Biomedical Engineering Reference
In-Depth Information
FIGURE 10.21
Resistivity versus temperature characteristics of a typical thermistor.
Mathematically, the resistance-temperature characteristic of a thermistor can be approxi-
mated by
1
T
1
T
0
R
T
¼
R
0
exp b
ð
10
:
16
Þ
where R
0
is the resistance at a reference temperature, T
0
(in degrees K), R
T
is the resistance
at temperature, T (in degrees K), and b is a material constant, typically between 2,500 and
5,500 K. A typical resistance-temperature characteristic of a thermistor is shown in Fig-
ure 10.21. Note that unlike metals and conventional resistors that have a positive tempera-
ture coefficient (as the temperature increases, the resistance increases), thermistors have a
nonlinear relationship between temperature and resistance and a negative temperature
coefficient. Increasing the temperature decreases the resistance of the thermistor.
EXAMPLE PROBLEM 10.11
A thermistor with a material constant b of 4,500 K is used as a thermometer. Calculate the resis-
tance of this thermistor at 25
C. Assume that the resistance of this thermistor at body temperature
(37
C) is equal to 85
O
.
Solution
Using the resistance-temperature characteristic of a thermistor (Eq. (10.16)) gives
1
298
1
310
R
T
¼
85
exp 4500
¼
152
:
5
O
The size and mass of a thermistor probe in a medical thermometer must be small in
order to produce a rapid response time to temperature variations. The probe is normally
covered with a very thin sterile plastic sheet that is also disposable to prevent cross-contam-
ination between patients.