Radiation DetectorsCIRCUITRY AND CIRCUIT ELEMENTSPeriodMetersandStartupRateIn many applications it is essential to know the rate of change of power. This ratenormally increases or decreases exponentially with time. The time constant for thischange is referred to as the period. A period of five seconds means that the valuechanges by a factor of e (2.718) in five seconds. Figure 34 shows a basic period metercircuit.Figure 34 Period Meter CircuitPlacing the signal through an RC circuit causes a voltage that is proportional to thereciprocal 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 toan infinite period. As the ion chamber output current changes, there is a voltage transientacross 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 bythe operator to monitor the rate of change of power during startup, it is termed startuprate. Startup rate (SUR) equates to reactor period using Equation 6-10.(6-10)SUR26.06tRev. 0 Page 61 IC-06
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