using your pressure offset for the velocity analysis (measurements with the 'high' pressure line disconnected)

A few astute students noticed that you may have a slight issue when you are finding your calculated velocity after you apply the differential pressure offset to your measurements. It is possible that you might get a negative measurement (obviously non-physical).

The issue boils down to the almost closed valve positions (low flow rates), because the differential pressure is so close to 0 with the high pressure line disconnected that you have to be much more accurate with your pressure reading. In reality, there is some variation in the offset with flow rate, but I only had all of you find one offset for all the data. If you think about it, even with the offset, the confidence you have in that measurement is probably fairly low because you are subtracting two relatively large numbers to try and arrive at a very small number (this means the uncertainty is significantly more than the average measurement!). I would note it in your report, and just not use these obviously non-physical values (which is a redundant measurement already), and state why you decided to remove them from the velocity calculation analysis.

Hey Josh,

Just as a sanity check, since I've spoken with students from other sections who have heard differently from their GSI, are we averaging our values with and without the second pressure sensor attached? I have heard that some sections were told to disregard the measurements taken without the second sensor on, but that our section is averaging the two values.

Is this correct?

Yeah, this measurement has been a real spur in many students' saddles, because the quality of the data was varied pretty widely from section to section, and even experimental setup to setup. The spirit of what the professors wanted the everyone to do was have both measurements and average them together to get the inlet velocity.

I have seen enough data at this point to convince me that doing this may result is some insane uncertainties. What I would recommend (and seeing as how I'm grading these reports, you should be safe in taking my advice) is if your calculated and measured velocities are quite different, just show me an example calculation and compare the two velocity values at some condition, and briefly explain why you decided to disregard the calculated velocity. So essentially it's your choice...you can use both values and average them together or just use the hot-wire anemometer measurement and explain why you didn't use the calculated inlet velocity based on a sample calculation. I am going to give people full credit for either to try and alleviate some stress in finishing this report.