CIRCUITRY AND CIRCUIT ELEMENTS – Logarithmic Meters
Logarithmic Meters
Radiation detection circuit currents or pulse rates vary over a wide range of values. The current output of an ionization chamber may vary by 8 orders of magnitude. For example, the range may be from 10-13 amps to 10-5 amps. The most accurate method to display this range would be to utilize a linear current meter with several scales, and the capability to switch those scales. This is not practical. A single scale which covers the entire range of values is used. This scale is referred to as logarithmic.
The logarithmic output meter must be provided with a signal which is proportional to the logarithm of the input signal. This is easily done by using a diode when the input signal is from an ionization chamber. The voltage across the diode equals the logarithm of the current through the diode. Using this principle, the simplified circuit, shown in Figure 33, is used to convert ionization chamber current to a voltage proportional to the logarithm of this current.
Period Meters and Startup Rate
In many applications it is essential to know the rate of change of power. This rate normally increases or decreases exponentially with time. The time constant for this change is referred to as the period. A period of five seconds means that the value changes by a factor of e (2.718) in five seconds. Figure 34 shows a basic period meter circuit.
Placing the signal through an RC circuit causes a voltage that is proportional to the reciprocal of the period. If the current output from the ionization chamber is constant, no current flows through resistor R, and the output voltage is zero. This corresponds to an infinite period. As the ion chamber output current changes, there is a voltage transient
across capacitor C, and current flows through resistor R. The more rapid the transient, the greater the voltage drop across resistor R, and the shorter the period. Rate information is displayed on a meter in decades per minute, and since it is used by the operator to monitor the rate of change of power during startup, it is termed startup rate. Startup rate (SUR) equates to reactor period using Equation 6-10.
where
SUR = startup rate in decades per minute
26.06 = constant
t = reactor period in seconds
The reactor operator adjusts control rods so that an upper limit, such as 1 DPM, is not exceeded. This allows an orderly increase in reactor power.
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