Detecting Liquid Media and Bubbles Using Optical Sensors

In my line of work in Life Sciences, we often deal with liquid media and bubble detection evaluation through a vessel or a tube. This can be done by using the absorption principle or the refraction principle with through-beam-configured optical sensors. These are commonly embedded in medical devices or lab instruments.

This configuration provides strong benefits:

    • Precise sensing
    • Ability to evaluate liquid media
    • Detect multiple events
    • High reliability

How does it work?

The refraction principle is based on the media’s refraction index. It uses an emitted light source (Tx) that is angled to limit the light falling on the receiver (Rx, Figure 1). When the light passes through a liquid, refraction causes the light to focus on the receiver as a beam (known as a “beam-make” configuration). All liquids and common vessel materials (silicon, plastic, glass, etc.) have a known refraction index. These sensors will detect those refraction differences and output a signal.

The absorption principle is preferred when a media’s absorption index is high. First, a beam is established through a vessel or tube (Figure 2). Light sources in the 1500nm range work best for aqueous-based media such as water. As a high absorption index liquid enters the tube, it will block the light (known as a beam-break configuration). The sensor detects this loss of light.

Discrete on-off signals are easily used by a control system. However, by using the actual light value information (commonly analog), more data can be extracted. This is becoming more popular now and can be done with either sensing principle. By using this light-value information, you can differentiate between types of media, measure concentrations, identify multiple objects (e.g., filter in an IV and the media) and much more.

There is a lot to know about through-beam sensors, so please leave a comment below if you have questions on how you can benefit from this technology.

The Challenge of Living in a Vacuum

We’ve all noticed that cloudy film that forms on the inside of our vehicle windshields. A major contributor to this annoying coating is outgassing. Of course, this is not the only situation where outgassing is an issue.

Semiconductors are vital in today’s world and can be found in all the devices that we have become so accustomed to. With features measured in nanometers, semiconductor production requires incredible accuracy. It is critical to remove contaminants from the process chambers and, therefore, the processes are done under vacuum. This presents a challenge when selecting components that must live in a vacuum. The vacuum environment causes materials to outgas which is the release of gasses trapped within the material. These stray molecules are a cause for concern as they can interfere with the semiconductor production processes. Adhesives, rubbers and plastics are common sources of outgassing but even metals and glasses can release gasses from cracks or impurities. However, for most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly.

Builders of equipment used to produce semiconductors must evaluate each component used in a vacuum chamber to mitigate outgassing as much as possible. To help them achieve this, they work with component vendors who have extensive experience in the industry and offer vacuum compatible products. These vendors can also provide highly customized products that ensure very high performance and quality, as well as addressing concerns with outgassing.

Precision sensors that will operate in high vacuum or UHV (Ultra High Vacuum) environments must be carefully constructed from materials with low vapor pressures in order to avoid outgassing. Also, some innovative methods are often utilized to address the challenges with precision sensors that are needed in a vacuum chamber. An example would be to use a precision photoelectric sensor with separate electronics. The electronics are contained in an amplifier which can be mounted outside of the vacuum chamber and the vacuum compatible sensor, with stainless steel housing, can be threaded into the chamber.

Fortunately, semiconductor equipment builders and their component suppliers are well versed in the challenges associated with outgassing and work together to overcome them. By conquering this, and many other challenges they insure we can continue to enjoy our high-tech gadgets.

Real-Time Optical Thickness Gauging for Hot Rolling Mills

An ever-present challenge in hot rolling operations is to ensure that the material being produced conforms to required dimensional specifications. Rather than contact-based measurement, it is preferred to measure the material optically from a standoff position.

Light band gauging station in hot strip rolling operation detects material thickness in real time.
Light band gauging station in hot strip rolling operation detects material thickness in real time.

In some instances, this has been accomplished using two ganged analog optical lasers, each detecting opposite sides of the material being measured. Through mathematical subtraction, the difference representing thickness could be determined. One difficulty of the approach is the need to put a sensor both above and below the material under inspection.  The sensor mounted below could be subjected to falling dirt and debris. Further, only a single point on the surface could be measured.

A new approach uses a scanning laser to create a band of light that is used to directly measure the thickness of the material.  An analog or digital IO-Link signal represents the measured thickness to a resolution of 0.01mm with a repeat accuracy between 10μm to 40μm depending on distance between emitter and receiver.  What’s more, the measurement can be taken even on red-hot metals. The illustration above shows a flat slab but the concept works equally well or better on products with a round profile.

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