A 20% investigation criteria for pocket ion chambers exceeds the measurement capabilities of the instrument at low doses. Reference the ANSI standard for pocket dosimeter calibration.
Pocket dosimeters have many inherent errors. The typical calibration criteria is only +/- 20% at full scale. A 20% linearity/accuracy criteria is measured at the low and high calibration points (typically around 40 and 160 mR). Therefore acceptance criteria for the linearity/accuracy evaluation is 40 +/- 8 and 160 +/- 32. The typical scale for low range is 0-200mR in 10 mR increments. Many people try to guess in between the lines, but the dosimeter was never designed for that level of accuracy. At best, each reading is +/- 5 mR accuracy. This means that for low dose exposures <10, errors could be +/- 500% to 100%. These errors are compounded by paralax error issues as high as 10 mR with unskilled users. Drift issues exist if used for long times. As with any instrument, the degree of accuracy improves with a skilled user and higher dose. The total error term for a single low dose expos! ure is well over 20%. Repeated use under low exposure conditions compounds these errors to very high numbers. It is highly probable that any repetative, low exposure user would exceed 20% error.
Prior to the 80's and 90's (prior to electronic dosimeters), pocket dosimeters were used in power plants as a tool to assist workers accumulate daily dose estimates so that they did not exceed maximum exposure limits. TLD versus pocket dosimeter readings were accepted to contain high false positive results well in excess of 20% which were ignored as the accuracy of the detector. Only the TLD reads that had significant dose and significantly exceeded pocket dosimeter estimates warranted evaluation. Most plants have now switched to electronic dosimeters because of these pocket ion chamber limitations.