High Temp Thermistor EGT Sensor
Thermometrics Corporation has built thermistor probe capable of measuring temperatures up to 1000°C. This sensor can be a direct replacement for thermocouple and rtd
sensors currently being used to monito exhaust gas temperatures.
The types of thermistors found in a vehicle exhaust environment will typically produce a negative temperature coefficient (NTC), meaning the resistance will decrease with increasing temperature. Thermistors offer a high sensitivity over a smaller range in temperature than either thermocouples or RTDs. At 0°C the resistance can be over 100,000 Ω,at 200°C 200 to 500 :, and at 800°C 50:. Thus,thermistors can achieve very high sensitivities over a particular range of temperatures. However, achieving nearly the same accuracy over a large range in temperatures is not possible (unless several pull up resistors are used) due to the highly nonlinear characteristic response.
Exhaust gas temperature is a measure of heat in the cylinder during combustion, and is most commonly measured close to the head. Since all metals melt, deform, or undergo transformation under excessive temperatures for that particular metal, it is easy to have too high of EGT and cause damage to engine/turbo components. Thus a tuner must measure the EGTs and keep them in check or suffer the consequences. High EGTs also contribute to knock. You cannot tune your car by EGTs alone, and you should not tune your car without measuring the EGTs.
RTDs vs. Thermocouples
Selecting the right temperature sensors for each application requires a close examination of the requirements.
Imagine you are an instrumentation engineer looking at the prints for a new
plant, and you see the many locations marked out for temperature transducers.
You wonder, what exactly has been specified to fill each space? Thermocouples,
because of the lower cost and familiarity, or RTDs, for the high accuracy and
Temperature transducer selection affects many aspects of the design and
installation of the equipment in the plant such as:
- •The type of wire that needs to be run.
- •The type of instrument that will be in the control room on the other
end of that wire.
- •Whether there will be local junction boxes with terminal strips or
transmitters. And, if so, what type of transmitter is required.
- •Whether any special piping considerations need to be made to install
or protect the sensor or to provide the required response time.
These are just some of the details, and the actual sensor selection and design
has not even been discussed yet. Looking at the big picture can help narrow
down the choices in a logical way. Look first at survival of the sensor and
then the finer points such as any performance requirements