TA I
NSTRUMENTS
TGA 2950 4–3
Description
of the TGA 2950
The TGA 2950 operates on a null balance
principle. Physically attached to a taut-band
meter movement, the balance arm is maintained
in a horizontal reference position by an optically
actuated servo loop. When the balance is in a
null position, a flag located on top of the balance
arm blocks an equal amount of light to each of
the photodiodes (the light is supplied by a
constant current infrared LED). As sample
weight is lost or gained, the beam becomes
unbalanced, causing an unequal amount of light
to strike the photodiodes. The unbalanced
signal, called the error signal, is acted upon by
the control circuitry and reduced to zero, or
nulled. This is accomplished by an increase or
decrease in the current to the meter movement,
causing it to rotate back to its original position
(null position). The change in current necessary
to accomplish this task is directly proportional to
the change in mass of the sample. This current is
converted to the weight signal.
The TGA 2950 has two weight ranges: 1 gm and
100 mg. Both ranges are continuous over their
weight loss operating range, which means that
the entire weight loss range can be viewed
without any loss of information. The weight loss
operating ranges are:
• 1 gm to 0 µg for the 1 gm range
• 100 mg to 0.0 µg for the 100 mg range.
Description of the TGA 2950
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