What are the units for variables in the imperial versions of Pneumatic Study Manual Equations 3.37 and 3.38? I believe these equations are taken from ISO 6358 (listed as ISO 6538 on page 3-55), and therefore the conversion factor (600) likely applies to the equation with metric units, and should be different for the equation with imperial units.
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Friday, March 14, 2025 2:57:55 PM
Pneumatic Study Manual Sonic Conductance Equations
- 3
- 11/10/2018
- Moderator
- 2
- 11/11/2018
Thank you for your question. First, I would like to thank you for pointing out the ISO reference typo. This has been identified and will be reflected in the next revision to the PS study manual.
Regarding Eq. 3.37 and 3.38, you are correct in that those equations are based upon the equations referenced and listed within the ISO standard as it relates to sonic conductance “C” and the critical pressure ratio “b”. The equations given in the ISO standard are in terms of mass flow rate in either [grams/sec] or [kg/sec], and not in normal air flow rates of volume per minute or [Nlpm or Normal L/min]. The specific equation used in Eq. 3.37 comes from SMC Solenoid Valves Flow Characteristics document due to common terms used on pneumatic components used within the industry. SMC was very instrumental in the development of the Pneumatic Specialist (PS) study manual and therefore, the equation shown gives the more common units used within the pneumatic industry, versus a more generic mass flow rate. The common units for sonic conductance “C” are [Liters (dm3)/(s*bar)], while the flow rate (Q) is in [Normal L/min]. Because of this there is a conversion from seconds to minutes and bar to MPa because of the units commonly used or given. The 600 is derived from those conversion of units.
If you are using imperial or U.S. Customary units, it is recommended to be certain that all of the units are given as such, including pressures, temperatures, and values of sonic conductance. It may be of best interest to check the units of sonic conductance values you are using and then perhaps convert pressures and temperatures to the metric equivalence and use the equation as it stands. Just be careful to not mix units when working with alternate systems. This is why the particular equations referenced (3.37 and 3.38) are only in metric units only. It is to align with the common terms/units found for values of sonic conductance.
Regarding Eq. 3.37 and 3.38, you are correct in that those equations are based upon the equations referenced and listed within the ISO standard as it relates to sonic conductance “C” and the critical pressure ratio “b”. The equations given in the ISO standard are in terms of mass flow rate in either [grams/sec] or [kg/sec], and not in normal air flow rates of volume per minute or [Nlpm or Normal L/min]. The specific equation used in Eq. 3.37 comes from SMC Solenoid Valves Flow Characteristics document due to common terms used on pneumatic components used within the industry. SMC was very instrumental in the development of the Pneumatic Specialist (PS) study manual and therefore, the equation shown gives the more common units used within the pneumatic industry, versus a more generic mass flow rate. The common units for sonic conductance “C” are [Liters (dm3)/(s*bar)], while the flow rate (Q) is in [Normal L/min]. Because of this there is a conversion from seconds to minutes and bar to MPa because of the units commonly used or given. The 600 is derived from those conversion of units.
If you are using imperial or U.S. Customary units, it is recommended to be certain that all of the units are given as such, including pressures, temperatures, and values of sonic conductance. It may be of best interest to check the units of sonic conductance values you are using and then perhaps convert pressures and temperatures to the metric equivalence and use the equation as it stands. Just be careful to not mix units when working with alternate systems. This is why the particular equations referenced (3.37 and 3.38) are only in metric units only. It is to align with the common terms/units found for values of sonic conductance.