Q: What is the aspirator for?
Q: Why is the aspirator air-flow directed away from the sensor? Shouldn't it blow directly on it?
Q: Why is there a separate temperature sensor? The 1-Wire chip used for the humidity sensor already has one built in. Why not just use that and save money?"
Q: Why shouldn't I use the external +5V power for the temperature sensor? Wouldn't that provide more stable operation?
Q: I can't seem to get the surface mount chip to stay put long enough to solder it in place. What should I do?
What is the aspirator for?
The aspirator is a fan that is used to keep the air moving in the radiation shield. It improves the accuracy of the sensors - especially relative humidity in the higher ranges. If moisture condensates on the sensor, it will be at 100% until it evaporates even though the true relative humidity is in the upper 80's or 90's. The sensor is reliable, but does easily saturate at the high end. Relative humidity is like temperature in that it varies widely in a given local area depending upon conditions. For example, at night dew may form on one side of your car while the other side is dry. The dry side is more than likely facing the wind. If there's no wind, then all sides will have dew (and there will be probably be fog in the area as well). Measuring relative humidity with a sensor is no different. The aspirator helps out by keeping the air moving and therefore helps prevent moisture from condensing on the sensor.
Why is the aspirator air-flow directed away from the sensor? Shouldn't it blow directly on it?
This decision to have the air-flow directed away from the sensors was based on the idea that if air blew directly on the sensors it might taint the readings (temperature cooler that it really is, humidity lower). You may wish to experiment, having the aspirator blow directly on the sensors. This fall I have noted that the sensor tends to saturate most nights. One of the early alpha testers of the sensor pulled his apart recently and found the board literally wet. Whether or not you experience this will have a great deal to do with your climate. If you experiment, be sure and keep records of what the highest humidity was overnight (if your system supports this) or at least the reading at a given time each night for a couple of weeks (say 10PM, for example). Repeat the observations after reversing the fan air-flow direction and compare the results for similar conditions (which should include temperature, wind and humidity). If you discover anything significant, pass the information along and I will post it here.
Why is there a separate temperature sensor? The 1-Wire chip used for the humidity sensor already has one built in. Why not just use that and save money?
Why shouldn't I use the external +5V power for the temperature sensor? Wouldn't that provide more stable operation?
Both questions are related, so they are answered together here.
The reliability and accuracy of the humidity sensor readings is substantially increased when external power is used. This is due to several reasons. First, the humidity sensor has a "settling-in" time once power is applied. It can take anywhere from 15 to 30 minutes for the sensor to stabilize. Second, when running in the bus mode and using long cables, the IR (voltage) drop on the long run can be enough to cause problems. This will vary with temperature. When running in the bus mode, power for the humdity sensor and the 1-Wire IC both must come from the bus. This mode is also time consuming in that the bus must be pulled high for about 1 second to insure devices on the bus have sufficient power to last during the command/read cycles that follow. In my experience I found that everything worked fine on the bench with short runs. When installed in the permanent location with a long run, it produced numerous read failures.
So why not do the same thing for the temperature sensors? The answer lies in a problem called "self-heating". Because of the I²R (power) losses in the devices, heat is generated if power is left applied all the time. The temperature in-accuracies in the humidity circuit are easily tolerated because the rated accuracy of the humidity sensor is far less.
However, power requirements for the temperature sensor are minimal and it operates reliably using bus power – even on long runs. The duty cycle is low enough that there is a minimal effect on the temperature reading. However, if you are experiencing numerous read errors or faults due to a long cable, you may wish strap the board for external power.
I can't seem to get the surface mount chip to stay put long enough to solder it in place. What should I do?
Too much solder may have been deposited on the traces during manufacturing. Take some solder-wick and clean the traces on which the SMD sits. By cleaning the pad with solder wick, it will become flat and placement of the part will be much easier.