In this article, we'll take a look at Sensirion's series of single-use liquid flow sensors that are designed specifically for biomedical applications.

Sensirion recently announced their new LD20 series of single-use liquid flow sensors. These high-performance sensors, which are intended for biomedical applications, are claimed to be safe, precise, and reliable.

According to Sensirion's website, due to their unique CMOSens® Technology these liquid sensors have such a high degree of accuracy that they are able to detect the smallest changes in the flow rate, so much so that when they're integrated into an infusion set the sensor can actually "feel the patient's heartbeat." Now that seems impressive!

As stated on the LD20 Series Flyer, there are three versions of the sensors in the LD20 series. When taking into account these three versions, flow rates ranging from a few hundred µl/hr to 1200 ml/hr can be measured. Unfortunately, datasheets are available for only two of these versions: the LD20-0600L and the LD20-2600B.

 

Figure 1. The LD20 flow sensors are available with barbed or luer-lock fittings. Images taken from the LD20-0600L and LD20-2600B datasheets (PDF).

 

The LD20-0600L and the LD20-2600B are quite similar in design and functionality; the primary difference between them is their full-scale flow rates. The LD20-0600L is limited to a full-scale flow rate of 20 ml/hr (0.3 ml/min), whereas the LD20-2600B can measure flow rates up to 1000 ml/hr (16.6 ml/min).

Take note (see the two figures below) that these specifications are based on the flow of water at 23°C.

 
Figure 2. The full-scale flow rate (20 ml/hr) of the LD20-0600L is determined using water at 23°C. Table taken from the datasheet (PDF).
 
Figure 3. The full-scale flow rate (1000 ml/hr) of the LD20-2600B is determined using water at 23°C. Table taken from the datasheet (PDF).

 

Keep in mind that both versions are listed as prototypes because, according to their datasheets, the LD20 series is still under development. So if you're planning to use any version from the LD20 series, be sure to coordinate your design efforts and/or timelines with Sensirion so they, hopefully, won't make any drastic product changes without giving you a heads up.

 

Bidirectional Liquid Flow Measurements

These LD20 series sensors are capable of measuring flow rates in both directions, as described in a video on Sensirion's YouTube channel, but the accuracy of the flow rates is higher in the positive direction than in the negative direction.

See the two plots below.

 
Figure 4. The flow rate measurements are less accurate when the medium is flowing in the negative direction. Plots taken from the LD20-0600L and LD20-2600B datasheets (PDF).

 

I2C Interface and Communicating with the Sensor

These liquid flow sensors utilize an integrated 5-pad contact scheme; they communicate via I2C, and they are compatible with Sensirion’s Sensor Viewer Software.

 

Figure 5. The interface and electrical connections for the flow sensors. Taken from the datasheet (PDF).

 

Sensirion has also provided an Engineering Guidelines document—call it an app note—that discusses the “base station,” which is essentially a test fixture that allows you to easily evaluate an LD20 device. It looks like a handy way to experiment with the sensor and assess its capabilities without investing the money and time required for a customized interface.

However, if you decide to use the sensor, you’ll have to invest the time and money eventually—because Sensirion clearly states that the base station is “meant for testing purposes only” and “will not be commercially available” as a standalone unit.

 

Figure 6. The LD20 base station and cable pinout, from the Engineering Guidelines document (PDF).

 

German Anyone?

Here's a tidbit that made me smile. In Section 3.1 (Sensor Start-Up and Warm-Up Behavior) of the LD20-0600L datasheet, you can find a bit of German: "Fehler! Verweisquelle konnte nicht gefunden werden." Fortunately, my good friend, Mr. Google, was able to help me translate this message into "Error! Reference Source Not Found." It's safe to assume that the final datasheet will require less translation.

 

Figure 7. German anyone? A minor though somewhat amusing documentation oversight in the LD20-0600L datasheet (PDF).

 

Have you had a chance to use one of these single-use liquid flow sensors? If so, leave a comment and tell us about your experiences.

 

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