We Build Underground Bomb Shelters

The Time-Averaging Method

The time-averaging method is used to compare the radiation levels between two or more locations in a bomb shelter when the radiation levels are climbing rapidly and when you have only one survey meter. If only two locations are to be compared and only a few second are needed to get from one location to another, the time-averaging method need not be used. The readings obtained at the two locations may be compared directly in that case.

The time-averaging method is a way to estimate what the approximate radiation levels WERE at several locations at ONE particular time. It consists of taking readings at different locations BEFORE and AFTER one particular time, then averaging those readings to get the reading at that particular time.

If only two locations are to be compared (locations 1 and 2), a reading is first taken at location 1. A short time later, a reading is taken at location 2. After another short period of time of EQUAL DURATION, whether 30 seconds or one or two minutes, a reading is taken at 1 again. The two readings taken at 1 are then averaged (add them and divide by two) and compared with the reading at 2.

If three locations are to be compared (locations 1, 2 and 3) with equal time intervals of say, one minute between readings, the readings are taken at locations 1, 2 and 3 and then at locations 2 and 1 again, IN THAT ORDER. The order of measurements, 1-2-3-2-1, must not be changed. The two measurements at 2 are taken ONE MINUTE BEFORE and ONE MINUTE AFTER the measurement at 3, the middle or CENTRAL measurement.

The two readings at 1 are averaged, and the two readings at 2 are averaged to give approximations of what the readings would have been at those locations at the same time that the reading at location 3 was taken.

To use the time-averaging method, you will need a wristwatch or clock that shows seconds as well as minutes. You should have an assistant to help you move quickly through crowds of people, watch the time, and help keep track of measurements.

Remember that the survey meter does not respond instantly to the radiation it is measuring when the range-selector switch is turned to “X0.1” You will need to allow a few seconds at each location for the needle on the meter to reach its final reading. Do not move, jiggle, or rotate the survey meter while the needle is settling down.

The survey meter should be held about three feet above the floor or at about waist level and about two feet away from the body when taking measurements. If you are taking measurements in a ground-level or below-ground bomb shelter full of people, it is important that all the people sit or lie on the floor while you take the measurement.

If people are standing, they will shield some of the gamma radiation from your instrument, and your survey meter will then show a lower reading than it would if people were sitting or lying down or if the room were empty. If you used this reading to compare with readings in other locations that are empty, you might conclude that the room with the people in it is safer, although it may actually be more hazardous.

If you plan to compare the readings at several locations, start the first reading where you think the reading should be the lowest, which should be where the people are located. Begin the readings 20 - 30 minutes after the needle reads about 0.1 R/hr in the safest location, after you have made your first rapid, spread-out survey.

If you start in another location, you may find that when you get to the estimated safest location, the radiation level may still not be high enough to read on the meter. You will then have to repeat the measurements later.

The 20- to 30-minute waiting period will allow time for enough fallout to settle on the ground so the readings will not be influenced much by radiation from falout particles still in the air.

You may wish to use this period to choose the exact locations where you will take measurements, mark these locations on your sketch and at the actual spot, and prepare a sheet of paper or a page in the RM log so your measurements can be written in the correct place when you take them.

You should have an assistant with you while you make these preparations so he or she will know what to do when you are taking the measurements. An example of the time-averaging method for comparing seven locations is shown in the Table below.

Table: An example of the use of the Time-Averaging Method

Room Location
Survey Meter
Time Reading (R/hr)
Before/After Before/After (Total) TIME-AVERAGE
Radiation Rate (R/hr)
(total divided by 2) COMMENTS
G 1 1040/1054 0.41/0.74 (1.15) 0.575 Lowest Rate
A 2 1041/1053 0.73/1.19 (1.92) 0.96 One-minute delay
B 3 1043/1051 0.69/0.95 (1.64) 0.82
C 4 1044/1050 1.01/1.29 (2.30) 1.15
F 5 1045/1049 1.32/1.55 (2.87) 1.435
E 6 1046/1048 0.79/0.86 (1.65) 0.825
D 7 1047 0.96 0.96 Central Measurement

* Table below shows where these locations are in the basement of the apartment floorplan sketch.

Note: This example results from an imaginary situation at the apartment floorplan sketch (earlier in this chapter) in which the time-averaging method is used to compare the radiation safety of various rooms when radiation levels are rising rapidly. The numbers are presented here as they might be entered by the RM in the RM log. The location numbers are entered on the sheet before starting. The columns marked “Before” under both the “Time” and the “Survey Meter Readings” are filled in from top to bottom as the measurements are made, and then the columns marked “After” are filled in from bottom to top. The numbers in parentheses in the column marked “Total” are obtained by adding the “Before” and “after” survey-meter readings at a location. The time-average radiation rate at a location, except for the central measurement, is obtained by dividing the total by two.

Figure: Locations of survey-meter readings (Plain View) for time-averaging in the basement of the apartment are shown by dots and are identified by numbers in circles.

The RM for the bomb shelter in the apartment, introduced in the discussion earlier, used the time-averaging method to compare the radiation safety of the seven rooms in the basement. The locations where the RM made the measurements are shown in the Locations Survey Measurement Figure above.

People were packed together in Room G, where the RM made the first and last readings. The choice of locations where readings were taken and the order in which they were taken was made before fallout arrived. In the example, fallout arrived at the apartment at 1009 hr.

Note: Twenty-four hour time is used to prevent confusion between AM and PM. This time notation is used by airlines and the military services. The first two digits indicate the hour of the day, starting with zero at midnight, and the second two digits indicate the minutes after the hour. The 24-hr time in the afternoon is obtained by adding 12 to the 12-hr time in the afternoon (hours past noon). Thus, 1:10 PM (ten minutes past one) becomes 1310 hr, 2:20 PM becomes 1420 hr, etc. See Appendix for a table to convert standard time to 24-hr time.

The first radiation reading inside the bomb shelter was made at location 1 at 1020 hr, as shown in the Figure. A rapid survey throughout the basement roughly confirmed that Room G provided the best radiation protection. It was decided the first series of detailed measurements for time-averaging would begin at 1040 hr.

The survey meter was brought to each designated location with enough time allowed to hold the meter in position for 10 - 15 seconds before the reading was taken. The first reading was taken at 1040 hr and the last at 1054 hr. Readings at location 1 were made seven minutes before and seven minutes after the central reading was taken at 1047 hr at location 7.

Readings at location 2 were made six minutes before and six minutes after the central reading at location 7, and so on. While moving from location 2 to location 3, the RM was delayed by a disturbance between some occupants of the bomb shelter, so the reading at location 8 was taken at 1043 hr instead of 1042 hr as initially planned.

In order to maintain the same time interval between the “before” and “after” readings at locations 1 and 2, the “after” readings at those locations were delayed a minute to 1054 hr and 1053 hr, respectively, instead of 1053 hr and 1052 hr as initially planned.

The two readings made at each location (except where the central reading was made) were added and divided by two to give an estimate of what the readings would have been at those locations at the same time the central reading was taken (1047 hr) at location 7. These time averages are listed at the bottom of the table (above).

From these readings, it was confirmed that Room G (location 1) provided the best radiation protection in the basement of the apartment.

Note: The readings at locations 1 and 2 almost doubled between the “before” and “after” readings.

Another series of measurements for time-averaging should be made as soon as practical, within 20 minutes after the first series, to confirm the results of the first series of measurements.

In the first series of measurements for time-averaging shows that there is an unoccupied area of the bomb shelter where the radiation levels are significantly lower, say 20 percent, than the area where the people are located, notify the bomb shelter Manager, and also inform him or her that you are going to make another series of measurements to check your results.

The bomb shelter Manager may wish to double-check your results. If your second series of measurements confirms the results of your first series, then the bomb shelter Manager will need to consider the possibility of moving bomb shelter occupants to this new location.

A number of factors should be taken into account before the decision is made to move or not to move. If the new location offers only a slight reduction (less than 20 percent) in radiation levels, a decision not to move may be made for several reasons, such as:

1. There may be less space, less desirable space, and/or not enough ventilation in the new location.

2. The location of the new space may result in higher radiation exposures to occupants while they walk to restrooms or to eating facilities.

3. Fire escape routes may not be as good.

If the new location offers substantially lower radiation levels, a decision to move may be made in spite of such shortcomings, especially if it appears that the radiation intensity may climb to such high levels that the accumulated exposure may result in radiation sickness.

Even if the current fallout is so light that radiation sickness is not likely, the bomb shelter Manager may decide that the occupants should move in order to be better prepared for the possibility of additional fallout from future attacks.

If a sudden squall or weather front with high winds and heavy rain strikes the bomb shelter while you are in the process of taking readings for time-averaging, you may need to disregard your measurements and wait until the weather settles down before you try the readings again. You may not be able to tell whether a decrease in reading from one room to another from the second room being safer or from a decrease in radiation level because fallout particles are temporarily being blown and washed away. The reading may change because of a combination of these two causes.

You should compare the radiation levels between the different areas at lest every 12 hours, or whenever anything takes place that might move the fallout particles around, such as a heavy rain or windstorm. After the fallout has stopped coming down and the rates are not changing rapidly, it won’t be necessary to use the time-averaging method for making these comparisons.