Plot Digitizer

To be perfectly clear, Plot Digitizer is not a tool I developed.  Nor can I claim to have discovered it;  it was shared with me by my friends at Carbon Lighthouse. But it is so darn handy that I had to include a link to it and a plug for it.
And I should mention that this is not the only tool out there that lets you do this, it just happens to be the first one I ran into and the one I use.  

Plot Digitizer is a share-ware Java application that allows you to load an image into its working window (upper image) and digitize it by first clicking on the end points of the X an Y axis and then clicking on the curve or curves you want to capture.  The result is a little Comma Separated Value (CSV) file that you can then load into Excel to recreate the curve. 
 
That can be very powerful because not only does it allow you to take something and format to your liking for a presentation.  Once it is in Excel, you can do calculations with it.

For instance, starting with a single rpm curve relating flow and static pressure for a specific fan speed, you can calculate all of the other rpm curves and plot them by using the affinity laws. You can do a similar thing for the brake horse power line or for different impeller sizes if you are working with a pump curve instead of a fan curve.
 
Then, you can take things a step further by adding the system curve or curves for the system you are analyzing.  Or, you can extrapolate additional information from your data using Excel's trend line feature.

 For example, I created the chart in the lower image by having our Office Admin use Plot Digitizer to capture a speed and brake horsepower line from the Greenheck curve in the upper image and load the raw data into Excel.  Once Dawn had done that, she sent the file to me and I:

• Added the formatting I wanted
• Developed system curves for a number of operating conditions created by the filters loading in the constant volume system served by the fan
• Used the affinity laws to project the fan speeds and brake horse powers required to hold the flow steady as the filters loaded
• Used a curve fit from those data points to develop a mathematical relationship between fan speed and brake horsepower
• Programmed that relationship into the control system where we knew the fan speed via an input and wanted to estimate the power being consumed in real time.

Pretty powerful and a lot faster than hand reading the points off of a chart or graph to get them into Excel.  And its free!  For an example of a pump curve that was developed using Plot Digitizer that can also be use as a template for your own pump or fan curve, take a look at the sample and instructions I provided on the Plot Digitizer Pump Curve Example page.

​And I should mention that this is not the only tool out there that lets you do this, it just happens to be the first one I ran into and the one I use.  

• Automeris.io is a free web based application that you can run online to digitize a curve.
• Plot Digitizer (a different one from the one I mention above) has a free online version as well as a more full featured version that you can download and run locally on your machine.