A new type of rainfall sensor (the intervalometer), which counts the arrival of raindrops at a piezo electric element, is implemented during the Tanzanian monsoon season alongside tipping bucket rain gauges and an impact disdrometer. The aim is to test the validity of the Poisson hypothesis underlying the estimation of rainfall rates using Marshall and Palmer’s (1948) exponential raindrop size distribution parameterisation. The latter is defined independently of the scale of observation and therefore implicitly assumes that rainfall is a homogeneous Poisson process. Our results show that 28.3 % of the total observed rainfall patches can reasonably be considered Poisson-distributed and that the main reasons for Poisson deviations of the remaining 71.7 % are non-compliance with the stationarity criterion (45.9 %), the presence of correlations between drop counts (7.0 %), particularly at higher arrival rates (ρ<sub><i>a</i></sub> > 500 m<sup>−2</sup>.s<sup>−1</sup>) and failing a chi squared goodness of fit test for a Poisson distribution (17.7 %). Our results show that whilst the Poisson hypothesis is likely not strictly true for rainfall that contributes most to the total rainfall amount it is quite useful in practice and may hold under certain rainfall conditions. Despite the non-compliance with the Poisson hypothesis, estimates of total rainfall amount over the entire observational period derived from disdrometer drop counts are within 2 % of co-located tipping bucket measurements. Uncorrected intervalometer estimates of total rainfall amount overestimate the co-located tipping bucket measurements by a factor of approximately 3. The overestimate is most likely due to poor calibration of the minimum detectable drop size (<i>D</i><sub>min</sub>). Intervalometer estimates of total rainfall when corrected for minimum drop size are within 1 % of co-located tipping bucket measurements. The intervalometer principle shows good potential for use as a rainfall measurement instrument and for determining rough estimates of mean drop size.