Thermo EPD Mk2, Technical Handbook

Page: 63 / 115

EPD/HB/40521/000
ISSUE 5
EPD TECHNICAL HANDBOOK Page 53
4.6.5 Dose and Dose Rate
The EPD calculates dose and dose rate. Dose is accumulated in 3 stores: Dose ,
Total Dose and ADS Dose . Dose rate is the current rate calculated by the EPD.
Dose
The EPD displays Hp(10) (penetrating or deep) dose and Hp(0.07) (superficial or
shallow) dose.
Dose is regarded as a short-term record of dose received, usually
associated with the issue of an EPD. It is this value that is compared with dose
alarm thresholds and an alarm raised if the value exceeds the thresholds. The EPD
records
Dose, Total Dose and ADS Dose in units of 1/64µSv, although this is
displayed as a decimal equivalent. Total Dose is a store providing a long-term
record of dose accumulated by the EPD.
The LCD display can be set to auto-
range between 1
µ
Sv to 16Sv (maximum resolution 1
µ
Sv).
Display characteristics that can be set are:
Units Sv or rem
Decimal Places: 2 or 3 decimal places
Resolution:
µ
Sv or mSv
Default Time: 10 to 99 seconds
Note : For Gamma dose, two counts are approximately equivalent to 1/64
µ
Sv and
for Beta dose one count is approximately 1/6
µ
Sv. To avoid statistical
inaccuracies doses should only be displayed in micro sieverts (or 0.1 rem),
but when summing a number of doses the full resolution should be used to
minimize rounding errors.
Dose Rate
This is the current dose rate as calculated by the EPD. It may be viewed on the
display or read via the IR communications link. Dose rates are calculated using an
averaging algorithm to reduce statistical variations to within ±20% under most
conditions. This results in inc reased averaging time constants at low dose rates.
Averaging time constants used to calculate penetrating dose rate Hp(10)/h maximise
at 60 seconds below 70uSv/h and therefore the statistical errors increase as the
dose rate drops below 70uSv/h. Additional processing is used to provide faster
response but with reduced accuracy when rapid changes in dose rate are observed.
The algorithm is a compromise that reduces statistical errors but gives a fast
response time. The dose rate displays on the LCD are limited to 2 significant digits.
Longer time constants have to be used when averaging the beta dose due to the
lower sensitivity of the beta detectors. The time constant used maximises at 100