Sorting NUMBER BASE OR CHARATER BASE

in my database When sorting on (sample height) number - the sorting is on character basis and not on number basis.

the problem that the numbers are treated as text and not as numbers?

There must be a way to have numbers formatted as numbers I do hope.

Example

http://demo.asprunner.net/obiekwe_kth_se/P...sample_list.php

(the password- admin, username -admin

in trying to sort the "Max Sample HeightZ(mm)"

30 > okay

but 30 < wrong