Method and apparatus for capacitively determining the uppermost level of a liquid in a container
View Patent ↗A liquid level sensor confirms actual physical contact between a probe and a liquid by verifying that any change in capacitance of the sensor is constant over a given time period and thereby is not caused by spurious electrical disturbances or other measuring irregularities.
1. A capacitive liquid level sensor, the sensor comprising:
an aspiration probe needle adapted to be vertically translatable relative to the uppermost level of a liquid in a container and having an effective capacitance therebetween;
a source of a fixed voltage applied to said probe through a fixed resistor forming a RC network;
a source of a reference voltage;
a comparator circuit adapted to compare the RC network and reference voltages;
means for repeatably discharging and charging the RC network;
a counter for counting the time required for repeatably discharging and charging the RC network;
a signal processor for averaging the charging time over a number of successive discharging and charging of the RC network,
said signal processor further adapted to identify any change in the averaged charging time that exceeds a pre-determined value as a consequence of the needle penetrating the uppermost level of the liquid.
2. The sensor of claim 1 further comprising means for confirming that such a change in the averaged charging time is stable over a pre-determined time period.
3. The sensor of claim 2 further comprising means for rejecting as invalid any changes in the averaged charging time that are not stable over said pre-determined time period.
4. The sensor of claim 1 further comprising interrogating means for subsequently verifying that the change in the averaged charging time exceeds a pre-determined value.
5. The sensor of claim 1 wherein the predetermined value is in the range of 2–5% of the averaged charging time.
6. The sensor of claim 1 wherein averaging the charging time over a number of successive discharging and charging of the RC network comprises averaging the charging time over 3 to 7 successive discharging and charging of the RC network.
7. The sensor of claim 1 wherein the comparator circuit comprises a pair of operational amplifiers.
8. An analyzer comprising the sensor of claim 2 , the analyzer having a computer receiving any change in the averaged charging time from the signal processor, the computer programmed to maintain the position of the needle below the uppermost level of a liquid in response to any said change in the averaged charging time.