Teledyne 328 Instruction Manual page 16

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4) Easy disposal.
5) Low cost.
The transducer functions as a fuel cell; in this instance, the fuel is oxy-
gen. Oxygen diffusing into the cell reacts chemically to produce an electrical
current that is proportional to the oxygen concentration in the gas phase
immediately adjacent to the cell's sensing surface.
Since the cell has a positive temperature coefficient, this variable must
be eliminated. The useful life of the cell is limited by the total amount of
current produced throughout its life. Taking these two facts into consider-
ation, together with a design goal of providing maximum accuracy, a combi-
nation of temperature compensation and temperature control is used.
Temperature compensation using thermistor circuits has been used from 30
to 125 °F (0 to 50 °C). This compensation alone will result in an accuracy
specification of ±2% of full scale or 5% of reading. By adding temperature
control starting at 25 °C (77 °F) and optimizing the thermistor compensa-
tion in the interval between 70 and 125 °F, accuracy has been increased to
±1%. Even greater accuracy could be achieved by using only temperature
control and controlling at 125 °F, but the cell life would be reduced by 50%
or more. Therefore, by temperature compensation and temperature control,
TBE has achieved reasonable accuracy consistent with long maintenance-
free operation.
2.3 Integral Cell Protection 2.3 Integral Cell Protection
2.3 Integral Cell Protection 2.3 Integral Cell Protection 2.3 Integral Cell Protection
An N-channel field effect transistor (FET) is incorporated in the cir-
cuitry across the cell to short circuit the cell whenever the power to the
instrument is off. The FET switches to a very high resistance instantly when
power is supplied to the analyzer, and is essentially no longer part of the
circuit.
The FET is necessary because of the characteristics of the cell and the
operational amplifier. When in operation, the amplifier input circuit looks
like a short of approximately one ohm to the cell. If the power were inter-
rupted without some means of short circuiting the cell, the amplifier would
appear as an open circuit to the cell. Since the cell is a current generating
device, these two circuit extremes (short circuit-open circuit) would neces-
sitate a prolonged period of equilibration. With the incorporation of the
FET in the cell circuit, the cell looks into essentially a short circuit at all
times and the instrument responds immediately when power is restored to
the circuit.
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Teledyne Analytical Instruments
Section 2 Section 2
Section 2 Section 2 Section 2