Apart from glass tube level gauges, transparent level gauges are always fitted with two plate transparentglasses between which the fluid is contained. The fluid level is indicated as the result of the differenttransparency of the two media and in some cases (for water steam), by conveying upwards on to the surfaceof separation (between liquid and gaseous substances) a source of light located at the back of the gauge,ÃÂÂ the rays of which are totally reflected down to the observer.