Effective surface tension (gamma(eff)) is a fundamental property of liquid marbles, i.e., particle-covered drops in an air environment. Recently, studies of monolayer nanoparticle-covered (mNPc) liquid marbles produced with a sol-gel film have significantly updated previous understandings regarding gamma(eff). Examining the derived explanations with the most common powder-derived liquid marbles should be meaningful. Here, the maximum height and pendant drop methods were employed for gamma(eff) measurements of liquid marbles produced with a polytetrafluoroethylene powder. Following the methodology proposed in mNPc liquid marble studies, the measurement processes were addressed in detail with the uncertainties in parameter identification as the main concerns, and the results were analyzed by highlighting the effective surface tension's equivalence nature and liquid marbles' surface solidity. By combining the present findings and conclusions related to both mNPc and powder liquid marbles, the key issues in measuring and understanding liquid marble gamma(eff) were summarized. (C) 2020 Elsevier B.V. All rights reserved.