# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 3 Calorimetric Characterization
### Calibration Step and Measurement Accuracy  
Calibration is a crucial part of a DSC measurement and its purpose is to link the measured properties to the actual values [19; 27]. A reference material with a certified value is measured by a calorimetric method. There is a difference between the known value and the signal received by a sensor; this relation is called signal-value function [6]. The establishment of the signal-value function is called calibration. There are some uncertainties in every measurement due to the different sources of errors, such as the operator, the machine and test conditions. Achieving a high precision in calorimetric experiments means higher cost and longer time. Therefore, there should be always a balance between the acceptable uncertainty of each measurement and the devoted time and costs [22]. The temperature and amount of heat are the properties that must be calibrated in calorimetric measurements with a reference material. Furthermore, reference materials can be used to compare the measurements and for validation of instrument and method. Measuring materials with known values from the literature (not certified value) can be useful to ensure the validity of test conditions, especially when the suitable reference materials are not available. As mentioned earlier, the DSC machines, which supply heat electrically, do not need to be calibrated. However, these DSCs are not widely available and they cannot be used for cooling. Thus, this section is focused on the DSC operated by thermal resistance heating. The value of thermal resistance should be known to assess the measurements. This value is usually determined by calibration using standard materials with known values. The test conditions, such as atmosphere conditions, heating rate and type of crucible, should be exactly the same during the calibration and sample measurement [37]. The procedure of calibration is different for each DSC from different manufacturers. Hence, the instructions in the manual of the machine have to be considered. Most calibrants used in DSC measurements are metals due to the sharp transition peak around the melting point and enthalpies measured by AC. Indium is one of the most frequently used metals for temperature and enthalpy calibration, and it can be used many times [27; 38]. [PERSON] et al. [27] experimentally studied the calibration of a DSC machine by comparing a metallic reference (indium) with an organic compound. In order to have a better assessment, both materials are selected from the same temperature range (indium, 156.61 \({}^{\circ}\)C) and (diphenylacetic acid, 147.19 \({}^{\circ}\)C). The measurements were conducted for both operating modes (dynamic and stepwise). Furthermore, the influence of the sample form (solid or powder) and the pre-melting option on the measurement were studied. In the case of pre-melting, it was found that if the sample pellet is not thin or flat inside the crucible, the melting temperature decreases from the first cycle to the second one. Therefore, pre-melting the sample would increase the reliability of the measurements [27]. Regarding the form and state of calibrants, due to different storage conditions and the presence of oxygen, it is recommended to use solid samples. [PERSON] et al. [27] concluded that by calibrating DSC with indium, one can obtain accurate results on organic compounds in the same temperature range. Furthermore, indium can be re-melted at least twenty times without a considerable change in its properties and, due to the smaller heat capacity, leads to less uncertainty compared to diphenylacetic acid [27]. [PERSON] et al. [39] calibrated a DSC by high purity metals, including gallium, indium, and tin, considering various regression models from 0 th-order (single calibrant) to 2 nd-order (three calibrants). The temperature and enthalpy of paraffin (n-eicosane) as a PCM was measured to evaluate the accuracy after each calibration.