The Hartmann & Braun dissolved oxygen meter for boiler feedwater measures and records the electric current generated by the action of dissolved oxygen on a voltaic cell. The calibration of this current in terms of dissolved oxygen by means of the ASTM referee chemical test is described. The instrument will record dissolved oxygen in three ranges, 0 to 0.03 ppm, 0 to 0.08 ppm, and 0 to 0.2 ppm. In the range 0 to 0.03, the meter checks the referee method within ±0.003 ppm. In the ranges 0 to 0.08 and 0 to 0.2 ppm dissolved oxygen, instrument values are within ±0.005 and ±0.01 ppm, repectively, of the referee procedure. The Hartmann & Braun meter requires about 7 1/2 min to respond completely to sudden surges of dissolved oxygen, and about 7 1/2 min to recover from such a surge. Temperature variation has a marked but reproducible effect on the dissolved oxygen record. Electrode fouling with corrosion products and leakage past the cathode gaskets affect sensitivity and reproducibility considerably.