TOPIC 2: Reading Results

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Survey meters generate electrical impulses.  These can be displayed on the survey meter in units of counts per minute (CPM), microroentgens per hour (µR/hr) or milliroentgens per hour (mR/hr).

If the survey meter is measuring the ionization in air from x-ray or gamma-ray … in a certain amount of time, It is providing us with “exposure rate”.  Exposure rates are always provided in units of roentgen per hour such as R/hr, mR/hr, or µR/hr.

To display the results in something usable for calculating dose we need our results in mR/hr.  The calibration facility interprets those pulses for X and gamma radiation only and provides a display in mR/hr or µR/hr. Survey meters that display only in CPM will have a conversion factor for mR/hr listed on the calibration sheet.

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The survey meter has an audio switch.  It is useful in the ON position when searching for a missing radioactive source or for contamination on equipment. An increase in the clicks from the meter indicates where radiation is located without having to constantly watch the needle movement.

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There is also a switch labeled F/S.  This indicates Fast and Slow.  In fast mode, the needle fluctuates quickly.  This is also helpful when searching for contamination or a lost source since a quick increase in the reading could indicate the missing source is in the direction the meter is pointing.  In Slow mode, the needle movement is average and slower.  Use this setting when writing down results for a survey.

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The Reset button takes all the voltage from the needle movement and allows the needle to settle to 0.  This is helpful if the needle to too far to the right and not responding fast enough when switching from one scale to another. If the needle appear stuck to the right, this verifies the needle has not been damaged and permanently locked to the right.

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Battery Check can be a push button, like Reset, or it can be a setting on the knob, like the one being shown.  When reading the needle location, be sure to know where the setting the knob is located as this is also critical information when determining the radiation field or if you are just in battery check.

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Cal stands for Calibration.  This is locked away so the user cannot make adjustments.  There is a potentiometer, or variable resistor, for each knob setting.  The potentiometer is adjusted to adjust the voltage which moves the needle.  When the detector is in a known radiation field, the potentiometer can be adjusted so the needle displays the correct exposure rate.

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Now that we know the different parts on a survey meter, how do we know what we are measuring?

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It doesn’t matter where you start, as long as you gather all the information.  Lets start at the top.  The needle is pointing to 2K which means 2000.

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Next we can look at the knob setting.  This is set at x10.

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So we have 20,000.  But 20,000 what?

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On the display we see COUNTS / MINUTE.

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Now its your turn.  What does this survey meter tell us?

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If you said 3,500 CPM, you were right.

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Let’s try one more.  What does this meter read?

Slow it down allow student to answer question.

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If you said 150,000 counts per minute, you were right.

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For those meters that are already calibrated in mR/hr, sometimes there is a slight discrepancy in the displays. Somewhere along the way of displaying the exposure rate, someone marked all the meters as “mR” and not “mR/hr.” Why is this the case? No one knows. But there are meters simply marked with “mR.” This should always be interpreted as “mR/hr.”

Knowing this, what does this survey meter display?

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If you said 0.35 mR/hr.  You are right.

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Survey meters are not very accurate.  They are within 20% of the correct reading. The humidity and temperature also affect the accuracy.  What they will tell you is that you are in either a higher exposure rate area or lower exposure rate area then the area you were just in.  They are very useful in letting you know if you are in a dangerous area though.

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For higher fields, with more ionization events, the way to limit the needle movement is to change the range.  As each higher scale is selected on the survey meter, the meter registers every tenth, or hundredth or thousandth count instead of every count. This limits the amount of charge on the capacitor that, in turn, limits how far the needle moves.

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For example, if the display shows 0-5 mR, the result should be the same on all scales but the needle will move as the scale is changed. Consider a result of 2.0 mR/hr:

SCALE  NEEDLE

X0.1  Off scale (this selection is too sensitive)

X1  2.0

X10  0.2  (very difficult to get an accurate reading)

X100  0.02 (impossible to see)

In this example, the best choice to read is the X1 scale.