The invention relates to a device for measuring the refractive index of a fluid, in particular intended for measuring the density of the fluid or the concentration of a substance dissolved in that fluid, comprising a rod which projects into the fluid and has a refractive index which is slightly larger than that of the fluid, which rod is provided, at the end not immersed in the fluid, with a light source which radiates into the rod and with a light detector, that part of the rod which is to be immersed in the fluid being partially surrounded by a casing which permits only total reflection of the light, while at the site of the part (or parts) of the rod not surrounded by said casing refracted light can, depending on the refractive index of the fluid, leak into the fluid.
A device of this type is known from Netherlands Patent Application No. 8500726.
In this known device the said casing consists of a reflecting gold layer. Contrary to expectations, it has been found that this gold layer is not resistant to the corrosive action of an aggressive fluid, such as concentrated sulphuric acid.
The aim of the invention is to overcome this drawback and for this purpose said casing consists of a layer, trapped in a transparent tube, of a medium which has a lower refractive index than said rod, in particular gas or vacuum.
Usually, a quartz tube will be used.
It is pointed out that German Offenlegungsschrift No. 3302089 discloses a device for measuring the refractive index of a fluid, said device comprising an optical fiber of small diameter to be immersed in the fluid, which fiber is provided, at the end not to be immersed in the fluid, with both a light source and a light detector. The angle of incidence of the light on the wall of the fiber will always be greater than the critical angle, so that the light can emerge only at the point of the part of the fiber which is to be immersed in the fluid. However, this point also serves to return some of the light by reflection. For this purpose, the point must be accurately ground, which is a costly operation. Moreover, the accuracy of the measurement will leave something to be desired because the light can leak away only over a very small part of the fiber, that is to say the ground point.