accuracy" should be expressed in terms of uncertainty. A good accuracy is, at bottom, a low uncertainty, but there are always small errors, both random and systematic origin.
Error
The error is not more than the difference between the counter output value and actual value of the flow at the instant when the measurement is made. A 1% error in the flow rate (usually expressed as accuracy of 1%) means that the output of the counter register, for example, 99 l / min when the actual value (determined from a reference standard) is 100 l / min. Since, in fact, the real value is never known, the error is, by definition, an unknown quantity.
Fig Definition of "uncertainty in measurement", x = Mean of all measured values, f (x) = Frequency, o-= standard deviation.
K-factor meter constant sensitivity and
To define the transfer characteristics of a counter with an output of linear or nearly linear momentum can be used two parameters. They are called "meter constant" and "sensitivity factor K" respectively. Both parameters are usually found printed on the nameplate of the meter.
K Factor Sensitivity
The sensitivity factor K is defined as the number of pulses per unit la.magnitud and determined in the laboratory. On the other hand, some manufacturers with the letter K denote the "constant of the counter." Others define the letter K as the ratio between output frequency and flow rate. In any case, the reader must know with certainty the exact use of the term in each application. In many cases, the K also defines a correction factor determined by calibration in the laboratory.
Meter constant
The meter constant is defined as the ratio between the actual volume and volume
registered. Unlike the K factor of sensitivity, the meter constant is usually
determined by calibration in the workplace, ie, by racking applications. In this procedure uses a standard reference value for the actual volume. Therefore, by definition, a constant good accountant should have a value close to 1. A control chart graphically representing the variations of the meter constant over time (control chart) indicates the stability of the counter.
All these parameters are commonly found in process applications racking that is calibrated and specifies the runtime system of a turbine meter, Coriolis displacement. Thus, a specification of, for example, "100.12 pulses / liter (± 0.1% of measured value) in the range of values from 10 to 100 liters / minute" means that the parameter value is between 100.02 minimum and a maximum value of 100.22 pulses / liter.
