Devices that are traditionally used to measure the body temperature which give measurements more of a static nature are not generally geared for continuous ambulatory temperature measurement. Non-contact sensors are unable to provide continuous remote temperature measurements if the patient is not in line of sight of the sensors. Among contact temperature sensors, most of the non-electric temperature sensors are not able to store temperature measurements in an electronic manner. In contact electrical sensors the most commonly used sensors are thermistors, thermocouples and RTD’s (Husain, 2012), (MINCO, n.d.). Therefore a comparison between these three sensors have been carried out to identify the most appropriate sensor for the application.
2.5.3.1 Accuracy
The accuracy of thermistors and RTD are high +/-0.1 °C and +/-0.01 °C respectively. Thermocouples are not as accurate as thermistors and RTD’s, however they too can have an accuracy of +/-0.5 °C (Baker, 1998).
2.5.3.2 Temperature range
The thermocouples have the highest temperature range when compared to RTD’s and thermistors. The temperature range of the thermocouples are from -272 °C- +2760 °C (Lipták et al., 2004). RTD’s can have a temperature range of about -250 °C- +900 °C. Whereas thermistors have a limited temperature range of about -100 °C to 450 °C (Baker, 1998). The measurement range of a human body temperature sensor can be considered as narrow, since the skin temperature does not vary over tens of degrees Centigrade.
2.5.3.3 Step Response time
The response time is the time taken to respond to a step change in temperature. The step response time is the smallest in thermocouples, they have a response time of less than 1 s. Whereas thermistors have a step response time of 1-5 s and RTD’s can have a step response time of 1-10 s (Baker, 1998). A step response time of a few seconds can be regarded as negligible for applications discussed in the thesis.
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2.5.3.4 Linearity
Linearity of a temperature sensor shows how consistently the output of the sensor changes with temperature. Thermistors are nonlinear therefore they provide very good accuracy over a narrow range of temperatures (Lipták et al., 2004). Since RTD’s use metal sensing they are more linear therefore they are stable at high temperatures (Anonymous, 1995).
2.5.3.5 Sensitivity
Sensitivity can be defined as the minimum change in temperature (input) that can bring about a detectable change in the output. Thermocouples produce a voltage difference corresponding to a difference in temperature and they have a medium sensitivity of 10s of µV / ˚C. In RTD's and thermistors the resistance changes as a result of change in temperature. Platinum RTD’s have a sensitivity of 0.00385 Ω / Ω / ˚C. Whereas thermistors have a sensitivity of several Ω / Ω / ˚C (Baker, 1998). Thermistors have the best sensitivity out of the three sensors (National Instruments, 2010). Since the sensor variations with respect to temperature is high and also the resistance change with regard to lead resistances is high in thermistors, a simple voltage divider circuit can be used instead of a bridge completion circuit when connecting the thermistor to an analogue to digital converter (Anonymous, 1995).
2.5.3.6 Drift/Stability
Drift/Stability is the ability of a sensor to give reproducible results for a period of time. It includes retaining the step response time, selectivity and sensitivity of the sensor (V. Bochenkov and G. Sergeev, 2010). A problem with low cost thermocouples is that they tend to drift when measuring high temperatures. However the ones made of nickel and platinum are more stable. Thermocouples are the least stable sensors (Anonymous, 1995). The stability of RTD’s is extremely high and they drift by less than 0.1 °C/year (Wilson et al., 2008). Thermistors are also known for good stability and glass bead thermistors have been reported to have RMS fitted drift rates as small as 0.19+/-0.08 µK per week (Lawton and Patterson, 2002).
2.5.3.7 Lead resistance
RTD’s have low resistances that changes slightly with temperature (0.4 Ω/°C). In order to measure such small resistances complex circuits are required and long leads will lead to errors. Thermistor can be used to provide remote measurements using long wires since the resistance of the wires are insignificant when compared to the high resistance of the thermistors (Wilson et al., 2008) (SENSORAY, n.d.). When fabrics are made using temperature sensing yarn the length of the leads will vary depending on the position of the sensor therefore thermistors are ideal for wearable applications.
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2.5.3.8 Electrical Noise
Electrical and magnetic noise can be a problem when using thermocouples to measure temperature, since they produce signals in microvolts and typically stray magnetic and electrical signals have magnitudes greater than the measurements. Small signals from an RTD is also prone to noise like thermocouples therefore precautions should be taken to prevent noise. Thermistors have the lowest effect from noise from the three sensors (pico Technology, n.d.).
2.5.3.9 Self-Heating
Self-heating is common in thermistors and RTDs. When powering an RTD it is important to find the smallest possible excitation current. With higher current levels RTD’s go into self-heating mode causing measurement errors and with lower current levels produce lower outputs susceptible to noise (Rajanish K. Kamat and Gourish M. Naik, 2002). When a thermistor self-heats the resistance of the thermistor becomes less than it is and this causes an error (Wilson et al., 2008).
2.5.3.10
Excitation
Thermocouples do not need any excitation whereas thermistors and RTD’s are powered by a voltage/current source (Baker, 1998).
2.5.3.11
Output
Thermocouples output a voltage whereas thermistors and RTD’s output a resistance (Baker, 1998).
2.5.3.12
Ruggedness
Ruggedness is the ability to withstand shock and vibrations. Thermocouples are known to be rugged temperature sensors (Rajanish K. Kamat and Gourish M. Naik, 2002) whereas thermistors are known to be most fragile out of the three. However most thermistors can handle mechanical and thermal shock and vibrations better than other types of sensors (Wilson et al., 2008).
2.5.3.13
Size
Size of the sensor is an important parameter for wearable electronic applications. Thermistors come in very small sizes (Wilson et al., 2008). Therefore thermistors can provide point sensing due to its small size.
2.5.3.14
Cost
Thermistors can be considered as the cheapest out of the three. Thermocouples can range from cheap to moderate. RTD’s are expensive when compared to the other two sensors (Baker, 1998). If the sensors are to be included in textiles, they must be reasonably priced.
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2.5.3.15
Summary
Table 2:3: Comparison of thermistors, thermocouple and RTD's (MINCO, n.d.), (Baker, 1998)
Thermocouple RTD Thermistor
Accuracy Low (±0.5 °C) High (±0.01 °C) Moderate (±0.1 °C)
Temperature range -260 to 850°C -270 to 1800°C -80 to 150°C
(typically) Response time Fastest (less than 1 s) Moderate (1-5 s) Slowest (1-10 s)
Linearity Moderate Best Poor
Stability Low Best Moderate
Sensitivity Low Moderate Best
Effects of Lead Resistance
- High Low
Ruggedness Most Rugged Moderate Moderate
size Bead diameter =
5 x wire diameter
Moderate Smallest 0.3 x 0.3mm
(package die)
Cost Low high Low to moderate
When comparing the temperature sensors it can be seen that for small temperature ranges (variation in body temperature) the thermistor is the most suitable sensor. It comes in small sizes and at very low costs. Also the high sensitivity and resistance of the thermistor ensures that complex circuitry is not needed to measure temperature using a thermistor and this can be beneficial because integrating complex circuits into a textile can be difficult. It also provides a good accuracy, relatively fast response time and a good stability at a low cost.