If I may add just a little to Dr. Tim's explanation.
For a very accurate reading you need a "positive displacement" meter. Its not just a spinning wheel or fan it actually takes an accurate measure of liquid and allows it to move from inlet to outlet of the meter. Then more accuracy is gained by spending $$$$ on temperature and viscosity auto calibration. We added air scrubbers and filters up stream from meters as Dr. Tim suggests. You will find very accurate meters typically in situations were the product being measured is expensive like petroleum products. Jay