6. Double Pan Balance
a. Typical construction (Figure 8).
(1) There are two double pan balances at the Secondary Reference level. One is
the Troemner 175-G and the other is the Voland 600. We will discuss the
Troemner 175-G in this lesson as the Voland 600 is very similar except it is
smaller in size and capability.
(2) The double pan balance is in reality an equal arm balance. That is, the arms of
the balance are made to be equal from the center point or Fulcrum to the knife
edges at the outer ends of the arm.
(3) The first items of interest are the knife edges. This is how the beam balance
scale gets its sensitivity. The knife edge at the top of the scale is the edge the arm
is balanced on. This edge is resting on a center plate which is flat and made of
agate. It is called knife edge because of its shape. Notice the "V" shape and how it
comes to a sharp edge. This shape reduces the friction of movement, thus
increasing the sensitivity and the accuracy. At each end of the beam you will
notice there is a weight tray or pan as it is normally called when working with the
balance. These pans are suspended on a knife edge also. The purpose of these
knife edges located at this point is the same as that of the knife edge in the center
of the beam: they reduce friction thus increasing the accuracy of the balance. Can
you imagine that the accuracy would be if the swivel points were a nut and bolt
type of construction? The movement would be restricted wouldn't it? This balance
is also precisely constructed; each part has been made for this balance alone. The
pans that are suspended from the beams are marked L and R for left and right and
they should not be interchanged. This aids in making up differences in weight of
each side of the balance. They are constructed to be as nearly equal in weight as
possible in order to achieve a high standard of accuracy in the balance. Another
important part of the balance is the pointer. This is the rod you see extending
down from the center of the balance arm and pointing toward a scale. This rod is
physically attached to the beam so that it will move as the beam moves. The
pointer will point to the lighter side of the balance if the weight is uneven. The
scale referred to is called the scale or the index. The index is graduated in
divisions starting at the center and marked off ten divisions to the left and ten
division to the right. This index increases the accuracy of measuring mass by
eliminating approximations in finding the rest point of the balance. There are a
few more items added to expedite the measurement and to prolong the life of the
knife edges. The pan lock steadies the pans when the arm is lowered. The pan
lock operated correctly will aid in preventing oscillations of the pans and the arm.
The equal arm balance as a whole is encased in glass. This case is for protection
against foreign matter and should be kept closed especially when taking readings,
since the slightest draft will keep the arms unbalanced and in motion. The equal
arm balance should be leveled and mounted on a solid surface. Equal arm
balances are manufactured in different sizes and ranges.
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