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Equations in Action:

The Calibration Mystery

What does a Rocket Scientist do?

OK, this isn't James Bond exciting, but it shows the reality of engineering.

We needed to remove hydrogen leaking into a large room by using air jets to mix, dilute, and push out this explosive gas. To test for mixing, we intended to use a source of nonflammable helium at a known leak rate--if you don't know the leak rate, you can't tell how close to well-mixed you are. So we bought a flow meter to measure the helium leak rate, and we sent it to the calibration lab to compare against a precise standard.

Gas flow meters often display the flow rate for air with a correction for other gases, including helium. The flow meter documentation said, "multiply the meter reading by 1.454 for helium", but the calibration lab compared against both air and helium and said, "multiply the meter reading by 2.43 for helium". OMG! We don't know what's going on, we could fail the test, and management will not be impressed!

This is where the detective work began! No car chases! No gunfights! Just mathematics and science. Feeling sleepy yet?

We re-read the documentation; it checked out. Could the calibration lab have it wrong? They haven't done helium calibrations before. Suspicious!

We took their data and analyzed it in the spreadsheet below. Put your cursor over the numbers in the spreadsheet, it tells you what each number is. The precise calibration standard is gas flow through a small circular tube called a nozzle. An interesting fact about gases is that through the smallest part of the tube (called the nozzle throat), the gas speed is exactly the speed of sound (for high speed gases). Scientists and engineers find that fact very interesting! The spreadsheet uses this fact to calculate the flow rate in the nozzle throat.

The spreadsheet calculation below showed the calibration lab was right! We talked to the calibration lab. They said, "The flow meter documentation agrees with our calibration. See, right here in the manual. This manual came with the meter."

Before purchasing, we reviewed online documentation which had an old value! Mystery solved!

If you made it this far--without falling asleep--you have the makings of a engineer.

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