Accuracy and Rangeability

If the Reynolds number (Re) and flow rate are both constant, the output signal of a head-type flowmeter will also be constant. However, if the Reynolds 

Post-Oil Energy Technology After the Age of Fossil Fuels 

Velocity Head Requirements of the Different Flowmeter Designs 

Flowmeter Type Permanent Pressure Loss (in velocity heads) 

The required metering accuracy should be specified precisely and at a realistic value. In some instances, absolute accuracy is less important than long-term repeatability. When the error limit is given as a percentage, it 

---

Most laminar flowmeters (also called capillary flowmeters) measure very low flow rates of gases in applications where other types of meters either give marginal performance or cannot be used at all. Commercial units use either capillary elements or matrix shapes (Figure 3.67). The pressure drop generated by these elements is in linear relationship with the process flow. When higher accuracy and rangeability is desired, thermal instead of d/p detectors are used with the laminar flow element. 

Solids flow measurement is more important in the control and optimization of coal-fired power plants than in alternative energy processes. The mass flow of solids can be detected by impact flowmeters, which are relatively low-accuracy devices (1-2% FS). Better accuracy and rangeability are provided by belt-type gravimetric feeders (0.5% AF over a 10 1 range), which measure both the speed and loading on the moving belt, as shown in Figure 3.89. 

As smart d/p transmitters became available, the orifice rangeability could be increased at a lower cost by using dual-span transmitters. Some smart d/p transmitters are currently available with 0.1% span accuracy, and their spans can be automatically switched based on the value of measurement. Therefore, a 100 1 pressure differential range (10 1 flow range) can theoretically be obtained by automatically switching between a high (10-100%) and a low (1-10%) pressure differential span (in practice, some overlap is recommended). The overall result can be a 1% of AF accuracy over a 10 1 flow range. 

Their disadvantages include their low accuracy, low rangeability, and the fact that they are suitable only for clean liquid, gas, or vapor services unless purged. Their principal limitation is that they measure the flowing velocity only at one point and, therefore, even after calibration, their measurement will be in error every time the velocity profile changes. Therefore, they are used only when low-accuracy volumetric readings are acceptable, such as in HVAC applications. To reduce the effect of velocity profile changes and... 

The chief advantages of gas PD meters are their high accuracy and wide rangeability. Their chief disadvantages are maintenance costs and the fact that wear and tear can degrade their performance. 

A program was initiated to evaluate several gas chromatographs on the basis of accuracy, repeatability and rangeability. These terms ate defined as follows ... 

The testing procedures were developed to determine the accuracy, repeatability and rangeability of the automated process gas chromatographs. These parameters were compared for several gas chromatographs and also to a calorimeter. 

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

Paddlewheel flowmeters are one of the least expensive ways of measuring liquid flow in larger pipes (Figure 3.94). The rotation of the paddlewheel can be detected magnetically or optically. Accuracies, pressure ratings, and temperature ratings are low, but rangeability is reasonable, as these units can handle both low and almost any maximum velocity. 

Rangeability. As can be seen in Figure 2, the accuracy of all the units is very good (0.2%) at heating values above 950 Btu/scf. Below 950, the accuracy of GC-A and GC-B decreases rapidly with decreasing heating value. In contrast, the accuracy of GC-C and the calorimeter remains fairly accurate even at the low heating values. 

---