Metria RST111, User Manual, Page: 5 / 12

5
Between the light-entering prism (1) and the refracting prism (2) there is a small gap, in which the liquid to be
measured is placed. When a light beam (natural light or incandescent light) enters the light-entering prism (1), a
diffuse reflection will be caused on its frosted surface; therefore, incident rays with various angles in the liquid to
be measured will, via the reflecting prism (2), form a light beam with reflection angles larger than the angle of
emergency.
The swinging reflecting mirror (3) guides the light beam into the Amici prisms unit (4), which consists of a pair of
Amici prisms, and plays a function of using a changeable dispersion to offset against the dispersion produced by
the refracting prism to different substance to be measured. Then the telescope objectives unit (5) images the
separatrix on the dividing scale (7), on which the graticule can be seen through the eyepiece (8), an image as
shown on the upper part of Figure 5.
lluminated by light beam via the condensing lens (12), the scale plate (11) links with the swinging
reflecting mirror (3) to form an integral whole, and gyrates around the graduation centre.
Through the reflecting mirror (10) and the reading lens (9), the parallel prism (6) images the indicating
values of the refractive indexes from different positions of the scale plate onto the dividing scale (7)
(referred to the image on the lower part of Figure 5)
4.2 Assembly (see figures 6 and 7)
The base (14) is a cradle for the instrument, on which the casing (17) is mounted. All optical components and the
main structure are enclosed in the casing except the eyepiece and prisms. The prism unit is mounted on the
casing, consisting of light-entering prism, refracting prism and prism base. Two prisms are fixed in the prism
holder with a special binding agent. (5) The light-entering prism holder and (11) the refracting prism holder,
linked by the rotation shaft (2) the light-entering prism can be opened or closed. When the two prism holders
are tightly closed and locked by the hand wheel (10), a uniform gap is left between the two surfaces of the
above prisms and the liquid to be measured should be filled in the gap. (3) the hood, (18) is the adapter
connector for four thermostats, (4) is the thermometer, and (13) is the thermometer holder, which can be
connected to the thermometer with an emulsive tube. (1) is the reflecting mirror, (8) is the eyepiece, (9) is the
cover plate, (15) is the refractive index graduation adjusting hand wheel, (6) is the dispersion adjusting hand
wheel, (7) is the dispersion graduation ring, and (12) is the condensing lens for illuminating the dial scale.
5. Operation
5.1 Preparation
5.1.1 Before performing the measurement, use distilled water or standard sample to check the readings. In case
of using standard sample, put one or two drops of naphthalene bromide onto the polished surface of the
refracting prism, then make it contact with the polished surface of the standard sample. When the indication of
the field of view is higher than the value of the standard sample, observe whether the separatrix is in the centre
of the graticule. If there is a deviation, slightly turn the screw inside the small hole (16) of Figure 7 with a screw
driver, making the objective to offset until the separatrix moves to the centre of the graticule. Repeat the
observation and correction until to minimize the indication of the initial error (including the collimating error of
the operator). After the completion of the correction, this location should be kept unchanged throughout the
process of measurement.
Generally, the instrument needs not to be checked in the routine measurement. In case of having doubted of the
indication of the measured refractive index, perform the checking procedure as specified in the above method
to make clear whether there is an initial error and make a correction if necessary.