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High density Polyethylene has a density of 0.95 - 0.96 grams /cc. (same as SG. Low density Polyethylene has a density of 0.916 -0.919. You can also use a melting stage (which is a little hot plate with a themometer attached. LDPE melts at 215 F. HDPE melts at 250F. Your problem with the LDPE rotocast tanks may be that they were using crosslinked LDPE which does not melt. You really can not weld that stuff.
An interesting way of accurately measuring SG on a small sample is to make up solutions of alcohol and water. Alchol has a lower density that water. What % ehtyl alchol = what density can be determine from googling it or using a hydrometer. If the part floats it is less dense less that the test solution. If it sinks it is more dense. If you use ethyl alcohol, you might celebrate by drinking the test solution.
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