12. Individual cells of an unsaturated group will have slightly different
values of EMF, so each cell must be measured in terms of a primary standard.
The manufacturer supplies, with each cell, a certificate specifying the
value of its EMF at a particular temperature.
The EMF will be slightly
higher than for the saturated type. New unsaturated cells ordinarily range
between 1.0190 and 1.0194 absolute volts.
13. The unsaturated cells, although less constant with age than the normal
cell, is still remarkably good.
Records, kept at one laboratory over a
period of 10 years, of a number of cells used by qualified personnel, showed
an average decline of about 0.003 percent (30 V) per year.
Statistics
based on the certification and recertification of many cells sent to the
National Bureau of Standards over a period of years give a much higher value
than the one above.
However, the conditions of use are unknown, and
undoubtedly range from careful use to abuse.
14. The internal resistance of a standard cell is very important. In the
unsaturated cell, it is normally between 80 and 500 ohms. If the internal
resistance increases, the electromotive force decreases when current is
being drawn.
In most instances, the internal resistance increases as the
cell becomes older.
15. There are several factors to be considered when you are using or
checking a standard cell. One of the most important of these factors is the
current drain which the load may cause. Since the current drain caused by
an ordinary voltmeter is detrimental to the cell, such an item should never
be used to determine the terminal voltage of a cell. If a cell is short-
circuited, its original equilibrium is permanently upset, because cadmium is
transferred from the amalgam to the electrolyte. If such a transfer occurs,
the cell cannot be depended upon for accurate measurements. Current up to
100 microamperes may be drawn from the cell for short periods without
permanently affecting the cell. Keep in mind that the current drawn from
the cell should be as small as possible, should be drawn for the shortest
period possible, and should never exceed 100 microamperes.
16. Excessive current drain (extending to accidental short-circuiting) is
the most common cause of deterioration of cells. When a cell is nearing the
end of its useful Life, the voltage will become increasingly unsteady.
A
cell's life depends, in addition to age, on the kind of treatment the cell
has received, so extreme caution should be exercised when using standard
cells.
Also, the cells should be mounted so they are not exposed to
extremes in temperature, drafts, or direct sunlight.
Learning Event 2:
DESCRIBE THE STANDARD CELL OPPOSITION MEASUREMENT TECHNIQUE
1.
You have learned that the standard cell is a very important standard in
the Army calibration system.
Not only is it a highly accurate DC voltage
standard used for calibration of other voltage standards, but it also provides
a link in the chain of traceability from the National Bureau of Standards to
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