Hello! Help me on how to constraint a number of characters in a fields.Char? 

and number must be minimum of 12 and maximum of 15 characters

here's my models.py

bir_id = fields.Char(size=15)

def _check_length(self, cr, uid, ids, context=None): 
     for record in self.browse(cr, uid, ids):
         if record.bir_id < 12: 
         return False 
     return True 
_constraints = [(_check_length, 'Error', ['bir_id'])]

when i enter less than 12 characters, there's no error. What do you think is the problem? 

Thank you!