Improve table lookup to allow both blanks and units

I am trying to pull control limits for many different types of measurements from a single table. Some control limits do not have a high limit and others do not have a low limit but I do not want a blank to be recognized as zero because that is misleading. The only solution I am aware of currently is to assign the values in the table as strings but read them as singles at the attribute. Values that are not a number are then read as "No Data". Although, this creates another issue in that these strings that are read as singles cannot be assigned a source unit of measure and therefore the table lookup cannot convert the units automatically. Possible solutions could include: - allowing you to assign a source unit of measure to a value that is converted from a string - allow null values in tables for data types like single - a variant data type for tables