SARAD GmbH
16
Manual_Smart-Radon-Sensor_EN_12-03-2020.docx
Analogue output
4..20mA current loop
The Smart Radon Sensor provides an analogue output with current signal from 4 up to 20mA
(so called „current loop”). This feature offers a reliable way for transmitting the measurement signal
over long distances. The terminal ports for the current loop are designated in figure 2 and table 1.
At the end of each integration interval, current value will be calculated according to the Radon
activity and the chosen measurement range. The output signal therefore will be updated periodically.
From the beginning of the measurement till the end of the first measurement interval the output
current value equals 4mA.
The maximal current value of 20mA corresponds to the value of ten times the Radon alarm
threshold. In the case of threshold value of 300Bq/m
3
(default setting) the 20mA output current
represents the Radon value of 3000Bq/m
3
. For Radon activity below detection limit the 4mA current
value will be held. The output signal of the current loop is realized with 12-bit digital/analog
converter. As a result the resolution of the output signal is equal to 0,0039mA.
Example:
The current output value will be set according to a following calculation with a Radon threshold of
300Bq/m³ and the Radon measurement equal to 1125Bq/m³:
I
OUT
= { 1125Bq/m³ • ( 20mA – 4mA ) } / { 10 • 300Bq/m³ } + 4mA = 10mA.
20mA output signal will be present for all measurement results above Radon range defined for the
current loop (10
•
Radon alarm threshold). The relationship between Radon alarm threshold and the
output range of the analogue signal allows for easy adjustment to the input range of the data
acquisition system.
Warning: If Smart Radon Sensor is being integrated in existing network or measurement system, first
an apparent resistance has to be considered. The entire apparent ohmic resistance ought not to be
greater than 475Ohm for 12V supply voltage. For 24V supply voltage the apparent resistance must
not excide 1075Ohm.
