Although the flow meter has been strictly tested before leaving the factory, because of changes in the environment, installation, running condition, etc, errors will occur, and test operation needs to be done using the following methods:
- Test using standard flow measure equipment
- Conversion between two flow media
The primary test medium in the factory is water or air. When a new non-water or non-air medium is
used to test the flow meter, the test result is computed as follows:
a. Flow: 
where,
Q — Flow meter display.（Kg）
Qo— True flow.（Kg）
p1— New medium density.（Kg/m3）
p— Primary medium density.（Kg/m3）
Example: A flow meter is tested by water, the new medium is light diesel oil (p1=750Kg/m3), and the true flow value is 500Kg. Then the value showed on the flow meter should be: 
That is, the re-test value is 433Kg.
b. Cubage flow: 
where,
Q — Flow meter displays flow value.（m3）
Qo — True flow value.（m3）
Example: With the same numbers as above, the result is: 
Remarks: It should be a little error between two different media test results because of different
medium characteristics, equipment conditions, installations, and other reasons.
3. Test using hang weight
Hang weight method can be used to test the flow meter as shown in the below figure.
The test result is computed as:
where,
Qm — Instantaneous quality flow value（Kg/h）
QV— Instantaneous cubage flow value（m3
/h）
α— flow coefficient
D — Flow meter diameter (mm)
F — Quality of hung weight（Kg）
p — Medium density（Kg/m3）
β — Target/ diameter ratio
Example: For quality flow, given α=12.52, β=0.7, D=100 and p=750Kg/m3 (light diesel oil), and the maximum instantaneous flow of the flow meter is 100t/h, so
It is known that the force on the target is 16Kg while maximum instantaneous flow occurs. So, if
using the hang weight method, the total quality of hung weights must less than 16Kg. Supposed 3Kg
weighs is used, the test result is:
Remarks: Hang weight method is not able to replace a strict experiment test, because of many
factors such as medium flow pattern, viscosity, press loss are ignored and errors will occur in hang
weight test process.