Due to the difficulty in determining polar compounds (PC) using the IUPAC method 2.507, simple and low cost devices for in situ determinations, such as Viscofrit, are commercialized for fried food processing establishments. This device is calibrated for oils but not for fats, such as beef tallow, since materials with a high melting point are rarely used worldwide. A large dispersion was observed in the measurements made with Viscofrit at 48 °C on thermoxidized beef tallow. The maximum emptying time was reached at about 6 hours of thermoxidation, at which point the fat still does not reach the maximum PC level allowed. In this test, a great dispersion in the data and a high percentage of “false values” were observed, which may be due to the presence of fatty solids at the measurement temperature, which affect the emptying time. In order to use Viscofrit for fats, such as beef tallow, it is suggested to perform a calibration at a temperature outside the range established by the manufacturer which ensures the absence of fatty solids, in this case 60 °C. Under these conditions, a maximum emptying time of 37 seconds was obtained.