An automated real time underwater weighing system.

Hydrodensitometry generally involves up to 10 trials as the subject becomes familiarized with the underwater weighing and breathing apparatus. The final result is usually an average obtained from trials with the highest underwater weights. This study describes a new underwater weighing system based on automated data acquisition and real time display of data during underwater weighing and in-tank residual volume measurement. Results are immediately available after each trial, making possible the use of percent body fat as the criterion for selecting trials for averaging. It is proposed that percent fat is a more logical choice than maximum underwater weight, because for the use of the latter to be valid, true residual volume must be accurately determined. Accuracy of percent body fat requires only that corresponding values of underwater weight and lung volume be known, without the necessity for the weight to be maximum or the volume truly residual. This also reduces the number of trials, which in this study averaged 4.3 +/- 1.0, because each trial was potentially useful regardless of the magnitude of the underwater weight.