No Oil Droplets Formation in Thermopile Flow Sensors

Used in Internal Combustion Engines

Tu XianZheng

Hot-film air mass sensors are commonly used for measuring an air-mass flow, which include a resistor for heating and one or two other resistors for temperature sensing. It has been reported that when hot film air mass flow sensors are used directly in the intake tract of the internal combustion engine or in a bypass channel to the intake tract of the internal combustion engine, oil may deposit on the sensor chips and, in particular, on the sensor diaphragms during operation or shortly after the internal combustion engine has been shut off. This oil deposit may result in undesirable effects on the measuring signal of the sensor chip, in particular because an oil film affects the thermal conductivity of the sensor chip surface, which results in corruption of the measuring signals or a signal drift.

As well known, condensation can produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condensed. As result droplets of water leave on the glass or can.

It is the same that when a thermal air mass flow sensor is operated at the border region of the heated measuring areas oil accumulates and over time results in oil droplets. The air flow drives the oil droplets on the surface up to the boundary of the heated measuring area, at which a stronger temperature gradient appears. The strong temperature gradient exerts a force opposite to the force exerted by the air flow. Oil droplets thus accumulate on the boundary line, which, when they reach a certain size, may be entrained again by the air flow to then contaminate the surface of the measuring area. In addition to the oil droplets, other contaminants also reach the surface of the measuring area due to this effect.

How to solve this problem? Back to the above mentioned condensation. Condensation of water occurs when water vapor within the air cools enough in order to change into the liquid state. A good example of condensation often occurs in the home during winter time, when water droplets form on the surfaces of cold windows. If open the window to let the cold air enter the room there will no any water droplets form on the surface of the window. So the only way to solve the problem is to reduce the operation temperature of the sensor. This can be done using a thermopile air mass flow sensor instead of a hot film air mass flow sensor.

A thermopile is an electronic device that converts thermal energy into electrical energy.

It does not respond to absolute temperature, but generates an output voltage proportional to a local temperature difference or temperature gradient. A thermopile air mass flow sensor is constructed with a heater for heating and several thermocouples for temperature sensing. The thermopiles are in series and so the output voltage due to temperature change is summed and increased over that of a single thermocouple.

With this advantage, POSIFA has developed thermopile air mass flow sensors with two group thermopiles positioned at the two side of the resistor heater and each group consisting of 40 thermopiles. Since the summed output of each group is so great that the operation temperature of the heater can be reduced as low as no condensation to take place. For example, the operation temperature can be set 10 to 20 degree Celsius higher than the air temperature. In this case condensation of oil vapor in the air is almost impossible.