When medical labs reduce errors they trim costs and improve processing uniformity. One way to make such gains is with automated laboratory equipment for handling liquids along with uniform analysis and sample preparations that deliver reproducible results.

Some labs still rely heavily on technicians to process samples and monitor operations. But as human involvement goes up, so does the likelihood of improper liquid handling, sample contamination, and inefficiency. Take measuring liquid levels for instance. Some labs still rely upon technicians to monitor the presence and levels of fluids, an imprecise and expensive alternative to automated monitoring.

Even automating the seemingly simple task of detecting liquid level is problematic. Some equipment relies on conductive electrodes to measure levels when the liquid comes into direct contact with the switch. When that happens, the switch sends a signal to a conditioning instrument that processes it into a switching signal. Because these sensing probes come in direct physical contact with liquids, labs run the risk of sample contamination and test failure. Even a single contaminated specimen can result in lost time and efficiency. At worst, contamination skews test results and lead to a flawed patient diagnoses.

A better way

To minimize contamination, lab technicians monitoring automated processes must interrupt operations to replace disposable probe tips. These must be replaced with every test-liquid change and for every patient sample analyzed. While it's effective to assess liquid level this way, disposable tips can be expensive and replacement causes costly downtime.

However, ultrasonic sensors on automated laboratory equipment can detect liquid levels quickly and accurately, and without touching the liquid. In addition, they are less expensive over time than contact probes and replacement tips. Reliable ultrasonic sensors offer a long operating life immune to damage from vapors, acids, alkalis, and other contaminants.

Ultrasonic sensors provide reliable feedback in automatic equipment that helps minimize errors. A special sonic transducer used in ultrasonic proximity sensors allows for alternating transmission and reception of sound waves. The time elapsed between emitting and receiving is proportional to the distance of the object (liquid surface) from the sensor. It recognizes the returning signal and generates an output that can be translated into a volume figure. Calculations are done in the sensor.

Sensor outputs come two ways. An analog output is a voltage signal that is a continuous function of the measured parameter. The second concerns positioning feedback. For example, if the task is to sense a small ball, you can set the sensor to a specific location so that the ball is sensed only when it reaches that spot. The repeatability is less than 0.5 mm on these sensors. That is, repeating the same test several times gives results within 0.5 mm. A teach function in the sensor lets users tell it where to take the measurement and position. For distance-measuring devices with an analog output, distance is proportional to the time between emitted and received signal.

The advantage of ultrasonic sensors is their ability to measure difficult targets. For instance, these sensors are not affected by color, transparency, or reflectivity of the target surface, and object texture has little effect. They are capable of reliably detecting solids, liquids, granules or powders regardless of color and opacity, and are resistant to dust, light, and humidity.

Even transparent and highly reflective items that cause problems with optical sensors can be reliably recognized by ultrasonic versions. Of course, ultrasonic sensors work best with targets that do not absorb sound, but even highly absorbent targets, such as foam or cotton, can be detected with the right models.

Sonic cones are another sensor characteristic that helps detect targets. The cones can sense different lengths away from the sensor. The sonic cone profile represents the active sensing areas for ultrasonic sensors. Size, shape, surface properties and the direction of the target being detected greatly influence the lateral detecting region of an ultrasonic sensor. The blind range represents the area in which the sonic cone does not sense. For example, the Sonus model from Baumer Ltd., has a standard sensing range of 10 to 200 mm from the front of the sensor. The blind range extends from 0 to 10 mm. No sensing occurs there. At its farthest reaches, the sonic cone has a sensing range of 20 mm laterally. One option, a sonic beam deflector, attaches at a 45° angle to decrease the blind range.

In automated laboratory equipment, sensors must detect liquid levels in narrow spaces. Some ultrasonic sensors come with a beam columnator at the sensor's face. It's a contoured nose cone that further narrows the beam profile for demanding applications.

The small size of ultrasonic sensors is key to making them useful in medical applications. Recent versions are as small as 27-mm long with 12 x 14-mm cross sections, so they can be placed close together. As a result of their proximity, miniature ultrasonic sensors can measure several rows of a well plate and take many measurements at once. Hence, a row of eight wells would require a row of eight sensors. When needed, sensors can be set for synchronized or multiplexed operation to suppress the interference occasionally generated when several are closely mounted.

Sensors and robots

When sensors are used on small robotic arms, sensor mass becomes an engineering consideration. Ultrasonic sensors are available with a mass as low as four grams (just 0.16 ounces), so they are easily built into robots. Miniature ultrasonic sensors provide analog or digital outputs and sensing distances of up to 200 mm. Their narrow sonic-beam profile lets them determine liquid levels in 96-well plates.

Reliability is important for molecular sample preparation such as DNA and RNA purification where prepared samples can tolerate no contact. Automated liquid handling combined with noninvasive, noncontact liquid level detection helps to ensure the quality of such samples.

The sensors at work

A few applications show how ultrasonic sensors improve lab processing.

On liquid handling robots, liquid levels in standard 96-well trays are often detected using conductivity liquid-level switches. While the method effectively monitors liquid level, it has drawbacks. For one, contamination is possible as electrodes contact the fluids. Because the switch cannot process a signal, equipment needs additional signal conditioning instruments.

Switching to a non-contact ultrasonic sensor improves the accuracy and performance of the robots, and prevents contamination. Eight miniature rectangular ultrasonic sensors each measuring just 10-mm across let the liquid handling robot simultaneously sense liquid levels in a row of wells and in less time than contact sensors need. The ultrasonic versions provide a continuous (non-pulsed) analog output that reports the liquid level in each well without additional signal conditioning.

In an automated contact-lens handler, lenses are individually placed into plastic wells along with a wetting solution. The well is then sealed with foil. The wetting solution level is a critical quality issue because lenses with insufficient solution would be defective upon receipt by the customer.

Fiber-optic sensors once used to detect the liquids' presence in the wells did not provide a reliable output. Readings were affected by the varying color of the contact lenses and subtle variations in reflectivity of the wetting solutions.

Replacing fiber-optic sensors with ultrasonic versions resolved the issues. Ultrasonic sensors are immune to liquid color, transparency, and reflectivity, so their data is reliable even with changes to liquid formulations or lens colors.

Drug-eluting stents help restore normal blood flow when inserted into a patients blocked artery. The stents are coated with a medication that helps ensure that arteries do not constrict again.

Drug coverage is critical to the effectiveness of the stent, so manufacturers must ensure that the device is completely immersed in the drug for appropriate periods. To fully cover the stent, all fibers must pass through a reservoir containing the drug. The device will not be adequately coated if the drug level in the reservoir falls below a minimum.

Liquid level is easily detected using an ultrasonic switch positioned above the reservoir. When the level falls below the minimum, controls activate a pump to refill the reservoir.