An optical setup that does not need in-line alignment of the object illumination and camera sensor, or requires a reflection mirror after the sample, is introduced to observe the optical phase in a heating liquid. This system uses an out-of-plane transmission digital holographic interferometer that uniformly adds speckle to the image holograms by means of a neutral phase screen. The way in which the liquid is illuminated allows having a variable magnification of the observed area of the sample. As a proof of principle, a thermal time-dependent distribution experiment is presented, whose resulting optical phase readily depicts the temperature-time-varying distribution in the liquid.