An improved strain-gauge device for continuous field measurement of stem and fruit diameter

An improved strain-gauge dendrometer was tested on apple (Pyrus malus L.) tree trunks and fruit to measure thickness dynamics. The sensor is similar to previous devices, but is more flexible allowing for a greater range of thickness change before it has to be physically reset. The practical resolution of the measurement system is given as one standard deviation about the mean of 51 observations taken on an Invar metal alloy in the field when temperatures ranged between 16°C and 17°C. The mean of these observations was 3.68356 mm with one standard deviation of 0.00947 mm. There is a positive correlation between temperature and sensor output having a slope of 2.96 μm °C−1. For a typical daily temperature change of 21 °C, this causes an apparent change of 62 μm. Stem thickness varied 0.7 mm diurnally with maximum values at night and minimum values during the day. Apple fruit diameter increased 3.3 mm over the 15 d period.