The oil in the oil filled thermometer why white?

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 oil filled bimetallic thermometer, as shown in figure: silicone oil filling bimetallic thermometer works and ordinary bimetallic thermometer, actually it is just in a header into the shockproof silicone oil or glycerin, to minimize the effect of vibration. When use, sometimes we will find 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 or silicone oil 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.