SARAD RTM1688-2 User manual

Manual
RTM 1688-2
Radon- and Thoron-Measurement equipment
Version 02/2023
Referenced documents:
Software manual Radon Vision
Software manual ROOMS 1.2.0
SARAD GmbH
Wiesbadener Straße 10
D-01159 Dresden
GERMANY
Internet: www.sarad.de
E-mail: [email protected]
Tel.: ++49 (0)351 / 6580712
FAX: ++49(0)351/6580718

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Content
Applications ............................................................................................................................................. 3
Short Description ..................................................................................................................................... 3
Important Hint ......................................................................................................................................... 4
Theory of Operation ................................................................................................................................ 4
Operating of the Instrument ................................................................................................................... 6
Power supply ....................................................................................................................................... 6
Input Filter ........................................................................................................................................... 7
Water ingress protection (optional) .................................................................................................... 7
Carrying out a measurement............................................................................................................... 8
Adjustment of the sampling interval ................................................................................................... 9
Operation Modes ................................................................................................................................ 9
Pump ............................................................................................................................................... 9
Alarm ............................................................................................................................................... 9
Fast/Slow Mode ............................................................................................................................... 9
Sniffing ............................................................................................................................................. 9
Data Handling .................................................................................................................................... 10
Data Storage .................................................................................................................................. 10
Printing a protocol ......................................................................................................................... 11
Data transfer.................................................................................................................................. 12
Version RTM1688-2 MODBUS ............................................................................................................... 13
Standards ........................................................................................................................................... 13
Bus timing .......................................................................................................................................... 13
Error management ............................................................................................................................ 13
Hardware ........................................................................................................................................... 13
Implemented MODBUS functions ..................................................................................................... 13
Statistical Error (for non-mathematicians) ............................................................................................ 14
Error Prediction ................................................................................................................................. 14
Detection Limit .................................................................................................................................. 16
Service ................................................................................................................................................... 17
Disposal instructions ............................................................................................................................. 18
Technical Data ....................................................................................................................................... 19
Scope of delivery: .................................................................................................................................. 20
Optional accessories: ............................................................................................................................. 20

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Applications
The universal radon and thoron monitor RTM 1688-2 can be used for the following
applications in geology, mineralogy, physics, chemistry, mining and construction:
• to determine the radon and thoron concentration in the air
• to determine the radon content in water samples and running water
• soil air measurements in construction projects and geophysical research
• localization of radon entry paths (sniffing) in buildings
• Emanation measurements
• Exhalation measurements
• As autonomous measuring station for monitoring buildings or systems
The above list of possible applications can also be expanded; the user can also use the RTM
1688-2 to solve other measurement tasks. You can find more detailed information on possible
areas of application in the section "Application Notes". If necessary, please also contact the
manufacturer's specialists for further information and advice on a possible solution to your
specific measurement task.
Short Description
The RTM 1688-2 is an absolutely universal radon / thoron monitor with which the entire range
of radon measurements can be covered. Its high sensitivity, combined with the measurement
method of alpha spectroscopy, allow the fastest possible measurements even with low radon
concentrations. The thoron concentration is determined at the same time.
Radon entry paths can be discovered by the “sniffing mode”. Soil gas sampling as well as radon
in water measurements are simple because of the built in pump.
The operation of the radon/thoron monitor is realised by only one button.
A serial printer may be connected to the interface of the RTM 1688-2 to present a protocol
directly on site. The radon/thoron monitor can be directly connected to a modem for remote
data transmission. The Radon Vision software (included in delivery) handles the remote
connection as simply as a direct cable link.
The measurement chamber is not sensitive against humidity variations; a drying tube,
commonly used in such instrument types, is not required.
Special attention was paid for the issue of quality assurance. Each stored data record contains
a complete alpha spectrum which shows the error-free operation of the instrument for each
single integration interval. Any number of measurement series may be created by

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starting/stopping the data acquisition. The data stored within the instrument can be read by
PC even if a measurement is in progress.
The small chamber volume of only 250 ml must be pointed out. The measurement of probes
taken from small and limited volumes is possible due to this property.
The radon/thoron monitor RTM 1688-2 can be powered either by a mains power adapter or
by the internal battery which gives an autonomous operation time up to 14 days.
The instrument is equipped with sensors for temperature, humidity and barometric pressure.
An integrated tilt detector will give a signal if the radon/thoron monitor has been removed
from its original position during the measurement.
The instrument comes with a calibration certificate issued by our DAkkS-accredited Radon
calibration laboratory. The calibration process fulfils the requirements of the DIN ISO EN
17025:2018.
Important Hint
The determination of the activity concentration of Radon is always a radiometric
measurement, meaning a counting experiment. This causes a number of specific
circumstances which have to take in consideration by the one who is carrying out this task.
Only the knowledge of those particularities allows the correct set-up of a test and avoids
misinterpretations of the achieved results.
Please read carefully the next chapters “Theory of Operation” and “Statistical Error” to
become familiar with this kind of radiometric measurements.
Theory of Operation
The Radon (Rn-222) gas concentration will be measured by the short living daughter products,
generated by the Radon decay inside a measurement chamber. Directly after the decay, the
remaining Po-218-nuclei become charged positively for a short period, because some shell
electrons are scattered away by the emitted alpha particle. Those ions are collected by the
electrical field forces on the surface of a semiconductor detector. The number of collected Po-
218 ions is proportional to the Radon gas concentration inside the chamber. Po-218 itself
decays with a half-life time of only 3.05 Minutes and about 50% (particles emitted towards
the detector surface) of all decays will be registered by the detector. The equilibrium between
the Radon decay rate and Po-218 detector activity is given after about 5 half-life times, say 15
Minutes. This time span defines the minimum achievable response time to a Radon
concentration step.
Now, the decay chain is continued by the both beta emitters Pb-214 and Bi-214 followed by
another alpha emitter, the Po-214. That means, each Po-218 decay causes one more
detectable decay by the Po-214 which is delayed about 3 hours because of the superposed

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half-life times of those nuclides. The emission energies of Po-218 and Po-214 are different and
therefore it is possible to separate both nuclides from each other by alpha spectroscopy. The
RTM1688-2 offers two calculation modes for the Radon concentration, one (Slow) includes
both, Po-218 and Po-214 decays and the other one includes Po-218 only (Fast). The advantage
of the “Fast” mode is the quick response to concentration changes while the “Slow” mode
gives the sensitivity twice as high compared with the fast mode. The higher sensitivity reduces
the statistical error of a measurement which depends on the number of counted decay events
only. The user should select the calculation mode carefully with respect to the application
specific requirements (see next chapter).
In case of Thoron (Rn-220), the direct daughter product Po-216 (which also underlies the
ionisation process) is used to calculate the Thoron activity concentration. The half-life of Po-
216 is less than 1s and therefore the equilibrium state between gas concentration and
collected activity on the detector is present immediately. The half-life of the Po-216 decay
products Pb-212 (beta) and Pb-212/Bi-212 (alpha) are too long to use them for Thoron
measurement. The single nuclides of the Thoron decay chain will be also separated by alpha
spectroscopy.

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Operating of the Instrument
Fig. 1: operation elements and connections
A
Display
F
External power supply
B
USB interface
G
RS 232 interface
C
Operation button (Toggle)
H
Air inlet
D Light I Air outlet
E
Connection water inlet protection
J
Dust filter
Power supply
Please insert the fuse at the backside of the instrument before using the instrument. The
instrument has no power switch because the power consumption of the electronic circuits is
less than the self-discharge of the battery. After inserting the fuse the instrument enters the
stand by status, the display will show “RTM 1688 SN:XXXXX”. It is necessary to set the real
time clock of the instrument in that case (see manual “Radon Vision”)
To maximise the capacitance and battery life time, the battery should be charged from time
to time (each 3 month) even if the instrument is not in use. Otherwise the battery may become
discharged deeply which can cause malfunction of the internal microprocessor. Remove the
fuse if battery maintenance is not guaranteed over long periods.
The instrument is powered either by the internal 12V/3.2Ah lead gel battery or by the AC/DC
wall adapter included in delivery. The battery allows an autonomous operation for more than
24 hours. The battery will be recharged as long as the AC/DC adapter is connected, the red
LED on the “LIGHT” button turns on, after a while the LED ” TOGGLE” is lighting, but it takes
about 8 hours for full charging in case of a totally discharged battery.
If the battery voltage drops below 11.8V, the display will show the phrase “LOW BATTERY!”
after finishing each integration interval. The measurement is still continued for several hours.
The pressing of the “TOGGLE” button leads back to the standard display output. If the battery
voltage decreases down to 11.2V the running sample will be stopped and the instrument
enters in the stand by status.

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Input Filter
The instrument is equipped with a high efficient multi-stage filter to prevent penetration of
daughter products into the measurement chamber. This filter will be protected against
contamination by an additional syringe filter fitted to the air inlet. Do not use the instrument
without that filter and replace it if required (becomes dark if loaded with dust). Take care for
the correct flow direction. Some filters can block the air flow completely when used in wrong
flow direction.
Water ingress protection (optional)
Please never measure radon in soil gases or in water samples without the stainless-steel
container connected to the water inlet protection! This prevents that water could enter into
the high-voltage chamber of the device. The penetration of water into the device can lead to
its failure and, as a result, to very expensive repairs!
SARAD GmbH offers to its customers a special stainless-steel container with floating contact
to prevent the penetration of water into the device when measuring radon in soil gases and
in water samples. For this purpose, a special stainless-steel container (see photo) is connected
to the air suction line of the radon measuring device between the soil air probe or water
sample and the measuring device and also connected with a plug. If water penetrates the
stainless-steel container, the pump and the air intake are automatically switched off, to
prevent the penetration of water into the high-voltage chamber and which results in a failure
of the device. To continue the measurement, pour out the water that has entered in the
stainless-steel container. The plug should be disconnected from the device and the stainless-
steel container should be removed for emptying.
In order to ensure the correct functioning of the water ingress protection system, the
stainless-steel container must be strictly in vertical position during the measurement!
If the device is used to measure radon in air, the supplied loose plug must be inserted instead
of connecting the special stainless-steel container. Otherwise the built-in pump would no
longer suck air.
fig 2: water ingress protection

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Carrying out a measurement
Press the push button to start a new measurement series. The pump starts and the display
will show the remaining time to complete the first integration interval.
RTM1688 SN:00001
Wait 120 Minutes
for first data!
The actual status and set-up information (see below) may be displayed by pressing the button
again.
If the first interval has been completed, five different display pages are available. The several
pages can be toggled by repeated pressing the push button. Depending on the selected system
of units, the concentration values are given either in ‘Bq/m³’ or ‘pCi/L’ (mbar/inHg, °C/°F)
The first page shows the actual Radon concentration (calculated for the last sampling interval)
with the statistical error for a 1 Sigma confidence interval. If “Fast” mode was selected, a
starlet is appended to the word “Radon” in the first row. Right beside, the time stamp is given
when the integration interval of the calculated concentration was finished
The bottom row contains at the left hand side the total number of integration intervals since
the last start of a measurement series. At the right hand side the pre-set integration interval
and the remaining time period of the actual sample is displayed.
RADON* 12:20
85Bq/m³±10%
#34 117/120Min
Page two gives the same information for Thoron (Rn-220)
Thoron 12:20
124Bq/m³±16%
#34 117/120Min
The readings of the additional sensors are shown at the third page. These values represent
averages which are derived from all “one Minute shots” of the entire integration interval.
Ambient 12:20
21.5°C 987mbar
46%rH 12.3V
The next page shows the average values of the Radon and Thoron concentration from the
start of the actual measurement series. The total sampling time is given in the first row.
Average 68.0Hrs
Rn: 314Bq/m³
Tn: 141Bq/m³

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The last page contains the status information, beginning with the date and time of the start of
the measurement series followed by the actual alert settings in the second line. The lower line
shows the selected pump and sniffing mode.
>>17.04.06 16:32
ALM: 250Bq/m³
CONT. SNIFF216
To finish a measurement series keep the push button down and wait for at least four beeps
from the buzzer. If the button is released, the sample will be stopped. If the button has been
locked by software, the button has to be unlocked before.
Adjustment of the sampling interval
The adjustment can be carried out as long the sampling is stopped. The “TOGGLE” button must
be hold down for at least 6 seconds (beeper). On the display appears:
INTERVAL: 1min
Now, the interval can be toggled by the button between 1, 5, 10, 15, 30, 60 and 120 minutes.
To accept the new setting, the button must be pressed again for at least 6 seconds.
Operation Modes
Pump
There are two different pump modes available, either continuous or interval pumping,
selectable by software. In continuous mode the pump is running during the whole
measurement period while the pump will switch off after the first five Minutes of each new
integration interval in case of interval mode. If the selected integration interval is less than
five Minutes the pump will run continuously too.
Alarm
If the measured Radon concentration exceeds the programmable alarm limit, the buzzer will
sound shortly each second. The alert has to be acknowledged by pressing the push button.
The alert check is performed after completion of each integration interval. If the alert is
enabled, “ALARM ON” will appear in the lower line of the status page.
Fast/Slow Mode
“Fast” and “Slow” mode will determine the kind of calculation of the Radon concentration.
Please refer to the chapters “Theory of Operation” and “Statistical Error”.
Sniffing
“Sniffing” means the search for radon entry paths, which are usually recognizable by locally
increased concentrations. Thoron can often be used as an indicator, since it is still available in
higher concentrations at such points (if any) and immediately decomposes in the measuring
chamber. In order to receive information as quickly as possible, the acoustic signal generator
can be set to "sniffing" mode. Each registered decay is indicated by a short signal.

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Depending on the setup setting, either only the decays of the thoron decay product Po-216 or
additionally those of the radon decay product Po-218 are signaled.
However, due to the relatively long half-life of about three minutes, the Po-218 is only suitable
for "sniffing" to a limited extent.
The procedure for the "sniffing" measurement would be as follows:
You should configure the device using the supplied software (Radonvision).
The interval can be set anywhere between 1 and 255 minutes via the software (only fixed
values can be selected on the device). An interval of 5 min can be selected for radon and 1
min for thoron. The configuration of the alarm can be determined. The pump mode should be
set to continuous. The measurement should be made for radon in the "fast" (FAST) mode
(display shows radon*).
The charging status of the battery should be checked (if possible >12V).
It should be noted that the pump only runs when the water ingress protection is plugged in
(or just the plug).
A sealing compound should be applied around the hose at the measuring point to prevent the
surrounding air from being sucked in.
The measurement can be started by pressing the toggle button.
If the measured values are below the limit to be detected, BDL (Below Detection Limit)
appears on the display.
Data Handling
Data Storage
All data are stored in a non-volatile memory using a circular architecture. That means, the last
2047 data records (data of last 2047 integration intervals) remain in the memory. Older data
will be overwritten if the memory exceeds the limit. Because the complete measurement data
are transferred during download to the PC, the memory should cleared after successful data
transmission and storage on hard disk. This will save time during the next transmission and
avoids redundant data storage.
Each data record is stored after completion of the integration interval and contains the full
information of this single integration interval:
• time stamp
• integration time
• alpha spectrum
• readings of additional sensors

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All sequential records with a time distance to the last record equal to the integration interval
are interpreted later as one measurement series. The measurement may be interrupted as
often as desired to finish the old and start a new measurement series. There is no limit for the
number of series. Single point measurements are also possible.
Printing a protocol
The RTM 1688-2 enables the direct printout of acquired data on the portable protocol printer.
Only the last acquired measurement series can be printed. Connect the printer to the RTM
1688-2 by the adapter cable (9-pin SUB-D male to 3-pin round connector) included in the
delivery. To switch on the printer press the right button at the panel. The LED is constant,
green. Note: after about 30 seconds, the printer switches in the sleep mode if no print job is
carried out. To start a protocol print, the instrument has to set in stand-by mode - stop the
measurement in case of a running sample. Start the printing out by pressing the “TOGGLE”
button for at least 5 Seconds (wait for four beeps). During printing out the display will show
the phrase “...print protocol”. If a new series was started unintended, the measurement may
be stopped again before completing the first interval. The print data remain valid as long.
The protocol header contains the serial number of the instrument, the start and the end of a
measurement series, the Radon and Thoron average concentrations and the time period used
for averaging (between start and end of the test). The header is followed by the records of the
measurement series containing time stamp, Radon and Thoron concentration and
environmental sensor readings.
_________________________
RTM1688 SN:XXXXX
Begin 06/12/23 12:34
End 06/12/24 17:34
Average 12.2Hrs
Radon 1234567Bq/m³
Thoron 1234567Bq/m³
_________________________
04/27/06 15:32
Radon 1234567Bq/m³±12%
Thoron 1234567Bq/m³±12%
25.5°C 56%rH 1002mbar
... more records
__________________________

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The printer is supplied by an internal rechargeable battery which can re-charged using a mains
power adapter. It takes about 15 hours to complete the charging process (full charged)
depending on manufacturer. During charging the LED lights orange and blinks short green. A
discharged battery is indicated by a short red blinking LED. The LED blinks also red in case
other problems.
To insert a new paper spool, the paper compartment can be opened by sliding the Lid Release
Button forward until the lid springs open. Unwind a small amount of paper from the roll and
insert the paper roll into the printer. Close the lid down, and the paper is loaded. For more
detailed information please look at the printer’s operation manual. Please follow always the
Manufacturers’ instructons!
Data transfer
The acquired measurement data can be downloaded to a PC using the USB or serial interface.
The easiest way to get data downloaded is to use USB interface on the front panel of the
monitor and USB cable supplied. Another possibility is to connect the PC interface cable (9-
pin SUB-D female to 3-pin round male) to both, instrument and PC COM port. If the PC is not
equipped with a standard COM port a USB to serial converter can be used. For further
information read the software manual.

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Version RTM1688-2 MODBUS
The MODBUS protocol offers a communication option in addition to the SARAD standard protocol. The
protocol type can – depending on the type of instrument – be selected either by jumper, switch or
menu. The MODBUS protocol implements only a part of the interface functionality and has been
implemented primarily for the cyclic reading of current measuring results. The adjustment of the
configuration parameters as well as the download of time distributions stored in the instrument is not
possible.
Standards
 MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b3
 MODBUS over serial line specification and implementation guide V1.02
 www.modbus.org
With respect to the standard following communication parameter are defined:
Baud rate: 9600bps or 19200bps (configurable)
Data format: 1 start bit, 8 data bits, 1 parity bit, 1 stop bit (total 11 bit)
Parity: even
Address range: 1…255 (configurable)
Bus timing
9600 bps
19200 bps
Min. period
between two frames (t3.5)
4.025ms
2.01ms
Max. period between two bytes within a frame (t1.5)
1.75ms
0.862ms
Response time
< 1s
< 1s
Error management
Incomplete frames or frames with invalid check sums will be ignored and result in a client time out.
Invalid or not supported function codes, register addresses and data length settings are responded by
the related exception codes:
 Invalid function: Code 0x01
 Invalid address: Code 0x02
 Invalid number of registers: Code 0x03
Hardware
 Instruments with native RS-485 interface (full bus functionality)
 Instruments with RS-232 interface with RS-232/RS-485 converter (full bus functionality)
 Instruments with RS-232 interface or internal USB/UART converter (point to point connection
without bus functionality – for example host which handles transmission over virtual (COM
port)
 Instrument specific implementation of the bus functions
Implemented MODBUS functions
Function code 0x03 (read holding register)

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Valid register addresses are:
Register
Address
Register content
Number of
registers
Format
0x0000
Radon concentration [Bq/m³]
2
Float
0x0002
Statistical error of Radon concentration [%]
2
Float
0x0004
Average Radon concentration since last start [Bq/m³]
2
Float
0x0006
Battery voltage [V]
2
Float
0x0008
Temperature
[°C]
2
Float
0x000A
Relative humidity [%]
2
Float
0x000C
barometric pressure [mbar]
2
Float
0x000E
Thoron concentration [Bq/m³]
2
Float
0x0010
Statistical error of Thoron concentration [%]
2
Float
0x0012
Average Thoron concentration since last start
[Bq/m³]
2
Float
IEEE 745 float values (4 Byte) are transmitted as two sequential 16 bit registers. The number of
registers to be read must be two. That means, only one value can be transmitted per frame. Other
values and not stated register addresses will cause an exception response.
Bus Settings:
 Address by INIT software
 Transfer protocol by push button menu at instrument
Statistical Error (for non-mathematicians)
The radioactive decay is a statistical process. That means, even if the Radon concentration is
constant over the time, the number of decays N counted within several intervals of the same
period will be different. N will vary around the mean value of all considered intervals.
Considering an infinite number of intervals would lead to an average which one indicates the
“true” result of N. For a single interval, the value of N will be either below or above the “true”
value. This observed deviation is covered by the term “Statistical Error”.
Therefore, each serious measurement contains beside the calculated Radon value the error
band for a stated confidence interval. The commonly used confidence intervals are 1, 2 or 3
Sigma () which refer to a likelihood of 68.3%, 95.45% and 99.73%.
For example, the correct interpretation of a measured Radon concentration of 780 Bq/m³ with
a statistical 1error of ±15% is:
The real “true” Radon concentration lies with a likelihood of 68.3% within the range from 663
Bq/m³ (780 Bq/m³ - 15%) to 897 Bq/m³ (780 Bq/m³ + 15%).
Error Prediction
The relative statistical error E for a chosen confidence interval of k-Sigma can be predicted
from the number of detected counts N by the equation:
E[%] = 100% * k * (N) / N

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The simple consequence is: The higher the number of counts the higher is the accuracy of the
measurement. From the opposite point of view one could ask: How many counts I have to
detect to achieve a predefined uncertainty?
Two items will affect the number of counted decays: The sensitivity of the instrument at the
one hand side and the time period used for counting process (integration interval) on the
other hand.
While the sensitivity is an instrument specific constant, the integration interval may be
expanded to the maximum acceptable value for the desired time resolution of a measurement
series.
The relationship between the measured Radon concentration CRn and the number of counts
N within an integration interval T is:
CRn = N / (T * S)
whereby S represents the Sensitivity of the instrument, given in the unit [cts/(min*kBq/m³)].
The sensitivity using the slow mode is double as high as in the fast mode (see chapter “Theory
of Operation”) and whenever the required response time is more than 2 hours the slow mode
should be selected.
For the following examples a fast mode sensitivity of 4 cts/(min*kBq/m³) shall assumed while
the slow mode sensitivity shall be 8 cts/(min*kBq/m³).
The first question could be: Which integration interval T has to set to get a statistical
uncertainty less than 10% at a confidence level of 1if the expected Radon concentration is
200 Bq/m³?
A 1error of 10% requires 100 counts (100%* 1 * (100) / 100 = 10%). Using the fast mode,
the integration interval can be calculated by
T(fast) = N / (CRn * S) = 100 cts / (0.2 kBq/m³ * 4 cts/(min*kBq/m³) = 125 min.
Because the required interval is longer than 2 hours, the slow mode is the better choice,
leading to the following result:
T(slow) = N / (CRn * S) = 100 cts / (0.2 kBq/m³ * 8 cts/(min*kBq/m³) = 62.5 min.
That looks pretty but makes no sense because of the longer response time. So we will set the
interval to 120 Minutes and ask for the statistical error in this case:
N(slow) = CRn * T * S = 0.2 kBq/m³ * 120 min * 8 cts/(min*Bq/m³) = 192 cts E(1) = 100 % * 1
* (N) / N = 100 % * 1 * (192) / 192 = 7.22 %
Now one could say 68.3% is not sure enough, I want to choose 2confidence interval to get
a more trustable result:

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E(2) = 100 % * 2 * (N) / N = 100 % * 2 * (192) / 192 = 14.44 %
For interpretation look at begin of this chapter.
Is an observed concentration change statistical significant or not?
If you have a look at the acquired time distribution you will see variations of the concentration
from point to point. The question is now: Is it a real change in the Radon concentration or only
a statistical fluctuation?
The test is very simple: Define a confidence level with respect to your needs and look at the
statistical error bands of the two points of interest. If the error bands do not overlap each
other, the change in the Radon concentration is significant otherwise it “can be or not can be”.
Example 1:
Reading 1: 1500 Bq/m³ ±10%  error band [1350 ... 1650 Bq/m³]
Reading 2: 1300 Bq/m³ ±13%  error band [1131 ... 1469 Bq/m³]
The upper limit of the error band of the reading 2 is higher than the lower limit of the error
band of reading 1. Because the “true” value could be placed within 1350 Bq/m³ and 1469
Bq/m³, the variation of both readings is not statistical significant.
Example 2:
Reading 1: 1500 Bq/m³ 10% error band [1350 ... 1650 Bq/m³]
Reading 2: 1000 Bq/m³ 15% error band [850 ... 1150 Bq/m³]
The error bands of the readings do not overlap each other. Therefore, a statistical significant
concentration change is given.
Two arbitrary points of a measurement series may be considered using this test. It is not
necessary that the points are direct neighbours.
Detection Limit
The term Detection Limit defines the smallest value of the Radon concentration which delivers
a non-zero reading of the instrument within a given integration interval (at least 1 decay per
interval). Because of the statistical behaviour a related confidence interval has to be stated.
Why is it necessary to know the Detection Limit? If the set integration interval is short and the
Radon concentration low, the expected “true” value of the number of detected decays may
be around or less than 1. Because of the statistical variations, intervals without any detected
decay will appear frequently. The most extreme situation would be a measurement series with
a lot of “zero” intervals and only one interval with one detected decay (because a decay
cannot be split).

RTM 1688-2
 SARAD GmbH 2023 Manual_RTM1688-2_EN_24-02-2023.docx 17
When calculating the Radon concentration by the given formula, the concentration value for
the interval with the one count is much too high while all other values show zero. Then, all
intervals have to be averaged to get a usable result. This procedure is nothing else than to
create an integration interval long enough to meet the Detection limit for the applied Radon
concentration. To avoid zero readings, set the integration interval with respect to the lowest
expected concentration level during measurement.
The mean („true“) value of the number of decays during an integration interval in case of a
Radon concentration in the surrounding of the detection limit is less than 16 and therefore
the statistical fluctuations have to be derived by the Poisson distribution. The stated
confidence interval gives the probability that the detected number of decays within the
interval is not zero.
Confidence Interval Required Mean Value for N at the Detection Limit
63,2% 1
95% 3
99,75% 6
Example:
Determination of the detection limit of the Monitor using the „Fast-Mode“ and an integration
interval of 60 Minutes. The confidence interval shall be 95% (that means in about 95 from 100
intervals a no zero reading should appear):
Required mean value (number of counts from the table): N = 3. Calculating the detection limit
by the formula:
C = N / (T * S) = 3 cts / (60 min * 8 cts/(min*kBq/m³)) = 0.00625 kBq/m³ = 6.25 Bq/m³ The
detection limit in this case is 6.25 Bq/m³.
Service
We offer a comprehensive repair, maintenance and calibration service for all our products.
Please fill out the form on our homepage (SARAD - Service) BEFORE SENDING A DEVICE and
send it to us. We need the device type and serial number as well as a short description of the
error that occurred or the expected service. You will then receive an initial cost estimate from
us as well as information on how to proceed.
The device check and the preparation of the cost estimate for repairs after the warranty has
expired are subject to a fee.

RTM 1688-2
18 Manual_RTM1688-2_EN_24-02-2023.docx  SARAD GmbH 2023
To use the calibration service, please also go to our homepage ( SARAD ) and fill out the
appropriate form.
Disposal instructions
Batteries and accumulators must not be disposed of in the garbage, but you are legally
obligated to return them to the appropriate waste collection centres. The measuring
instruments must be disposed of in the electronic waste or handed to the manufacturer at the
end of their service life for proper disposal. If necessary, they have to be decontaminated
before.

RTM 1688-2
 SARAD GmbH 2023 Manual_RTM1688-2_EN_24-02-2023.docx 19
Technical Data
Measurement Range
Radon/Thoron 1 Bq/m³ … 10 MBq/m³
Temperature -20 °C … 40 °C
Humidity 0 … 100 %
Bar. Pressure 800 mbar … 1200 mbar
Response time (95%)
Radon fast 15 Minutes
Radon slow 120 Minutes
Thoron 1 Minute
Sensitivity (fast/slow) 3 / 6.5 cts/(min*kBq/m³)
Measurement accuracy 7%/5% @1kbq/m3; 1h
Sample Interval 1 … 255 Minutes (adjustable)
Memory 2047 data records (circular structure)
Pump rate 0.3 l/min
Internal volume (chamber/air loop) approx. 250 ml
Power supply
Battery operation > 160h (continuous pumping)
> 10 days (pump in interval mode)
Recharge Time 8 Hours
AC/DC adapter 100-240V/AC, 50/60Hz, 18V/DC1A
User interface Display 3 x 16, 2 push buttons, buzzer
Interface USB, RS232, RS485 (optional)
Protocols SARAD proprietary
MODBUS RTU (9600/19200bps)
ASCII Protocol print
Dimensions 232 mm x 182 mm x 135 mm
Environmental condition
Temperature 5 °C ... 40 °C
rel. humidity 0 … 95%, not condensing
bar. pressure 800...1100 hPa
Weight , net 3.5 kg
Tamper detection if instrument is moved > 8 Seconds

RTM 1688-2
20 Manual_RTM1688-2_EN_24-02-2023.docx  SARAD GmbH 2023
Scope of delivery:
Article no. Name Quantity Remarks
22050020 RTM 1688-2 1
Adapter 110-240V/AC, 50/60Hz
/18VDC
1
80020002
dust filter 40
µ
m
2
fuse
2
PVC tube
(6,35 x 3,18)mm
1,5
m
USB cable
1
RS232 cable
1
Suitcase for transport 1
Could be substituted
by measuring case
Manual RTM 1688
-
2
1 CD*
Radon Vision Software, manual
1
CD*
Actual version
Certificate of calibration acc ISO17025
1
Warranty certificate
1
*also available on our Homepage https://sarad.de
Optional accessories:
No.
Article no.
Name
Quantity
Remark
1 20050025
Measurement case *
incl. 1x tube 2m (6x3mm)
2x adapter 5/10mm
1x tube 1m (10x6mm)
1
2 20050021 Transportation case for RTM 1688-2 with
IP68 (Peli™)* 1
3 10010003
Aqua Kit 500ml
incl. 1x tube 1m (6x3mm)
2x adapter 5/10mm
2x tubes 15cm (10x6mm)
1set
to measure Rn in
water
4 10010004 Gas transfer membrane 1 set
to measure Rn in
water
5 10010007
Soil gas probes
incl.
2x adapter 10/13mm,
2x adapter 5/10mm,
1x tube 1,5m (6x3mm)
1x tube 1,5m (10x6mm)
1x tube 15cm (14x10mm)
1 Set
6
10010017
Sharp tips for soil gas probes
1 set
100 pcs
7
80020005
Simple retractor for soil gas probes
2
pc
s
8
10010010
Special retra
c
tor for soil gas probes
1
9
20040003
Monopak
,
borehole packer probe + pump
1
set