How Do Optical Sensors Detect Leaks? CMR Electrical Explains

In this article we will explore how optical sensors detect leaks, and discuss how they have improved over time.

A History of Leak Detection Sensors

Introduction of Spot Probes

In the early 1990s, PTC (Positive Temperature Coefficient) thermistor sensors were a popular choice for leak detection in various industrial applications. PTC thermistors are resistors that increase their resistance with rising temperature. This characteristic made them ideal for applications where precise temperature control and monitoring were necessary.

Initial Setup:

• The PTC thermistor sensor was designed to operate within a specific temperature range, typically between 60 to 120 degrees Centigrade.
• A small voltage would be applied across the thermistor to heat it up to the desired working temperature. Once the sensor reached this temperature, the current drawn by the sensor would stabilise, typically around 1mA.

Leak Detection Mechanism:

• The sensor would be monitored by an alarm unit that continuously checked the current being pulled by the thermistor. In normal conditions, the current remained steady, indicating that the sensor was maintaining its set temperature.
• When a leak occurred, the leaked fluid, usually water, would come into contact with the heated thermistor. The heat from the thermistor would cause the water to evaporate.
• As the water evaporated, the temperature of the thermistor would drop due to the cooling effect of the evaporating fluid.

Response to Temperature Drop:

• A decrease in temperature would cause the thermistor’s resistance to drop. To maintain its operating temperature, the sensor would then draw more current.
• This increase in current would be detected by the leak detection panel, as it would be calibrated to recognise such changes.
• Upon detecting the increased current, the leak detection system would trigger an alarm, alerting operating staff to the presence of a leak.

Development of Optical Leak Sensors

Spot probes have improved drastically over the last twenty years, and modern sensors now use an optical sensor to detect leaks. An optical liquid leak detection sensor contains an infrared light emitting diode and a phototransistor. This type of sensor also has the advantage of not having any moving parts, increasing the life expectancy of the probe.

Examples of these modern sensors include our optical leak sensor type OSP – engineered to detect leaks of various liquids. There is also the Type OSPW, which is adapted to detect non-conductive liquid leaks (like diesel or oil).

Optical Sensor in the air

The infrared light from the LED undergoes total internal reflection within a prism tip. Most of this light is directed back to the phototransistor. This results in a strong signal output because the phototransistor receives a high amount of light.

Optical Sensor in liquid

When the prism tip is submerged in liquid, the refractive index changes at the boundary. This causes infrared light to scatter, reducing the amount of light that reaches the phototransistor. Consequently, the output signal decreases, indicating the presence of liquid.

Conclusion

In the early 1990s, companies widely used PTC thermistor sensors for leak detection, appreciating their simplicity and sensitivity to even minor leaks. However, as technology advanced, several limitations of PTC thermistors became apparent, leading to their eventual obsolescence. These sensors were prone to inaccuracies due to fluctuations in ambient temperature, and their requirement for sustained high temperatures significantly shortened their lifespan. These limitations often led to false alarms or undetected leaks as well.

The introduction of optical liquid leak detection sensors has largely resolved these issues. Modern optical sensors offer greater accuracy, require less maintenance, and boast a longer operational lifespan, making them a superior choice for reliable leak detection today.