Which surface temperature sensor is best for your application?

At Campbell Scientific, we redesigned our back-of-module temperature sensors and launched the CS241 and CS241DM purpose-built sensors to optimize performance on bifacial photovoltaic (PV) modules and help you collect data as precisely as possible. These newer sensors have some similarities to our previous sensor models, but they also offer important improvements. Moreover, there are differences between the CS241 and the CS241DM. How should you decide which model to use? First, we’ll discuss the key features both models share. Then, we’ll explore what makes them unique and why one model might be better suited to your application than the other.

What are the similarities?

Small sensor footprint

The CS241 and CS241DM are known for their small footprint that covers less than two percent of the area on a full-sized panel. Both models come with a precision 1000-ohm Class A platinum resistance thermometer (PRT), Pt-1000. The aluminum disk that protects the PRT has a diameter of 2.54 cm (1.0 in.) and a maximum height of 0.419 cm (0.165 in.), and both models ensure there is minimal impact on solar panel performance. Because the sensor meets the IP68 rating, both the CS241 and CS241DM are safe for monitoring floating solar panels.

Class A Pt-1000 sensing element

The sensing element of the CS241 and CS241DM is a Pt-1000 Class A PRT encased in a specially designed low-mass aluminum disk. The slim profile and geometry of the disk reduces any effect the sensor has on the temperature of the cell being measured. This means that the CS241 or CS241DM won’t act as a heat-sink, something we have seen with other sensor designs. The Class A sensing element used in these sensors provides a PRT accuracy of ±(0.15 + 0.002T)°C and excellent stability over a wide range of temperatures when combined with the sensor head.

Optimized for bifacial panels

Both the CS241 and CS241DM were designed for use on bifacial PV panels for performance assessment and monitoring. (Bifacial solar panels are rapidly gaining adoption in the solar energy market as an advantageous option because they collect light from both sides of a solar panel and increase the density of power production.)

Simplified strong, reliable adhesive

The adhesive for the sensor's disk has been updated to have maximum sensor-to-module bonding and a thermal conductance greater than 600 W/(m²*K), exceeding the IEC 61724 standard that requires conductance to be greater than 500 W/(m²*K). Combined with a reduced mass cable, the improved adhesive allows the sensor to be mounted without tape or additional epoxy. This improvement not only decreases the impact on the solar panel, but also makes installation more convenient.

What is the difference?

Analog versus digital sensor

The key difference between the two sensors is that the CS241DM provides a digital Modbus RS-485 output, whereas the CS241 has an analog output that can be configured as either a two-wire or four-wire measurement depending on the measurement device and the measurement channels available.

The CS241DM is an addressable Modbus remote terminal unit (RTU) and can be configured to provide data to many Modbus client devices. The digital measurement (DM) board supports daisy-chaining multiple CS241DM sensors, resulting in simplified connections needed for monitoring multiple temperatures required by IEC 61742. This can reduce cable length and simplify communication topology. The CS241DM also contains a configurable termination resistor that can be enabled for the final sensor in a daisy-chain.

The DM board of the CS241DM measures the sensor using a four-wire measurement with a precision resistor and 24-bit A/D. The board is housed in a rugged overmolding with an M12 connection for reliable environmental protection. The board also has built-in surge protection to protect the measurement electronics from surges that can be common at solar power plants.

In contrast, the CS241 is an analog sensor with a built-in, high-precision completion resistor. The sensor can be measured using a two-wire or four-wire configuration depending on the desired accuracy in the measurement and the available analog channels on the measurement device. For the highest accuracy, a four-wire measurement should be used. 

How to choose

Choosing the best sensor for your application can depend on several factors, including those mentioned briefly below:

• The CS241 is suitable for use with existing meteorological stations equipped with a data logger that has available analog channels. Two-wire bridge measurements can be used for shorter cable lengths (typically shorter than 6.1 m [20 ft]). For cable lengths up to 45.7 m (150 ft), Campbell Scientific recommends using a four-wire measurement. 
• The CS241DM works well with supervisory control and data acquisition (SCADA) systems. Because of its Modbus over RS-485 communications ability, the CS241DM can support much longer cable lengths. This sensor is also a good choice when analog channels are not available.

Campbell Scientific sales and application engineers can help you select the sensor that will provide you with the lowest total cost of ownership for the system in which you are integrating the sensor. For assistance with project specifications or questions about which sensor to use, contact Campbell Scientific’s Renewable Energy Group at 435-227-9020.