Did you know that you can get a device called a liquid withdraw valve and use a "wet leg" connection to fill your own 20 lb propane tanks from a larger tank yourself? So if you have a 500 lb tank for garage heating, you can use that tank to refill your grill tank. And much cheaper than getting your grill tank filled or exchanged.
the liquid withdraw valve is built on the tanks. all asme tanks 250 gallon and up have them, The liquid withdraw valve is not made to fill your 20# tanks, its put on larger tanks to "withdraw liquid" from the tank if there is an issue so you can empty the tank.
so, can you fill your 20# tank? yes..... but code is you have to fill by weight, so you can not over fill.... some will say BUT IT HAS AN OVER FILL PROTECTION VAVLE. yes it does, but they can fail and over fill your tank. and if you do not know that its over filled, well you just sent liquid into your grill.
the OPD on the tank is not built to use as a stopping device for filling tanks. its used an emergence device in case its over filled. This is why they have scales to weigh the tank when filling. does the employee use the scale? most of the time no.
and this is why they come to me when there 20# tanks are overfilled and we have to evacuate them to a safe level.
do it enough times and the tank will over fill every time because now you just wore out the valve.
So they sell the device to attach to the tank, yes. should it be added.... not really.
ppl do it all the time. they will continue to do it. no one can stop you... im just giving info, you can make your decision on what you want to do
safest way is to run a gas line to the grill from the tank.