Do you know your fluid? Is it what you think it is? Is it from a known source?
Viscosity, varies with temperature. Is flow measurement going to be affected by viscosity change due to temperature anyway? Might be if the temperature range is large and it’s a Variable Area meter… Will the fluid be changed through the life of the system, introducing different viscosities; meter choice is important here.
Viscosity change over time. due to volatility of light compounds it’s likely, especially if exposed to the atmosphere, that viscosity will increase over time. Possibly if water is leaking into the flow stream or condensation in the process that the viscosity will decrease.
Viscosity changes due to pressure. These are known but fairly small changes compared with temperature effects. Viscosity can double between atmospheric pressure and 2,500bar.
Specific Gravity, Density. These are often quoted in Material Safety Data Sheets (MSDS). For some flowmeters it’s irrelevant, especially if the measurement principle is volumetric; for others, like VA it’s fundamental. And remember density changes with temperature. In general, if you want a mass flow rate or total then use a mass flowmeter (and vice versa).
Thixotropic? A shear sensitive liquid can be tricky for some measurement principles. To preserve the fluid at normal viscosity the rate may have to radically reduced. Typical thixotropic liquids encountered are paints. When stirred they change from a ‘gel’ to a more free flowing liquid.
Corrosion issues: chemical compatibility. Perhaps the first property that is investigated in meter selection is the chemical nature of the fluid being measured. Is it going to corrode any of the components or will it react with the materials and change some dimensions or shape? If a table found on the internet indicates that polypropylene is ‘compatible’ with fluid X will it be suitable for some close fitting parts where just a 1% expansion will stop the meter going round. 1% may indicate, to some people, that it is compatible.
Build up, formation. Slow or fast deposition on the inside of the pipe and other, more sensitive parts, inside a flowmeter may affect the internal diameter used for rate calculations on velocity based devices or the weight of a rotating part or simply stop a part meshing or rotating.
Solids content and solids size. Generally expressed as a percentage, the amount of particulate and the size of that particulate will govern the metering method. And it may not be obviously so. Some of the latest paints have small amounts of additive to give the paint a special quality. These will block a tightly toleranced PD meter or it’s bearings.
Filter size. Is it filtered? Is the filter mesh in the filter bowl or has it been removed because it keeps clogging up?! What level of filtration, NAS class, mesh size, is designed in and what level has been achieved. Is it well filtered but then stored in an open container?
Lubricity. This parameter is frequently ignored and frequently not known. It can have an effect on some flowmeters.
Homogeneous? It’s usually taken for granted that fluids are homogenous i.e. the same consistency at any point. A typical non-homogeneity is air entrainment, perhaps a few bubbles or a stream of bubbles. In extremis, this might be slugs of air passing through. Most flowmeters can’t cope with this phenomena but some make a decent estimation and more than a few will recover after the air passes.
Anodic acceleration of corrosion. This problem occurs when the fluid acts in concert with two dissimilar materials in the pipeline – for example, the flowmeter body and the pipework. The measured fluid acts as an electrolyte, depositing or removing material depending whether the materials act as anodes or cathodes. In some instances another wetted part may see accelerated corrosion.
All in all, consult the specialists. www.litremeter.com