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Linearity defines the degree to which the output of a medical temperature sensor changes continuously over the temperature range. NTC thermistors are exponentially nonlinear and exhibit higher sensitivity at low temperatures than at high temperatures. Over time, as microprocessors became more widely used in sensor signal conditioning circuits, sensor linearity issues became less important. Both thermistors and platinum resistance temperature detectors require careful consideration of the power dissipated in the sensor element when energized to prevent self-heating. For platinum resistance temperature detector sensors, the measurement current is specified in the applicable standard. There is no such standard for NTC thermistors, it is just up to the design team to determine the appropriate current levels to confirm that there is no significant self-heating while providing a sufficient signal-to-noise ratio, these currents are usually in the microamp range.
The response time, or how fast the sensor indicates temperature, depends on the size and mass of the sensor element (assuming no predictive methods are used). Among them, IC temperature sensor has the slowest response, platinum thermal coil element is the second slowest, platinum film, thermistor and thermocouple have small packages, so the response time will be fast, and glass microbeads are the fastest responding thermal sensitive resistance. Response time itself is an ill-defined feature, and better methods of measuring thermal response often employ the time constant, the time it takes for a medical temperature sensor to record a change in temperature as it travels between two different temperatures. As the name suggests, this is a constant value for a given medium, independent of the starting and ending temperatures, and it is based on the fundamental physics of heat transfer. There are different time constants for the different media being measured, for example, the time constant measured in still air is 10 times longer than that measured in oil.
Errors due to electrical noise in temperature indications are a major problem due to the weak millivolt signal of thermocouples, but NTC thermistors have very high resistance and electrical noise errors will be much smaller.
Lead resistance can cause error drift in resistive devices. This effect is more pronounced in low-resistance devices. The relatively low temperature coefficient of the platinum RTD sensor complicates the problem, so use a 3-wire or 4-wire lead configuration to subtract the lead resistance from the measurement. For thermistors, usually choosing a higher resistance value will eliminate this effect. Thermocouples must use extension leads and connectors of the same material as the leads themselves, otherwise errors may occur.
The cost of medical grade temperature sensor elements should not be a major consideration when choosing the type of sensing technology to use. Each type, NTC, RTD, thermocouple or semiconductor, has its advantages and disadvantages. In each technology, there is a cost associated with sensor characteristics such as accuracy, stability, temperature range, and ambient resistance. Of course the most important thing is to understand the total cost of the program to determine the most efficient solution.