Perfluoro-2,4-dimethyl-3-isopropyl-3-pentyl is an extremely stable free radical due to six bulky CF₃ substituents protruding above and below the radical center like 'picket fences'. Two 'picket fences', each comprising three trifluoromethyl groups, protect the radical center against various active molecules. Due to inert nature of the radical it could be used as an internal standard for the electron spin resonance (ESR) measurement. The highly congested 'picket fence' geometry hinders the three C₃F₇ groups from rotating around the three bonds next to the radical center. As a result, the ESR spectrum has a beautiful hyperfine structure (Fig. 1). Since this hyperfine structure is significantly influenced by magnetic field homogeneity and/or modulation amplitude, we propose this radical as the indicator testing for both spectral resolution and sensitivity of the ESR spectrometer.