This indicator checks exhaust gas ways within gas heatings:

The indicator reacts upon humidity in exhaust gas streams. In case the outlet is blocked, a repulsion proceeds to the control valve. The gas removal increases the atmospheric boiling point and causes water condensation. The indicator's probe reacts upon the condensated humidity of this repulsion and is indicated optically and acoustically on the indicator.

Type AP-2 (Exhaust gas tester).
Complete instrument with flexible probe,
9 V battery,
cable and
protective case.