Digital Humidity/Temperature Sensors

“Humidity” refers to water vapor content in air. Humidity – in combination with ambient temperature – is a significant environmental characteristic affecting living organisms as well as buildings, machines, etc. Poor indoor air quality has negative impact on human health and comfort, but also in industrial environments where sensitive material is handled or processed (e.g. paper, leather), it will be required to keep humidity within a certain range. See also Ref 3. Digital humidity sensors are key components to maintain a prefect working environment.

In fact, ambient humidity and temperature have an impact on electronics, too. These parameters are changing electrical behaviour of semiconductors, and any out-of-recommended-range operating conditions might decrease reliability or even damage a system (e.g. by a shortcut caused by moisture or by electro-static discharge ESD) .

The ideal relative humidity for human health and comfort is about 40–50%, so it should be kept at in this range for occupied areas and industrial environments (e.g. using air conditioning). This is where electronic devices feel comfortable, too :-). But for unattended devices (incl. IoT) smart remote sensing might be crucial in order to ensure proper device operation resp. quality of received data. This article reviews a selection of typical commercial humidity/temperature sensors with digital output, e.g. I²C interface.

Target Applications

Digital Humidity sensors play a key role not just for monitoring air condition, but also for control of equipment used to modify air condition. Typical target application for humidity sensors is stand-alone equipment like air conditioners (HVAC), refrigerators or home weather stations.

But in our networked world remote sensing is getting increasingly important, esp. for mobile devices which which are subject to relocation and changing environments, e.g. a smart phone or navigation system or a drone or wearables for medical/fitness applications . Also for home automation, surveillance and industrial purposes it might be useful to track ambient air humidity and temperature, inform operator (e.g. to predict maintenance of equipment) and/or adjust device operation accordingly, if required. For these kind of remote applications (IoT) the overall system would look like this:

Figure 1: Remote Humidity Sensing Application (© chip-info.com)

Basics

The amount of water vapor needed to achieve saturation depends on ambient temperature: Air can hold more moisture at higher temperatures, i.e. maximum water storage in air increases as the temperature increases. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C. The ratio of moisture in the air to the highest amount of moisture at a particular air temperature is called relative humidity (“rH”), measured in percent, i.e. will always be in the range of 0-to-100%.

Different methods have been developed to measure humidity. Start was done in ancient China (Shang dynasty, 1556 to 1046 BC): They used a bar of charcoal and a lump of earth: its dry weight was taken, then compared with its damp weight after being exposed in the air. The differences in weight were used to tally the humidity level (see Ref 1).

Today, capacitive hygrometers are used where cost, space, or robustness are relevant. In capacitive hygrometers, the effect of humidity on the dielectric constant of a polymer or metal oxide material is measured. The dielectric absorbs or desorbs water molecules depending on surrounding conditions. See also Ref 2). Due to nature of semiconductors and corresponding production processes, accuracy of capacitive sensors varies even if originated from same batch. In order to improve rH accuracy to few %, all sensors reviewed here are factory-calibrated one-by-one. In general, the incremental change in the dielectric constant of a capacitive humidity sensor is nearly directly proportional to the relative humidity of the surrounding environment. The change in capacitance is typically 0.2–0.5 pF for a 1% rH change, while the bulk capacitance is between 100 and 500 pF at 50% rH at 25°C.

Commercial Products

Digital humidity/temperature sensors contain elements for both measurement parameters plus a mixed-signal circuit in a fully monolithic way, i.e. implemented on same die. Used packages are not quite standard because part of the die (where sensor is located) must be exposed to ambient air, but they are all surface-mountable (SMD) and reflow-solderable:

Figure 2: Capacitive humidity sensor, example (© Sensirion)

In general, the integrated humidity sensor is driven by chip-individual factory-calibration data. Both sensor outputs are digitized by a shared Analog-to-Digital-Converter (ADC) and forwarded to a serial host interface, usually I²C.

Figure 3: Block Diagram, generic (© chip-info.com)

Criteria for competitive comparison

Besides technical data, availability and price of a digital humidity/temperature sensor, also public access to comprehensive product documentation and online support services are important. In particular, this applies to small/medium size projects, i.e. typically to distribution customers.

Sensor performance and technical data

Besides important electrical characteristics like power consumption, manufacturers of digital humidity sensors are also specifying relevant and common parameters which can be used for technical assessment:

Accuracy: Deviation of sensor readings against a high-precision reference at time of calibration.
Hysteresis: difference of measured values of the same sensor at a certain log point when coming from a dry environment or from a humid environment .
Long-Term Stability: The aging of a sensor may lead to drift of the measured value compared to reference. Long term drift is not predictable and may move to the upper or to the lower limit. It is specified by rH per year(rH/yr) resp. °C per year (°C/yr).
Response Time: For a sensor exposed to an abruptly changing environment it will take some time until the sensor output value has followed this step. Typically, the rise resp. fall time is taken for a sensor to reach 63% of the final value . This period is called “𝜏 63%”.

In general, you cannot not easily compare sensor performance data one-by-one. In fact, given values are difficult to compare because – for each criteria – manufacturers might refer to different conditions.

For example, specified accuracy (e.g. ±3 rH) might apply to a limited humidity range (e.g. 20…80 %) only, i.e. accuracy at higher humidity level (e.g. 90%) might be worse.
Product comparison gets even more tricky if “typical” values are provided (instead of “maximum” values). For these parameters applied conditions are usually provided, but sometimes not.
- An example is average power consumption where manufacturers use “typical” operation scenarios, e.g. one measurement per second at low resolution (if programmable) resulting in attractive-looking numbers which might not correspond to real-world target application.
Similar problem applies to other sensor parameters which depend on ambient temperature or power supply voltage.

All these observations have been incorporated into our product overview table as best as possible, but in case of doubt you will have to contact the manufacturer for more information (directly or via online support).

Other technical criteria used for comparison are start-up time and measurement duration.

Sensor Location

By nature, the integrated silicon-based humidity sensor has to get in touch with ambient air. For this purpose manufacturers are using a special package featuring a sensor window. In most cases, this window is located on top of the package. For protection during production process and during operation in the field, some manufacturers are offering a factory-installed chip cover.

As an alternate approach, Texas Instruments HDC2010 has located its sensor at the bottom of the package. In any case, each surrounding PCB layout will have to support proper chip operation and requires extra attention.

Some sensor ICs have an integrated heating element which can be used to test the sensor, to drive off condensation, or to implement dew-point measurement.

Command Interface

After power-up (or hard reset) the sensor IC enters idle state and waits for a command to be received via I²C (or SPI) host data interface, e.g. for chip configuration (e.g. set measurement resolution, if available) or to start measurement.

Some chips offer an auto-measurement mode to perform measurement readings on a periodic basis (e.g. once per second), thus eliminating the need to initiate a measurement request through an I²C command and eliminate associated response time. In combination with a reduced sample rate and/or resolution such configurations will have an impact on overall power consumption.

Another interesting feature is an interrupt/alert pin some products are offering. It will be activated if a programmable condition is met, e.g. measured rH value reaches a certain threshold. This signal can be used to trigger an action – without involving the host MCU.

Availability, Chip Price and Online Support

Product availability is not a critical selection criteria because all products are offered by multiple distributors – even in small quantities. Nowadays, a customer can compare distributor stock quantities and prices, and place orders immediately – 100% online at distributor websites.

In general, purchase prices are subject to frequent change (e.g. due to currency exchange rate), and depend on quantities and negotiation. Consequently, in our product overview table we provide a price indication only – based on actual budgetary quotes published by manufacturer and/or distributors at publication date. For a reliable quotation you will have to contact your sales partner anyway. Price range of reviewed sensor products is around 1 to 4 EUR for 1000 units. Average priced products (around 2,50 EUR) are marked “◇”, less expensive products (1,50 EUR or below) are marked “▽”, more expensive products are marked “△”.

For technical support big customers might have direct access to manufacturer application and product experts, but distribution or online customers rely on decent online support services and documentation, incl. application notes, sample source code, discussion boards, etc provided by manufacturers. If manufacturers do not properly address these needs of mentioned customer group it might be too risky for them to select their products. See “Online Support” category in product overview table below.

Product Overview

Our selection of humidity sensors is limited to candidates with integrated temperature sensor and a digital output. These are

Note: Embedded hyperlinks refer to corresponding manufacturer web pages.

Important Note: This article is introducing and comparing typical products representing the market of humidity/temperature sensors sensors available today. Please note that – besides products mentioned in this article – some manufacturers (esp. Sensirion, Renesas, TE Connectivity, Silicon Labs and TI) are offering additional versions, e.g. for different operating temperature ranges or with different accuracy levels at different prices.

Note: In our product overview table we consolidated public information originated from websites and manufacturer documentation. If no appropriate information has been found, respective field is left blank.

For better visibility and further use you can download above table as a PDF from here:

Product-Overview-Table Download

Conclusion

For automotive applications you will probably have to focus on digital humidity sensors offered by Sensirion or by TE Connectivity which are AEC-Q100 qualified. Sensirion SHT35A sensor performance is best in automotive class and offers useful features like an alert pin, auto-measurement mode and programmable resolution, but unfortunately is quite expensive. Overall technical data of automotive-certified TE HTU31D is slightly ahead of Sensirion SHTC1 and available at lower prices, but no evaluation kit or any online support is being offered. At least from this perspective Sensirion products might be better choice for distribution customers.

Besides Sensirion SHT35A and TE HTU31D also Renesas HS3001 offers best-in-class rH accuracy of <2% typ., but its price is also at top end of list.

Silicon Labs Si7021 offers just medium sensor performance at a medium price level, but very low power consumption and good online support services.

With its additional pressure sensor Bosch Sensortec BME280 is positioned aside from the rest of our sensor selection, but is still reasonably priced. BME280 is certainly a good choice for weather forecast or other applications requiring actual ambient barometric pressure. BME280 is offering good overall sensor performance and unique functions (IIR-filter and oversampling) which are configurable using Bosch’s BME280 API. On top of this, measurement cycle timings are adjustable according to customer requirements.

STMicroelectronics HTS221 is not new (initial data sheet has been released May 15, 2014) and it offers only average performance, but very good price and excellent online support. It should be highlighted is that HTS221 Evaluation Kit is based on ST’s popular STM32 MCU, which might be beneficial for customers who have selected STM32 for use as host MCU.

If SPI data interface is mandatory (instead of I²C), you will have to short-list ST HTS221 and Bosch Sensortec BME280.

For battery-powered devices pre-selection of suitable candidates is not straight-forward because overall power consumption depends on application requirements. Specified power consumption of Texas Instruments HDC2010 and Silicon Labs Si7021 are lowest, but other products (incl. TI HDC2010, Bosch BME280, Renesas HS3001,TE HTU31D and Sensirion SHT35A) are allowing to adjust related parameters like sample rate, resolution, etc. and might be worthwhile to evaluate, too.

Unquestioned low-price-leader is TI’s HDC2010. Besides average sensor performance it offers lowest power consumption and features an alert/interrupt pin (like Sensirion SHT35) for offline action triggering. In combination with TI’s online support services HDC2010 qualifies as a short-list candidate for most projects.

References

“Hygrometer“, Wikipedia
“The Capacitive Humidity Sensor”, Technical Note, ROTRONIK Measurement Solutions
The Importance of Controlling Humidity in Commercial and Residential Environments, White Paper, TE Connectivity
other online references (URLs) are embedded in text