This is part 3 of a three-part series covering Minimum Detectable Activity (MDA) time constants. It covers microprocessor controlled ratemeters.

Part 1: Determining MDA Time Constants - Linear Analog Ratemeters

Part 2: Determining MDA Time Constants - Logarithmic Ratemeters

This article addresses response times for microprocessor (µP) controlled ratemeters. A variable time constant is produced by the µP to keep the ratemeter readout within a prescribed standard deviation based on the count interaction rate. LMI models which incorporate the µP ratemeter response variables listed in the table below, are the Models 2350, 306, 2240, and 2241. Models 2221 and 2224 follow the tables to an extent. The Models 2240 and 2241 also incorporate a selectable fixed response time mode. The Model 2221 uses the Fast or Slow fixed response when the x1 - x1k ratemeter multipliers are selected (analog meter only). The standard Model 375 Area Monitor has a fixed time constant of 1 second when in the monitoring mode. When in the calibration mode, a variable time constant applies. Please note that certain variations of the Model 375 have different firmware versions and therefore may have different response times.

Obviously, it is not possible to present one or two tables of time constants that apply to all (or even most) LMI ratemeters under all conditions. For response specifications for a particular model, consult the appropriate instrument instruction manual or contact us.

The Time Constant variables listed in the table below are in seconds. Recall from Part 1 that "one time constant is the length of time required to reach 63% of the full charge or discharge"; therefore, it will take approximately 3 time constants for the ratemeter to reach its final value. Example: if the incoming counts are between 0 and 1.20k counts per minute (cpm), then the ratemeter time constant will equal 10 seconds for the fast mode. The ratemeter will reach the count rate final value in approximately 30 seconds. The majority of LMI instruments with digital ratemeters update the digital display every two seconds. The display update time is independent of the ratemeter response time; therefore, a ratemeter time constant of 1 second may require 4-6 seconds (2-3 display updates) before the final count rate value is reached.

Since the ratemeter uses a variable time constant, the rise time from zero may take longer than the predicted approximate 3 time constants. This longer time is due to the time constant starting out high and continuing to the desired time constant.

For example, if the incoming counts are 3000 cpm and the ratemeter is zeroed, the Model 2350 will start out with a time constant of 10 (Fast Response). As the incoming counts continue to accumulate, the time constant will be adjusted down to a time constant of 4. After the ratemeter settles at 3000 cpm and the counts change within ±300, then the ratemeter will respond as predicted (within approximately 3 time constants).