Measuring urine S.G. using a refractometer
A refractometer is an instrument that measures the refractive index of a liquid. The more particles there are in a liquid the more a beam of light will be bent (refracted) as it passes from one medium to another e.g. from air to urine. The result is the formation of a shadow line between the illuminated and dark areas. The result is read from where this shadow line crosses the scale on the refractometer.
Components of a refractometer (refer to figure 1):
Measuring prism
Illuminator flap/cover
Eye piece
Bimetallic strip (placed internally)
Calibration screw
Scales
A traditional handheld refractometer consists of a measuring prism that you place a few drops of the liquid to be examined on with a flap to cover the sample. It has an eye piece that you look through as you hold the instrument up to a light source to obtain a reading.
The eye piece can be twisted to achieve a sharply focused image. This makes it easy to obtain an accurate reading of the result. Pressing the flap down with your finger may also help to obtain a clearer image.
There is an adjustable screw behind the stage. This moves the bimetallic strip and so allows the refractometer to be correctly calibrated with distilled water before it is used to test a urine (or plasma) sample. It is important to do this regularly, as the refractive index varies widely with the temperature of the environment.
Veterinary clinical refractometers typically have two or three scales (figure 2). The scale used to measure specific gravity is normally found on the right hand side and is typically labelled as U.G. (urine gravity) or S.G. (specific gravity) with a range of 1.000-1.030 or 1.000-1.040
