Sensors that perform multiple functions—such as pressure sensors that provide temperature output or sensors that monitor both temperature and relative humidity—can provide many benefits to designers in a variety of industries. Among them, medical device designers who are trying to reduce device size to improve portability are particularly interested in saving board space; designers of HVAC HVAC devices are welcoming fewer external interconnects in the device (they are causing performance) Higher reliability due to the potential source of loss. Most designers like the design concept of “a sensor that implements the functions of the original two sensors” because it means that the test and calibration workload of the product is greatly reduced. By comparing the design stages, designers can determine whether a combined sensor has a greater advantage than two independent sensors.
Designers should consider the following when deciding whether an application should use a combined sensor: - board size and available space - accuracy and reliability - total cost of design and production
Board size and free space
Perhaps the most obvious advantage of choosing a combined sensor instead of two separate sensors is "to provide more functionality in a single sensor-sized package." Providing two outputs (such as the Honeywell HumidIcon digital temperature and humidity sensor) in a SOIC-8 SMD package is a big advantage when board space is limited. However, if the designer wants to measure the temperature or relative humidity externally, a SOIC-8 SMD package is not enough. Add a package with a size of at least SOT23.
Sensors that are calibrated, temperature compensated, and amplified over the entire temperature range can reduce the number of components required for signal conditioning on the PCB, further saving space and reducing weight. By adding a digital interface option to the sensor, designers can also remove external resistors, capacitors, and amplifiers on the board. The digital interface provides a direct signal path from the sensor to the microprocessor, which saves board space and reduces the problems that can be caused by multiple signal conditioning components on the PCB.
However, not all applications are suitable for combined sensors. For example, if the humidity sensor and the temperature sensor are not at the same process temperature, or where there is a certain distance between the temperature measurement and the location of the humidity sensor on the board, it is not suitable for the multi-function temperature and relative humidity sensor. In both cases, it is necessary to use two separate sensors.
Precision and reliability
As long as a component is added to the board, the number of external interconnects and potential sources of signal failure will increase, reducing the overall reliability of the system. Combined sensors offer higher reliability than multiple independent sensors because they have fewer components and joints that can be damaged. Also, as described above, using the digital interface option can reduce the number of components on the board and the connection points where signal errors can occur.
The total error band (TEB) is a true measure of sensor reliability. In addition to accuracy, TEB takes into account all the factors that determine sensor reliability. The smaller the TEB, the better the sensor will avoid independent sensor testing and calibration, support system accuracy, optimize system uptime, and provide superior sensor interchangeability. For reference, the Honeywell HumidIcon® digital temperature and humidity sensor has a total error band of ±5 % RH and an accuracy level of ±4 % RH. A slightly higher total error band reflects considerations for factors other than accuracy. Mean Time Between Failure (MTTF) can also be used to determine sensor reliability. Because sensors vary in quality, reliability, and precision, it is important for designers to carefully evaluate performance specifications, product quality, and manufacturer's reputation when selecting a combined sensor.
For applications that require the highest level of accuracy, if a combined sensor does not meet the overall accuracy tolerance of the application, then it is necessary to use two separate sensors. Single-function relative humidity sensors typically have an accuracy rating of ±3 % RH to ±3.5 % RH.
Total cost of design and production
In many cases, integrated sensors save more on actual production costs and application costs than multiple independent sensors.
The savings in actual production costs depend on the sensor and its function, I/O (input/output) mode. Taking a combined temperature and humidity sensor as an example, it can temperature compensate the relative humidity (RH) measurement and provide a second independent temperature output - providing the user with two outputs. This saves money when using integrated sensors instead of two digital I/O sensors.
The application cost saved by the combined sensor may be much greater than the actual production cost saved. The time saved in researching, purchasing, installing, and testing multiple sensors, which may come from different manufacturers, can help increase productivity. Because compensation and calibration are not required, designers can often perform capital-intensive testing in advance, resulting in shorter production cycles. In addition, productivity advantages apply to installed applications because a single sensor has fewer interconnect points (potential sources of failure).
Combined sensors are a very useful tool in demanding design applications, reducing design, test and installation time while providing reliable output. The above advantages of the combined sensor are particularly significant in applications where board space is limited.
Sleep ventilators, respirators, and other medical devices can use a combination sensor to provide accurate relative humidity and temperature measurements to provide warm, humid air for better comfort, safety, and treatment.
Multiple output sensors can be applied to critical processes and experiments to provide optimal temperature and relative humidity levels for improved process efficiency under expected weather conditions.
The Honeywell HumidIcon digital temperature and humidity sensor can be used to provide accurate relative humidity measurements in compressed air lines, allowing the system to remove all condensate; dry compressed air is critical to customer process control measurements.
Precision relative humidity and temperature sensors in communication cabinets and HVAC systems help maintain proper temperature and humidity levels in the cabinet, providing maximum system uptime and maximum performance.