Do you know what caused the oil in the oil filled double metal thermometer white?

Oil filled the thermometer is in the ordinary course of the bimetallic thermometer header filled with silicone oil or glycerin, have the effect of shock absorption. Oil filled bimetallic thermometer mainly used in industrial equipment, some of the pressure vessels working pressure is higher, and using the environment vibration, this time ordinary bimetallic thermometer can't satisfy the use requirement, need to use silicone oil filling bimetallic temperature, as shown in figure: as we know, oil filled bimetallic thermometer works actually like ordinary bimetallic thermometer, it is just in the header into the shock of silicone oil, to reduce the influence of vibration. Occasionally when in use, we found that the oil in the thermometer pale, what reason is this? We made a summary, may be the following two main reasons: 1. Manufacturers use glycerin is too cheap, impurity is in the oil; 2. Oil filled the use circumstance temperature of the thermometer, undermining the oil internal composition, causing the oil physical properties change and the metamorphic pale.