Exhaust manometer rises, then reverses

[Roverbeam]

IMG_6021.jpg

Hi, I built my bench a while ago, and it sat unused for a while as a worked on other projects. I'm now trying to get it sorted out and fully functioning.

It does a great job with intake, giving consistent numbers that closely align with this heads elsewhere-published flow.

When I switch to exhaust though, it behaves in a way I can't understand. As the static pressure builds, the flow manometer initially climbs "in step" as I would expect. But as I bring the flow up to 28", the manometer completely reverses and bubbles back into the fluid pot.

I've swapped the Clippard valve, rechecked the hose routing multiple times, checked for leaks, and I'm stumped!

30 second video shows the problem:
https://youtube.com/shorts/p9Wv7lUieEo

[saltfever]

Chad: For some reason your video clip is non-op. YT says it has been removed by the author. Have you used a thread or piece of cloth to see if the flow has actually stopped or reversed?

[Roverbeam]

I’ve got the exhaust valve mid way open, so I can feel strong flow from a couple feet away. I’ll check the yt link and make sure it works. Thanks for letting me know!

Edit, hope this works:
https://youtube.com/shorts/Q1UwbzBoeiQ?feature=share

[Roverbeam]

So I figured it out. The clamp-top jar I used for the manometer fluid reservoir is great in suction (intake testing) but didn't seal under pressure. It took me a while to think through what was happening - while the manometer is measuring a difference across the orifice, both of the inputs to the manometer are positive pressure when testing exhaust. It's easy to think that it's a positive/negative pressure situation - at least that's my excuse.

Here's my solution: add a clamp to the clamp-top jar...

IMG_6064.jpg

[Brucepts]

Sorry I totally missed your original post! Glad you got it sorted out!