This work presents the development of a solar thermal test loop to assess the performance of small-sized parabolic-trough collectors (PTC) under real outdoor conditions, as well as the results of a set of experiments aimed at comparing the testing conditions specified in the existing standards for solar thermal collectors (EN 12975-2:2006, ASTM E905-87:2013, SRCC 600 2014-17:2015 and ISO 9806:2017). The aim is to contribute to establishing the correct certification scheme for this specific type of collectors, which owing to their suitability for covering the thermal energy demand in the temperature range between 100 and 250 degrees C (mainly in the field of solar heat in industrial processes) are currently experiencing significant development both in terms of commercial options and operating projects. In general, an absence of consensus among the standards was detected. According to the results obtained in the specifically designed test loop, some alterations to the existing standards are issued: for example the correct sampling rate should be 5 s, the uncertainty required for the temperature difference is too restrictive, the maximum wind velocity imposed is difficult to fulfill, and variations permitted to solar irradiance and ambient temperature could be more constrictive. (C) 2018 Elsevier Ltd. All rights reserved.