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Principle of Variable Flow Meter(Rotameter)
- Principle of Rotameter ( Float Type) 【Example】
[In the case of a liquid]
Equation 6 is used as it is. When a flow meter for water is used for kerosene with different specific gravity, the reading will change as follows:
[In the case of a gas]
In the case of a gas, the gas density ρ is negligibly small with respect to the float density ρf, and so Equation 6can be simplified as follows:
1.
In the case of a gas, C" is regarded as constant when the Re number is close, and the flow rate is determined based on the gas density ρ.
2.
The gas density changes, of course, according to the type of gas, and it also changes according to the pressure and tem-perature. Therefore, when the same flow meter is used for measurement under different conditions, the following rela-tionship exists theoretically:
3.
The density is inversely proportional to the absolute temper-ature T, while it is proportional to the absolute pressure P. When these conditions are combined with Equation 7, the following equation holds:
Example 1: When the type of gas differs
When a flow meter for H2 is used for He, the equation will be as follows when the above equation is substitu-ted because P1, P2, T1, and T2 are the same:
Example 2: When the pressure condition differs
When a flow meter for N2 with the atmospheric scale (P1G = 0 Pa) is used for measurement of N2 • 0.3 MPa (= P2G), ρ1, ρ2, T1, and T2 are the same and the follow-ing equation holds:
Example 3: When the temperature condition differs
When a flow meter calibrated at 1 atmosphere at 20°C is used at 25°C:
4.
The above corrections are approximations, and the meas-ured flow rate frequently differs from the theoretical value un-der actual conditions. Please use the correction equations, keeping in mind that theory and practice are not always the same.
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