Thermometrics Corporation has developed a thermistor probe capable of measuring temperatures up to 1000°C, making it a suitable alternative to traditional thermocouples and RTD sensors for monitoring exhaust gas temperatures.
In vehicle exhaust systems, thermistors typically exhibit a Negative Temperature Coefficient (NTC), where resistance decreases as temperature increases. Thermistors offer higher sensitivity over a narrower temperature range compared to thermocouples or RTDs. For example, a thermistor’s resistance may exceed 100,000 Ω at 0°C, range between 200 to 500 Ω at 200°C, and drop to around 50 Ω at 800°C. This provides exceptional sensitivity within a specific temperature range. However, maintaining the same level of accuracy over a broader range is challenging due to the thermistor’s nonlinear response unless additional resistors are used.
Exhaust Gas Temperature (EGT) Monitoring
Exhaust gas temperature (EGT) measures the heat generated during combustion, typically recorded near the cylinder head. Excessive EGTs can cause metal components like engine or turbo parts to melt, deform, or suffer damage. High EGTs also increase the risk of engine knock. Therefore, tuning a vehicle requires careful monitoring of EGTs to prevent component damage, though EGT monitoring alone is not sufficient for complete tuning.
Comparison of Sensor Types:
Thermocouples: Thermocouples, particularly type K, are cost-effective sensors but require expensive cold-junction compensation to adjust for voltage variations due to changes in wire material. Thermocouples offer quick response times because of their low mass, but in exhaust environments, they need a protective sheath that reduces this response advantage.
RTDs: Resistance Temperature Detectors (RTDs) operate by measuring changes in a metal’s resistance with temperature. In vehicle exhaust applications, platinum is preferred for its ability to measure across a wide temperature range while maintaining near-linear output. Standard RTD resistance is 100 Ω at 0°C, but for exhaust applications, 200 Ω at 0°C is typical, which doubles sensitivity and reduces the impact of vehicle wiring.
Thermistors: Thermistors are highly sensitive within specific temperature ranges but are less accurate across broader ranges due to their nonlinear response. With resistances over 100,000 Ω at 0°C, dropping to 200-500 Ω at 200°C, and 50 Ω at 800°C, they provide excellent precision for targeted temperature monitoring in vehicle exhaust systems. However, additional components may be needed for more accurate readings over a wide temperature range.
Thermometrics’ thermistor probe offers a reliable and sensitive solution for high-temperature exhaust monitoring, providing an alternative to more traditional thermocouples and RTDs in automotive applications.