THERMAL SENSORS


Marine engine monitoring


Electronic sensor reliability is allowing for better configuration and higher performance of high horsepower marine engines, says Patrice Flot, chief technical officer at the CMR Group


M


arine engine temperature monitoring provides important information on running behaviour, safety margins and the need for maintenance on critical components. Even mobile parts of the


gear train can be monitored for temperature thanks to wireless technology. Temperature sensors are therefore usually the


most numerous sensor type, often found at several locations and circuits on the same engine, measuring a range of parameters including oils, fuels, water, gases and exhaust. The three established sensor technologies of thermocouples, resistance temperature detectors (RTDs) and thermistors are often combined on the same engine, each with their own advantages and drawbacks.


THERMOCOUPLES Thermocouples have a high temperature capability ideal for measuring exhaust gases, cylinder head outlet and turbocharger temperatures. They can also be used on fluids and engine components such as bearings, liners or exhaust valve seats. Thermocouples give the shortest response times


due to a small thermal inertia. They also offer the smallest size intrusion and are very robust against vibration and thermal shocks. The drawback however is with accuracy as the relationship between temperature and voltage is not linear and depends on material quality and purity. Another disadvantage is the extra cost of fitting a compensation cable to overcome the “cold junction”.


RESISTANCE TEMPERATURE DETECTORS RTDs mostly employ platinum elements and are used either at PT100 or PT1000, where the number is the ohms value at 0°C. Resistance value is directly proportional to temperature. They are also low-cost and accurate and extension


cables for connection to the HMI use inexpensive copper lines. PT100 remains popular for fluid temperature measurement but must be connected by three or four wires to obtain the required accuracy. PT1000 sensors are becoming increasingly prominent as connection line resistance is comparatively negligible and does not affect accuracy.


42 /// Environmental Engineering /// September 2018 Sturdy sensor


housing and installation for RTDs are highly important as the platinum element can be sensitive to vibrations. RTD sensors are mainly used for fluids, compressed air and bearings as high temperature capability is limited and response times can slow due to thermal inertia.


THERMISTORS While their lead-wire resistance does not affect sensor accuracy, thermistors are sensitive to temperature, typically having much higher nominal resistance values than RTDs. These low-cost ready- for-use electronics are often used in automotive applications. Not all automotive sensors can be used on industrial engines, which use much larger pipes and tubing and require much higher lifetimes. For example, marine engines with 4,000 hours of


operation represent only around nine months of running. Yet, 4,000 hours of running for a car will bring them close to the end of their life cycle. With time between major overhauls for industrial engines varying from 30,000 to 50,000 hours, it is a challenge for fragile sensors to last this long.


INDUSTRIAL ENGINE SENSORS Instrumentation manufacturers are working with engine OEMs to develop products to meet the reliability and lifetime challenges for industrial engine sensors, while trying to minimise engine delivery cost. The evolution of engine technology makes the


importance of reliability and long lifetime for exhaust gas temperature sensors increasingly critical. As reliable sensors are now mandatory for today’s common rail and electronic gas engines, any sensor failure, even for a few milliseconds, is unacceptable – failure of exhaust gas temperature sensors can lead to the immediate shut down of the engine. Exhaust gas temperature sensors are now


mandatory for smaller engines with electronic injection, offering considerable business opportunities to a few instrumentation makers that are able to produce sensors with excellent reliability. EE


❱❱ Industrial engine sensors need


to meet long lifecycle and reliability challenges


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The


evolution of engine technology makes the importance of reliability and long lifetime for exhaust gas temperature sensors increasingly critical


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