In this study, we examined the thermal stability of Relatherm HT-3 Heat Transfer Fluid versus five other liquid phase heat transfer fluids by comparing their vapor pressures at an elevated temperature. Of all samples tested, we found Relatherm HT-3 Heat Transfer Fluid to possess the lowest vapor pressure and by extension the highest thermal stability.

Figure 1.0 – Vapor Pressure of Heat Transfer Fluids obtained with ASTM D323 Test


We collected approximately 100ml/3-4 oz. of six common heat transfer fluid products. One of those virgin samples, Relatherm HT-3, was drawn from the inventory of Relatherm Heat Transfer Fluids. Virgin samples (approximately 100ml/3-4 oz. each) of five competitor heat transfer fluid products were obtained from end-users and retail distributors. The samples collected are as follows – Sample A, manufactured by a major Canadian lubricant manufacturer based in Ontario; Sample B, manufactured by a global energy company headquartered in Texas. Sample C, manufactured by a major Pennsylvania-based heat transfer fluid manufacturer; Samples D & E, manufactured by a global chemical manufacturer located in Tennessee.

All the samples were transferred into clearly labeled glass beakers.


We utilized the Knudsen Effusion Method, a dynamic gravimetric technique widely used for the determination of vapor pressures. 50ml of each heat transfer fluid sample were charged into a Vapor Pressure Analyzer (VPA) sequentially. The analyzer was used to measure the rate of escape of vapor molecules into a vacuum at 288OC/550oF through an orifice in the Knudsen cell. The rate of mass loss through the orifice was measured by the Surface Measurement Systems’ Ultra Balance within the VPA system. Vapor pressure readings were taken from the analyzer for each heat transfer fluid sample.


Relatherm HT-3 was found to have the lowest vapor pressure while Sample E had the highest vapor pressure. The vapor pressures obtained for the six heat transfer fluid samples are presented in Table 1.0 below.

Heat Transfer Fluid

Vapor Pressure (Bar)
Relatherm HT-30.055
Sample A0.069
Sample B0.14
Sample C0.17
Sample D0.32
Sample E0.35

Table 1.0 Heat Transfer Fluid Vapor Pressures



In this study, Relatherm HT-3 was found to possess the lowest vapor pressure  at the test temperature when compared with the popular brand name heat transfer fluids that were analyzed. Its formation of the lowest vapor fraction at a high temperature indicates a higher heat capacity and thermal stability.