logarithmic equation

[dginbuffalo]

Hello,

I've been told that this is the way i need to calculate airflow from a DP transmitter. I am far from a math genius and trying to find out if this is even a valid equation? This is regarding air flow across an orifice plate and not the way I recall it is calculated. Any math wiz's out there? :site:

 

[Ken Moore]

You can only have SCFM if it's temperature compensated, otherwise it' just CFM.
I'm no expert, but It is my understanding that you are using DP to measure velocity. So the pipe diameter has to be in the calc somewhere. I have never done it your way.

 

[dginbuffalo]

You can only have SCFM if it's temperature compensated, otherwise it' just CFM.
I'm no expert, but It is my understanding that you are using DP to measure velocity. So the pipe diameter has to be in the calc somewhere. I have never done it your way.

Yes- The formula provided was from the orifice manufacturer I'm told. I know it must be temp.compensated for SCFM but it seems like I need pipe diameter, flow coefficient,etc to even get CFM. Maybe his formula works but I still have to figure out how to get that into an AB Micro 850 PLC!

 

[kamenges]

I can't say if the equation is correct or not. My biggest issue with this is that "y" is in milliamps and I have no idea what that equates to.

The only thing I can imagine is that this is a package deal. It is an orifice plate installed in a flanged pipe section with a specific DP installed across the orifice. If this is the case then everything you need is built into the constants in the equation.

Ultimately you are looking for CFM so you need to solve the equation for "x". You will know "y" since this is the input from the DP. The equation rearranges to:

x = (ln(y/3.125))/0.0019.

When I solved this for the 20mA case I came up with 977 CFM.

Keith

 

[dginbuffalo]

Ultimately you are looking for CFM so you need to solve the equation for "x". You will know "y" since this is the input from the DP. The equation rearranges to:

x = (ln(y/3.125))/0.0019.

When I solved this for the 20mA case I came up with 977 CFM.

Keith

Keith- I can't seem to get the same answer with your 20mA case? Can you explain this a bit? ALso- Here is a table of points and what the flow should be. Do these points work out with the re-arranged formula?

 

[Tom Jenkins]

This is surely a "one-off" equation for a specific orifice, pipe size, and dP transmitter scaling. The general equation for an orifice plate and dP transmitter includes the square root of the dP and many other terms.

https://www.engineeringtoolbox.com/orifice-nozzle-venturi-d_590.html

If you are using the system that that equation was developed for you are OK. You can convert the function to a polynomial by putting it into a spreadsheet, charting it, and using the trend function to get the equation. Use at least five significant digits in the equation. Or, you can use this, which I got by backsolving the original equation:

x= 526.315789 ∙ ln(0.3199488 ∙ y)

If all things are not the same as the original set up, you need to do some digging.

 

[dginbuffalo]

Thanks Tom! I think we may have it dialed in.