1292 THERMAL EFFECT ON A POTTER BABY SCALE

Abstract

The objective of this study is to describe a significant source of error in the measurement of insensible weight loss (IWL), due to thermal effects on a Potter Scale. The scale was balanced and stable at room temperature. It was then placed in a pre-heated incubator, servo-controlled to maintain air temperature of 36°C. After 5 minutes, the scale began to register a large negative weight change. The maximum rate of this weight change was -15 g/hr at 15 minutes, and it was -7.5, -3.2, -0.9, -0.3 and -0.1 g/hr at 30, 60, 120, 180 and 240 minutes respectively. It was 5 hours before the scale was again stable. A total weight change of -10g was recorded. 49%, 73%, 91% and 94% of this weight change occurred by 30, 60, 120 and 150 minutes respectively. The temperature recorded from a site inside the scale rose 12.8°C. Hence, overall, there was a -0.78g weight change recorded for every °C rise in scale temperature. On cooling, the scale regained the registered weight loss. In vivo measurements of IWL in newborn babies confirm that erroneously large weight losses are recorded while the scale temperature is rising. The described bias due to heating will cause significant overestimations of IWL. The error will be magnified for smaller babies because they usually require higher incubator (and therefore scale) temperatures. In situations where IWL is measured with a Potter scale in an unstable thermal environment, temperature effects on the scale itself should be allowed for.