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Fly ash (FA) has potential applications in geotechnical and geoenvironmental projects, where its water retention characteristic curve (WRCC) is important. The main objective of this study was to establish the inherent range of measured suction of FAs using four different tools: tensiometer (TM), matric potential sensor, equitensiometer (EQT), and WP4 dew point potentiameter with a varying measurement range. Based on the measured results, the variability of WRCC equation parameters of four FAs collected from different thermal power plants were studied. Further, the sensitivity of a different range of measured suction on WRCC equation parameters of FA was investigated. The study indicated that the possible range of measurable suction for FA is 1 to 1000 kPa, which can be measured conveniently with an EQT. The parameters of FA were not influenced by high suction measurements (>1000 kPa) of WP4. The suction data (>1000 kPa) obtained using the WP4 can be appended with a low-suction-range instrument such as a TM for a better representation of the WRCC of FA. However, the study indicated that WRCC parameterization of FA is influenced more by the low range of suction.