LoRa IoT Smart Room Sensor User Guide
T0006338_UG
Version 1.5
TEKTELIC Communications Inc.
Page 17 of 30
of the External Connector reports voltages in mV, which can then be converted to
temperatures. The conversion formula (voltage to temperature) can be obtained in one of the
following ways:
1.
Calibrating Thermistor and Performing Curve Fitting:
A number of voltage-temperature pairs can be obtained for a given thermistor, then the
following curve fitting formula can be used:
𝑇 =
−𝐵
ln (
𝑎
𝑉 − 𝑏)
− 273.15
where
T
is in °C and
V
is in mV, and where
B
is the B-value of the thermistor (e.g. 3380 K
for CWF3AA103G3380 or 3984 K for NTCAIMME3), to obtain the best
a
and
b
based on a
desired criterion, e.g. MMSE (minimum mean square error) or minmax criterion.
For example, for CWF3AA103G3380 and NTCAIMME3, using the MMSE criterion, the
conversion formulas
𝑇 =
−3380
ln (
0.00314
𝑉
− 0.0000018)
− 273.15
and
𝑇 =
−3984
ln (
0.000416
𝑉
− 0.00000024)
− 273.15
are obtained, respectively.
2.
Quick and Approximate Conversion Formula:
Another easier, though less accurate, way to quickly characterize any NTC (negative
temperature coefficient) thermistor is to use the following conversion formula:
𝑇 =
−𝐵
ln (
26.43
𝑉
−
1
68.1) + ln (𝑅
0
) −
𝐵
𝑇
0
− 273.15
Where
T
is in °C and
V
is in mV, and where
B
is the B-value of the thermistor, and
𝑅
0
is
the reference resistance, in
k
Ω, of the thermistor at the reference temperature
𝑇
0
, in
K
.
For example, for CWF3AA103G3380,
𝐵 = 3380
,
𝑅
0
= 10
and
𝑇
0
= 273.15 + 25 =
298.15
.
3.
Steinhart
–
Hart Equation:
In this method the temperature of the thermistor, in K, is given as:
