PD meters, or positive displacement meters, work by measuring the volume of liquid that is displaced by a rotating chamber or piston. These meters are accurate as well and have been used for many years in this type service.

Turbine meters work by measuring the rotational speed of a turbine placed in the liquid flow path. As the liquid flows through the meter, it causes the turbine to rotate, and the resulting rotational speed is used to calculate the flow rate.

Tank gauging methods involve measuring the level of liquid in the railcar tank. Tank strapping and sticking are two common methods used for tank gauging.

Tank strapping involves measuring the distance between pre-determined points on the tank, such as the top and bottom, using a tape measure. The distance measurements are used to calculate the volume of the tank. This method is less accurate than metering, with an accuracy of up to +/- 0.5%.

Sticking involves inserting a graduated stick, or dipstick, into the tank until it reaches the bottom. The level of the liquid is determined by reading the measurement on the stick where it intersects with the liquid surface. This method is also less accurate than metering, with an accuracy of up to +/- 0.5%.

In terms of percentage differences in accuracy, as previously mentioned, it is common for metering to be up to twice as accurate as typical tank gauging methods. The choice of a flow meter or tank gauging method will depend on the accuracy required for the specific application. While coriolis meters and PD meters are highly accurate, they may not be necessary for all applications. Similarly, while tank strapping and sticking may be less accurate than metering, they may be sufficient for some applications.

As an example, a producer using coriolis meters to load the product and an end user or storage terminal using a tank gauging method where the product is unloaded, will commonly see discrepancies in the measurement of the various products because coriolis meters are highly accurate, with an accuracy of up to +/- 0.1%, whereas tank strapping can be up to 5% inaccurate in comparison to that of a coriolis meter. However, the frequency and magnitude of these discrepancies will depend on several factors, including the accuracy of the meter or tank gauging method, the calibration of the equipment, and the conditions under which the measurement is taken.

To minimize the chances of discrepancies, it is important to ensure that the equipment used for both metering and tank gauging are calibrated and that the operators are trained to use the equipment correctly. It is also important to monitor the equipment regularly to ensure that it is functioning correctly and to identify any potential issues before they become a problem.