When using biochemical analyzer to analyze sample

such as blood

the liquid carrying capacity of the sampling needle surface will lead to carryover

which affect detection precision of biochemical analyzer.In order to reduce the sample carrying amount in the sample needle and assure precision of sampling

a highly reliable liquid level detection system is put forward.The system regard sampling needle as capacitive sensor of liquid level

using phase-locked loop circuit CD4046 to detecte change in capacitance of capacitive sensor

signal filtering and output of control signal

which achieve goals of reduction of cross-contamination and improvement of performance of instrument.Clinical detection datas show that the liquid level detection sysem has features of good stability and high reliability.