TJ, because Silvers and Norways are often lawn trees, they don't have to compete for sun and water etc. like forest trees. That can often result in pretty decent sugar levels in their sap. My soft maples are decent producers and never register below 2%. Yesterday's reading was the best yet. I think my trees were presenting a peace offering to me for jerking my chain for the past month and a half. ;)
I use a sap hydrometer that I purchased from ebay. Here's the link if you are interested. I love it.
http://www.ebay.ca/itm/0-10-Portable...item4d1a489414