The Isochronous Mass Spectrometry (IMS) is a powerful technique developed in heavy-ion storage rings for measuring masses of very short-lived exotic nuclei. The IMS is based on the isochronous setting of the ring. One of the main parameters of this setting is the transition energy $gamma_{t}$. %The transition energy $gamma_{t}$ plays an important role in the isochronous mass spectrometry (IMS). It has been a challenge to determine the $gamma_{t}$ and especially to monitor the variation of $gamma_{t}$ during experiments. In this paper we introduce a method to measure the $gamma_{t}$ online during IMS experiments by using the acquired experimental data. Furthermore, since the storage ring has (in our context) a relatively large momentum acceptance, the variation of the $gamma_{t}$ across the ring acceptance is a source of systematic uncertainty of measured masses. With the installation of two time-of-flight (TOF) detectors, the velocity of each stored ion and its revolution time are simultaneously available for the analysis. These quantities enabled us to determine the $gamma_{t}$ as a function of orbital length in the ring. The presented method is especially important for future IMS experiments planned at the new-generation storage ring facilities FAIR in Germany and HIAF in China.
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