More than one RHT sensor is needed on a single I2C bus, however the I2C addresses are the same. How can two or more RHT sensors be used on the same I2C bus?
The RHT sensors do not have a programmable I2C address. Using a multiplexer and a device select line, multiple RHT sensors can use the same I2C bus. A single NMOS is placed on the SDA line with the drain on the host side and the source on the slave side. 10k pull-up resistors should be placed on both side of the NMOS. The MCU can control the SDA line by the device select.
RH data is overflowing and rolling over. What can be done to fix this?
In cases of severe contamination such as the presence of conductive dust or liquid, readings can become significantly higher and will wrap around. To protect against this, it is recommended to use the factory-installed ePTFE cover. Generally, after contamination a bake and hydrate process will recover accuracy however in severe contamination cases, accuracy may be irrecoverable.
The cover is designed to be compatible with normal convection reflow soldering. The cover is not designed to be replaced. We have seen some customers that use excess heat in soldering and the cover shrinks to the point that it is no longer effective. Some customers have had issues with some conformal coating material drying between the cover and the protective tape used while conformal coating is applied. This results in the protective cover peeling off with the tape when the tape is removed after the conformal coating dries. Both of these issues should be avoided. While the cover can be replaced with care to avoid sensor surface contamination, the recommended rework procedure for a device where the cover has been damaged or has peeled off is to replace the device.
Sensor's RH readings at high humidity drift over time. After exposure once the humid environment drops to a lower RH, there is a shift in the sensor's accuracy.
Prolonged exposure to high humidity will cause gradual drift of the humidity sensor. All Si70xx sensors have on-chip heaters that can be used to counter local high humidity, help prevent condensation, and reduce the drift. Please refer to AN607, pg.28, for a more detailed explanation on how to estimate RH with heating. For example, turning on the heater with control setting 0x3 heats the sensor about 5C (depending upon system design) which results in approximately 23% drop in the local RH reading.
The sensor’s I2C bus is stuck. How can this be fixed?
A common error when there is noise or a host that is not robust is that the sensor can get in a state where it is driving SDA low expecting to be sending data and the host thinks it is sending data with SDA high. This creates bus contention and can hang the bus. In this case of bus contention, we recommend that the host clocks SCL until SDA is released and then sends a stop and new start.
Can the RH sensor measure humidity below 0C? And how can it be tested?
Yes, the sensor works below 0C. The accuracy drift/temperature must be considered. Testing the RH Sensor below 0C is difficult as most RH chambers cannot control the RH level accurately if not at all when temperature is lowered to 0C or below. To check for accuracy when measuring below 0C, a chilled mirror hygrometer is recommended.
Can the RH sensor’s PCB be cleaned with Isopropanol?
Sensor will become inaccurate if exposed to the fumes from Isopropanol. Generally no clean PCB is recommended. If cleaning is required, use deionized water after using water soluble solder flux with the sensor’s protective cover in place. Water pressure must be kept low to maintain the integrity of the protective cover and not allow entry of water into the sensor cavity, there is a 15 PSI max limit. Leave some PCBs uncleaned so that the cleaned boards can be compared for an accuracy check.
Datasheet states the RH sensor cannot go through hot air rework due to possible alteration of the device’s characteristics and damage to the cover during hot air rework. Can hot air rework be used to replace a device on a pcb?
Solder joint touch up can be done with a limit of 5 seconds per lead. It is generally recommended that any other rework be done with a fresh device. The problem is hot air rework can reach temperatures much hotter than the peak temperature of the device. When the cover is heated too much it will shrink very quickly. This happens very easily when air flow is directed from the top of the device. If it is possible to use the hot air tool from the below the PCB there should be no damage to the device or the cover.