jfholm wrote:

I have been following some threads on the sf-1020 and how their readings do not coincide with the earlier sf-600. Stef, you may have hit onto the issue. I did not know they had an adjustable odd shaped orifice. I have also read where the readings you get with the sf-1020 will not work with the formulas for finding other information from the airflow readings. I wish I could remember more detail about them.

Malvin and I have been talking about the cd of orifices and we both feel that the cd changes with test pressure and diameter of the orifice itself. in example a 1" orifice may have a cd of .587 and a 2" a cd of .597 and so on as you get larger. Malvin and I have both tried to photograph this phenomena but so far have not been successful. We are using a dye that shows up under UV or black light but so far have not had success. When one of us gets it to work we will post the results and videos.

John

John, you are correct. The Cd value is made up of a number of factors; beta ratio, pressure ratio (Reynolds number), plate thickness, roughness of the bore, eccentricity of the plate to the wall, pressure tapping positions, etc.

Orifice standards set out to control plate construction, placement, pressure tap positions, etc. allowing these factors to be included in the Cd calculation so that only the beta ratio, the Reynolds number (pressure ratio) and perhaps an expansion factor are the variables.

If you build a flow bench using an orifice in a pipe you must use an iterative calculation method to determine the flow rate. You start with a "standard" Cd value, calculate the flow rate and then determine the Reynolds number through the orifice. This is used to then calculate a Cd value. The flow rate is calculated again and the value compared to the previous calculation. The calculations are repeated until the variation between flow rates is a minimum. It normally takes only 2-4 iterations to compute the "exact" Cd value and flow rate and can be easily accomplished on a spreadsheet.

The good thing about building a flow bench with large plenums is that when the beta ratio is very small, under 0.125, the Cd value vs Reynolds number is very stable over a large range. According to BS ISO TR 15377:1998, section 5.3, it means that you can use a constant Cd value of 0.596 for the flow rate calculation. The uncertainty value of this is 1% so Cd could vary between 0.590 - 0.600.

As the orifice size goes up and thus the beta ratio, the more likely the Cd value will change with the pressure ratio/Reynolds number.

I think its best to calc the biggest orifice size that you think you would need for testing and then determine the size of the plenums based on limiting beta ratio to less than 0.125.