The following two methods may be helpful in tests where it is necessary to mix the water sample with chemicals (usually
indicated by the instructions, "swirl to mix")(fig. 2-2).
Mixing in a Square Sample Cell: Grasp the neck of the sample cell(l) with the thumb and index finger of one
hand. Rest the bottom of the cell on the tip of the index finger on the other hand. Rotate the cell quickly, first
one way and then the other, to mix the samples.
Mixing in a Graduated Cylinder or a Flask: Grip the cylinder(2) (or flask) firmly with the tips of three fingers.
Hold the cylinder at a 45 degree angle and rotate the wrist several times as though stirring a pot. Reverse the
direction of rotation. Repeat a few times.
Figure 2-2. Mixing Solutions.
18.104.22.168. Using Graduated Cylinders.
General. There are two identical 25 ml graduated cylinders in the WQAS-PM. They are used to measure water
samples for tests. Only clean dry cylinders should be used for any test procedure. Before using a graduated
cylinder, rinse it two or three times with a small quantity of the fluid to be measured. After use the cylinders
should be cleaned thoroughly before returning them to the WQAS-PM.
Reading the Cylinder. The wall of the cylinder is
marked in millimeters. For accuracy, the level of the
liquid in the cylinder should be determined by reading
the meniscus at the top of the liquid level.
Reading the Meniscus (fig. 2-3). The top surface of the
water in a container or tube is not flat. Some of the
water is pulled-up against the inside walls of the tube.
This makes the surface concave. This curved water
surface inside the tube is called the meniscus(l). For
accurate measurements, the surface level must be read
at the bottom of the meniscus. To do this hold the flask
or cylinder so the water surface is at eye level. Hold it
up against a uniform bright background. (A good
technique is to hold a piece of white paper at a slight
angle behind cylinder to help highlight the meniscus.)
The meniscus will appear as a region at the top of the
fluid with a thickness equal to the amount the water is
pulled up the sides of the tube. Read the water level at
the bottom of this region.
Figure 2-3. Reading the Meniscus.