Change the filters regularly. Dirty filters restrict air flow, reducing efficiency and worse case, can cause the evaporator to ice up. Disposable fiberglass filters should replaced. Electrostatic or electronic filters need to be washed regularly.
To increase accuracy, some woodworkers avoid using the zero end of a rule or tape and use the 1-in. mark instead. You can also use other starting points, such as 2 in. or 3 in. However, when using this technique, you must remember to subtract that number from the actual reading further down the rule.