If you declare a variable  -let's say an integer- The variable value will be
automatically set  to 0 until you change that in your code.
if you declare a variable as a string. its value will be ""
As you may see, you will check  the value of the integer with the following
dim i as integer
if i = 0 then .....
and the value of the string
dim str as string
if str = "" then ....

but you may as well use
if i = empty then...
if str = empty then ...

what I meant when I asked where do you initialize your PreLowVal variable is
that I did not see you giving it a value. So as per the above its value will be
0 (and of course not "").
As I already told you I did not follow this thread and I do not really know
what you are trying to do.