The Rego manual mainly focuses on large tanks, 500 gallons or so.  Here is
the relevant passage for our tanks:

DOT Cylinder Purging
1.  Exhaust to atmosphere any air pressure in the container
2.  Pressurize the cylinder to 15 psig propane vapor
3.  Exhaust vapor to atmosphere
4.  Repeat four more times

The pressurization with propane vapor to 15 psi requires special equipment.

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Mike S
Sent: Wednesday, October 03, 2012 4:58 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: propane tank question

On 10/3/2012 7:09 PM, Robert Mac Dowell wrote:
> Let me put it this way. I had all the valves and bungs out of my tank
> when I painted it. All this purging bull is just that,bull.

Which is, of course, why the National Propane Gas Association, which doesn't
know as much as you do about propane, publishes a bulletin covering the
proper procedure for purging, Bulletin #133-89 (a).

And, the idiots over at Rego, an international manufacturer of propane
equipment, who not as wise as yourself, say things like (
http://www.regoproducts.com/PDFs/LP-Gas_Servicemans_Manual.pdf ):

"The Importance of Purging
A very important step which must not be overlooked by LP-Gas distributors is
the importance of properly purging new LP-Gas containers. Attention to this
important procedure will promote customer satisfaction and greatly reduce
service calls on new installations. Consider the following:
. Both ASME and DOT specifications require hydrostatic testing of vessels
after fabrication. This is usually done with water.
. Before charging with propane, the vessel will contain the normal amount of
air.
...
Both water and air are contaminants
They seriously interfere with proper operation of the system and the
connected appliances. If not removed, they will result in costly service
calls and needless expense far exceeding the nominal cost of proper purging.
...
If the natural volume of atmosphere in the vessel is not removed before the
first fill, these problems will result:
. Installations made in spring and summer will experience excessive and
false container pressures. This will cause the safety relief valve to open,
blowing off the excess pressure.
. The air mixture present in the vapor space will be carried to the
appliances. This may result in as many as 5 or more service calls from pilot
light extinguishment.
. If a vapor return equalizing hose is not used, the contained air will be
compressed above the liquid level, resulting in slow filling.
. If a vapor equalizing hose is used, the air, and any moisture it contains,
will be transferred from the storage tank to the transport."