Totalizer Flow Meter

Fluid – What are you measuring?
In order to identify what type of flowmeter is suitable to a certain application, it is important to know what state the fluid being measured is in; fluid or gas. Gases compress and can't be measured with a liquid meter. This is a vital information to know right from the start. This article focuses on selecting a meter for liquid measurement. You can find further information on gas flowmeters here.
Once fluid has been identified, it is vital to assess if it is clean. A dirty fluid contains solid particles, typically called a slurry, while a clean fluid will be particle free. For example, flowmeters that have wetted moving parts, such as the positive displacement (volumetric flowmeter) or turbine (velocity flowmeter) would not be suitable to dirty fluids, as it will be more susceptible to mechanical wear, plugging or erosion due to the presence of solid particles. Hence, the flow meters that have wetted moving parts are substantially applicable to clean fluids only. On the other hand, dirty fluids would be appropriate to run in an non-contact meters such as electromagnetic (velocity meter), ultrasonic (velocity meter) or Coriolis (mass meter). These also have limitations on them, but handle particles better.

Viscosity and Flow Profile – How thick or thin is the liquid?
One of the principal parameters to consider when selecting a meter is the fluid's viscosity, or how thick the fluid is. Since the fluid to be measured has been identified, it is now possible to look at its properties related to flow such as the viscosity. This is defined as the measurement of resistance to flow or alternatively, it is the internal friction of a fluid, the amount of friction the molecules create as they flow over each other. The importance of this parameter in flow measurement is that it determines how well mixed a fluid is and thus how repeatable the reading may be.

Flowrate Information – What is the maximum and minimum flow rate?
This parameter is equally important as the prior parameters to determine the right size of the meter that will suit the application. Flowrate is the volume or mass of a fluid flowing/moving per unit time. You can convert from mass to volume through the density (the amount of volume a fluid takes up per unit mass) or specific gravity ( the ratio of the density of the substance to the density of water or how much a litre of the fluid would weigh divided by the weight of the same volume of water).

Temperature and Pressure Rating – What is the maximum allowable?
Other key parameters in sizing of flowmeters are the temperature and pressure. Similar to the flowrate, which represents the size capacity of the meter, the temperature and pressure parameter classifies the meter's material capability to withstand the effect of thermal energy and forces exerted by the flowing fluid.
Since viscosity is a function of temperature, it is indeed important to take this parameter into consideration when performing the sizing, in the same way as the viscosity. Furthermore, the operating temperature is crucial to the wetted components of the meter, particularly the sealing, as seals have temperature limits and some materials are not able to withstand extreme temperatures or extended periods of time. Lastly, temperature helps to decide if an electronic instrument can be mounted directly onto the meter or it needs to be remote mounted, because the electronic components have a temperature limitation.

Accuracy/Repeatability/Linearity – How accurate and precise?
Some applications may specify and require high accuracy meters, like for those being used in dosing applications or custody transfer (applications where a consumer is being charged based on the reading). Inaccurate readings result in financial loss or quality issues on a product being manufactured. It is important to select the meter to meet the desired accuracy of the process.
Accuracy relative to flow metering is the measurement of how close the measured value is generated by the device/instrument to the actual flow rate. It can be expressed as percent of Full Scale or percent of Reading. Accuracy over range or Full Scale accuracy implies that the error is consistent over the full range of the flow rate for the meter. For example, a meter with flow capacity of 100L/min and 1% Full Scale accuracy would have an error of 1 L/min no matter whether the reading is 10 or 100 L/min. On the other hand, percentage of reading accuracy calculates from the actual reading. A meter with flow rate range of 10-100L/min and 1 % reading accuracy would have 1L/min error at 100L/min and 0.5L/min at 50L/min. Hence, it is apparent that a meter with accuracy calculated over reading will be more accurate at low range readings than a meter with full scale accuracy specified.