March 19, 2009
PTC thermistors: Exact level sensing
EPCOS has developed two new PTC components with stainless steel packages for sensing the level of liquids. The B59050D1100B040 type is suitable for measuring water levels and can be operated within a temperature range of between +10 °C and +65 °C. The B59050D1120B040 variant is designed for liquids with a thermal conductivity of λth >0.12 W/mK such as oil or fuel. The permissible temperature range in this case is between -25 °C and +50 °C.
To prevent overheating, the PTC elements should be operated at a voltage <18 V. The tip of the PTC element can emit different thermal losses. The transition from liquid to air and vice-versa causes these losses to change very quickly, and as a result, the resistance of the PTC element. This allows the device to record very simply and exactly whenever a liquid goes above or below a certain level.
In contrast to level measurement with mechanical contacts, PTC-based systems operate with no moving parts. Moreover, complex electronics such as those required by ultrasonic or optical systems are unnecessary.
Further information on this topic can be found at www.epcos.com/ptc
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