Comet T8148, T8248 Instruction Manual

T8148

Programmable indoor transmitter of temperature

and CO2 concentration with 4-20 mA output

T8248

Programmable indoor transmitter of temperature

and CO2 concentration with 0-10 V output

Instruction Manual

2 IE-SNC-T81(2)xx-01

General description

The transmitters are designed for online measurement of carbon dioxide concentration and

temperature of air in a building interiors.

device type

construction type

output

T8148

ambient air

4 – 20 mA

T8248

ambient air

0 - 10 V

Principle of CO2 concentration measurement is based on Non-Dispersive Infrared Technology
(NDIR). It is maintenance free because of the patented autocalibration feature, compensating for the
effects of aging and therefore a guarantee for outstanding long-term stability. If the device is in
operation permanently, an auto-adjustment will start automatically after 24 hours operation each.

Measured values can be read in „SLOW mode“ (filtered, averaged) or in „FAST mode“ (current

values without averaging). SLOW mode has advantages in applications like climate control because
of filtering short time peaks. As an example exhaled air from an employee passing the sensor could
affect the climate control negatively with a short response time because the control would trigger a
change of the ventilation based on this one-time measurement. On the contrary in „FAST mode“ no

software filter is used for calculating the output value. This fact adds a noise of typ. ±30ppm which

has to be considered in terms of accuracy. Of principle measurement is the measured value of CO2
concentration depends on the value of air pressure - altitude at the installation site. For this reason, it
is suitable for accurate measurement to set the altitude of the installation site by TSensor software ­see below.

Measured values are displayed on dual line LCD display. The visual indication of CO2
concentration is provided by three-color LED.

After power up of the device starts internal test. During this time (about 20s) LCD display shows
(----) instead of CO2 concentration value.

Devices with 4-20 mA outputs can be connected to circuitry by means of galvanically isolated or
galvanically non-isolated current loop. Outputs 0 - 10 V are galvanically non-isolated.

All transmitter setting is performed by means of the PC connected via the optional SP003
communication cable (not included in delivery). Program TSensor for transmitter setting is
available to download free www.cometsystem.cz/products/reg-TSensor.

Models marked TxxxxZ are non-standard versions of the transmitters. Description is not
included in this manual.

Please read instruction manual before the first device connection.

IE-SNC-T81(2)xx-01 3

Device setting from the manufacturer

output 1: T8148 - range 4 to 20 mA corresponds 0 to 2000 ppm
T8248 - range 0 to 10 V corresponds 0 to 2000 ppm
output 2: T8148 - range 4 to 20 mA corresponds 0 to 50ºC
T8248 - range 0 to 10 V corresponds 0 to 50ºC

measurement mode: SLOW
display: switched on
LED indication: up to 1000 ppm lights green LED, between 1000 and 1200 ppm lights

yellow LED and over 1200 ppm lights red LED

altitude: 300 m above see level at the installation site
Modification of the setting is possible to do by means of the PC and TSensor program.

Installation of the transmitter

Devices are designed for indoor applications. It is recommended to mount them on universal
wiring box (common installation box KU68) with using two enclosed mounting screws. For correct
function there is necessary to find proper device place. It shouldn’t be placed at places where it can
be affected by sunshine, near radiators, heating elements and other heat sources, air handlers,
windows, doors, into racks and shelves and similar places. For buildings with less thermal
insulation there is not suitable to place them on external walls of building. If there are
communication conductors placed into conduit, there is strongly recommended make it caulk, to
restrict air flow around device.

For transmitter connection it is recommended to use a shielded cable. Maximum cable lenght of
the current loop is 1200 m, maximum voltage output cable lenght is 15 m. The cable should not be
led in parallel along power cabling. Safety distance is up to 0.5 m otherwise undesirable induction
of interference signals can appear.

Firstly mount the rear part of the device on the wiring box with two screws. Connect cables to
terminals (terminals are self-clamping and can be opened by a suitable screwdriver). By jumpers J1
and J2 select galvanically or non-galvanically isolated current output (T8148), see „Typical
application wiring”. Finish the installation by inserting the front part of the unit.

Installation and electrical wiring should be performed by qualified personal only.