Understanding and Controlling the Sources of Variability in pMDI Plume Temperature Measurements

Thermocouple-based measurements of pMDI plume temperature are used as in‑vitro descriptors relevant to patient-perceived cooling and device development; however, reproducibility can be compromised by uncontrolled ambient conditions. This study investigated major sources of variability in minimum plume temperature (Tmin) measurements by enclosing a custom plume temperature tester within an environmental chamber (ClimateZone). Plume temperatures were recorded using four centreline Type‑K thermocouples positioned at 25, 50, 75 and 100mm downstream of the actuator under a fixed sampling flow (28.3L/min). Three pMDIs (two commercial products and an HFA‑134a/ethanol placebo) were tested across a staged temperature-humidity matrix, with three replicates of six actuations per condition. A regression model quantified contributions of product, ambient temperature, RH and probe distance. Ambient temperature and probe distance were the dominant drivers of variation in Tmin, with RH exerting a secondary effect. At 25mm, increasing ambient temperature from 19°C to 40°C increased Tmin from ∼2°C to ∼15°C at 25%RH and from ∼3°C to ∼24°C at 70%RH. Increasing distance shifted Tmin upward, consistent with progressive mixing and thermal recovery. Under uncontrolled laboratory conditions, inter‑day variability exceeded intra‑day variability by ∼2-3 × at 25mm (IQR 1.518°C vs 0.488-0.716°C), motivating explicit environmental control. These findings demonstrate that plume temperature is a configuration‑dependent metrology endpoint that is confounded by ambient conditions; therefore, defined and reported environmental control is required for robust inter‑day and comparative plume temperature assessment. Model-based extrapolation to 37°C/100%RH is provided as hypothesis‑generating context and should not be interpreted as validated physiological prediction.