Conventionally, a level sensor for detecting, for instance, the level (liquid level) of a liquid is known as an example of a sensor for detecting the state of a liquid, an electrostatic capacity-type liquid meter as its example is used in the measurement of the residual quantity of fuel in an automobile, for example (e.g., refer to patent document 1). In this electrostatic capacity-type liquid meter, a capacitor is formed between an elongated tubular electrode (outer cylinder electrode) made of a conductor and a tubular electrode (inner electrode) provided within that tube along the axial direction (hereafter referred to as the “gap”) so as to measure its electrostatic capacity. The electrostatic capacity-type liquid meter is mounted in a tank for accommodating the liquid such that the axial direction of the outer cylinder electrode becomes the rising and falling direction of the level of the liquid. Since the electrostatic capacity of a portion which is not immersed in the liquid is dependent upon the dielectric constant of air in the gap, and the electrostatic capacity of a portion which is immersed in the liquid is dependent upon the dielectric constant of, the measured electrostatic capacity becomes larger as the level of the liquid becomes higher. For this reason, it is possible to detect the level of the liquid on the basis of the electrostatic capacity.
In recent years, there are cases where an NOx selective reduction catalyst (SCR) is used for an exhaust gas purifying apparatus for reducing into harmless gas nitrogen oxides (NOx) emitted from, for example, a diesel powered automobile, and a urea aqueous solution is used as its reductant. In the case of measuring the level of a liquid which exhibits electrical conductivity as in the case of this urea aqueous solution, a level sensor is used in which an insulating film made of a dielectric is coated on the outer surface of the inner electrode, for the purpose of preventing short-circuiting between the outer cylinder electrode and the inner electrode of the aforementioned level sensor. The electrostatic capacity of a portion which is not immersed in the liquid becomes a combined capacity of the electrostatic capacity of an air layer in the gap and the electrostatic capacity of the insulating film of the inner electrode. Meanwhile, since the electrostatic capacity of a portion which is immersed in the liquid becomes the electrostatic capacity of the insulating film since the electrically conductive liquid assumes a potential substantially equivalent to that of the outer cylinder electrode. Then, the total of the two electrostatic capacities is measured as the overall electrostatic capacity of the sensor. Since the thickness of the insulating film is sufficiently small as compared to the thickness of the air layer, the amount of change of the electrostatic capacity accompanying a decrease of the portion which is not immersed in the liquid is sufficiently small as compared with the amount of change of the electrostatic capacity accompanying an increase of the immersed portion. For this reason, the overall electrostatic capacity of the sensor becomes directly proportional to the increase and decrease of the electrostatic capacity of the portion immersed in the liquid, so that it is possible to detect the level from the relative magnitude of the measured electrostatic capacity.
If such a level sensor is used by being assembled to a UUU, it is possible to issue a warning or the like in the case where the residual quantity of the urea aqueous solution is small, making it possible to inform the driver that it has become impossible to properly perform the reduction of nitrogen oxides by the exhaust gas purifying apparatus. Incidentally, it is known that the urea aqueous solution has a proper range of concentration (concentration of urea contained in the solution) in effectively reducing the nitrogen oxides. For this reason, even if the residual quantity of the urea aqueous solution is a proper quantity, there is a possibility that the reduction of the nitrogen oxides cannot be effected satisfactorily in cases where the concentration of the urea aqueous solution has deviated from a proper range due to such as a change over time and in cases where a liquid (light oil, water) other than the proper urea aqueous solution is accommodated in a urea water tank. Accordingly, it has been proposed to juxtapose a concentration sensor for detecting the concentration of the urea aqueous solution in the urea water tank and to issue a warning or the like in response to the respective outputs of the level sensor and the concentration sensor (e.g., refer to patent document 2). It should be noted that, as such a concentration, one has been proposed which is used in a indirectly heated-type concentration detecting unit including a heat generating resistor and a temperature sensing element (e.g., refer to patent document 3).
Patent document 1: JP-A-9-152368
Patent document 2: JP-A-2002-371831
Patent document 3: JP-A-2005-84026