Originally Posted by Murf2u
So you're saying they don't weigh cylinders with OPD's to determine when they're full anymore?
I have found in Canada propane tanks are still weighed for "fullness". I have found in the US tanks are filled with volume, and the vent is opened until "propane" starts to release. The cost is based on liquid propane metered into the tank until the vent releases propane.
The icing on the external piping around the regulator in the OPs post is due to propane expanding as the pressure is reduced - it is an endothermic expansion of a gas, known as the Joule-Thompson expansion. Not Physics - but the laws of Thermodynamics.
As mentioned, it "sucks in" heat from surrounding materials/tubing. The metal gets cold and condenses the moisture in the high humidity air on the outside of the tubing/regulator creating the ice. When it is higher outside ambient temperatures, the air temperature is usually hot enough to compensate the outside metal temperature helping to prevent the formation of ice.
Turbo expanders in commercial gas plants use this principle to condense natural gas liquids (NGLs) - high pressure compression followed by a large pressure drop - the resultant cold mixture condenses butanes, propanes and sometimes ethanes. I have designed many of these facilities.
EDIT: Interesting story - I worked in Norman Wells, North West Territories in my earlier career, where the ambient temperature was -55C one day (-67F). I had a plant operator drain the lube oil from a propane chiller and he actually had liquid propane (at atmospheric pressure) in a bucket (the chiller was outside). He then brought the bucket into the maintenance shop and it started to "boil" as the temperature of the metal bucket started to raise about the -44C temperature and bubble point (boiling point) of liquid propane at atmospheric pressure. True story (but not relevant to the OP posting LOL).