On this page:
http://www.pentairpool.com/pool-pro/...series-169.htm

I found this note:
"3: PLD50 requires more D.E. than the PLD70 because of precoat design and flow characteristics!"

I think that answers your question. BTW, thanks for the feedback on the signatures. I discovered that when I upgraded vBulletin version, the signature visibility of new members got turned off. It's going to take me awhile to fix that, but you should be able to see my signature now.

Ben