The friction factor, Eq. (2), can be determined by measuring the pressure drop (Δ*p*) across the test section, the density of the working fluid (*ρ*), the test section inner diameter (*D*), the pressure drop length of the test section (*L*), and the mass flow rate ($m\xb7$). Each of these measurands is accompanied with quantifiable uncertainty, Table 1. Using the description of Kline and McClintock [27] to express experimental uncertainty, and the procedure described by Moffat [28], the uncertainties associated with the measured friction factor results were estimated to be between ±5.5 and ±8.5%. Among the five measurands listed in Table 1, the uncertainty of the tube inside diameter contributed the most to the uncertainty of the friction factor. This is due to the friction factor that is expressed with the inner diameter (*D*) being raised to the fifth power, Eq. (2).