Datasheet

DATA SHEET
Technical changes reserved
0141 0316-115 07/2009
Thermocouple module
with digital I²C-Interface - THMOD-I²C
Thermovoltage Typ -1360
Numeric value via I²C Interface 0x 0000...7FFF
dec. 0...32767
Physical value -12,500...85,804 mV
Measuring range 98,304 mV
Resolution 6 µV
Termperature channel all models
Numeric value via I²C Interface 0x 0000 ... 7FFF
dec. 0... 32767
Physical value -32...95,996 °C
Scaling T (°C)=V / 256 - 32
Determination of temperature
The modules are universal to use, and in principle, suitable for all
thermocouples. The included thermocouple is a K type with material-
combination NiCr-Ni. The following examples refer to the module -T1
and included thermocouple. The connection of the thermocouple must
be done with the correct polarity, otherwise the temperature will be
measured incorrectly. From the respective module variants, the usable
voltage range is so selected that the specied temperature measuring
range can be realised with Fe-CuNi thermocouple (type J), as well as
with NiCr-Ni thermocouples (type K). Since the NiCr-Ni thermocouples
produce a lower Seebeck voltage as compared to Fe-CuNi (type J)
thermocouple, the measuring range is larger with NiCr-Ni thermocoup-
les, with somewhat lower temperature resolution. Because of this, for
example, the module type-800 with application of NiCr-Ni thermocoup-
le can be used up to approx. 1200 °C. The rst channel measures the
thermovoltage. The scaling is dened, that 16 bit integer arithmetic can
be applied for the calculations and table interpolations. The scaling of
measured values depends on the used module and is linear to ther-
movoltage. The second channel measures the absolute temperature
at the connection point with a Pt1000 resistance thermometer. The
scaling is optimised on the integer processing in the micro controller
and is linear to the temperature. The temperature is to be calculated
mathematically. For this purpose, both channels must be calculated
together. First the thermovoltage is measured, which is read over the
rst two bytes at address 0x78 over the I²C-Bus. The rst byte is the
MSB, and the second byte is the LSB. The uppermost bit is used for
error detection and is not included into the calculation. Afterwards, the
temperature is determined at the connection point, for which the se-
cond and third bytes are read. The second byte is the MSB, and the
third byte is the LSB. The uppermost bit is used for error detection and
is masked. The value corresponds to the temperature in 1/256 °C, with
offset shift of -32 °C.
Corresponding to the measured temperature at the connection point
and depending upon the used thermocouple, a correction value is
worked out (for type K see table 1, right 3 columns). Then the cor-
rection value is added to the numeric value of the thermovoltage, so
that the temperature at the connection point is added with correct sign
quasi as thermovoltage on the measured thermovoltage.
With the intermediate results as shown in table 2, the temperature of
the measuring point is determined through interpolation.
Example: This is for a Module -300 with NiCr-Ni Thermocouple (Type
K). The 4 Byte I²C-Data at the address 0x78 in HEX is read like this:
60 85 3E 00
The thermovoltage (0x6085, dec 24709) is 12.209 mV. The tempera-
ture value of the connection point (0x3E00, dec 15872), results in a
temperature value 30.0 °C (see table 1). For this temperature and the
Module -300, the correction value is 1203 digits. Added to the numeric
value of the thermo voltage results in 25912 digits thermovoltage. With
this value, one can now interpolate in table 2 and the measured value
comes out as 330 °C at the measuring point.
Correction value of connection point
Digit Temp. mV -300 -800 -1360
512 -30 -1,156 -1156 -578 -385
3072 -20 -0,778 -778 -389 -259
5632 -10 -0,392 -392 -196 -131
8192 0 0 0 0 0
10752 10 0,397 397 199 132
13312 20 0,798 798 399 266
15872 30 1,203 1203 602 401
18432 40 1,612 1612 806 537
20992 50 2,023 2023 1012 674
23552 60 2,436 2436 1218 812
26112 70 2,851 2851 1426 950
28672 80 3,267 3267 1634 1089
31232 90 3,682 3682 1841 1227
B+B Thermo-Technik GmbH | Heinrich-Hertz-Straße 4 | D-78166 Donaueschingen
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