New gas hauler

I’m fairly new to it, 6 months, but I use a combo of sight glass, tapping the tank, and before/after veeder root. If all else fails, you can climb up top, pop the lid open and take a look inside.

 

sight glass and bucket

 

Drain compartment into a bucket…..

open the valve very carefully

 

Hopefully you have sight glasses. You can also tell by lifting the drain hose, and listening. If you concerned about every last drop out, don’t worry about it.

 

The tapping has never worked for me on anything other than dry bulk like frac sand.

Best method is final reconciliation using veeder root or stick, this step is critical since it shows that you didn't cross dump or have a lot of left over product. Kinda tricky if you have a lot of active fuelers like at Costco.

You gotta play around with your tractors air bags if you're on an angle, even if you do the bucket it could still have a lot of product that won't get out because of the angle. The bucket test only works on flat ground.

 

Many solid answers given....
Trailer angle can contribute to compartment retention, occasionally its terrible.
Being aware of that angle, checking sight glass, listening for the whoosh sound as the compartment drains, and finishing with a bucket draw down are all good answers.
I endeavor to keep the rear of my trailer lower than the front when delivering.
Then I really don't need to fuss with anything else.
I get a good drain, little retention, and the customer gets all their product delivered into their tanks.
I'll even pull onto a curb to raise that front end to get a good drain.

 

Back in my day we just listened for the whoosh and watched the sight glasses on the API head and the drop fittings. I think the newer Scullys have retain sensors in addition to overfill. Tapping on a liquid tank is just about worthless IMO, but feeling for the temperature change line (like you do on a propane tank) might work if the product is considerably cooler than ambient air.