|
Peter_4059,
Finally got around to comparing the Simple Approximation and the Better Approximation for dew point calculation. Below 50% RH, as stated for the Simple Approximation, the numbers get significantly out of wack. Example (for our somewhat extreme observing conditions on the praries of Canada - during the winter):
Ambient Temperature: -20 C
Relative Humidity: 10 % (not uncommon in the winter)
Simple Approximation: -38 C (frost point - not dew point)
Better Approxiation: -44 C
And of course the number diverge even more as the temperature drops further and the humidity gets lower.
I like observing during the winter months better than the summer as I can have the scope setup and be viewing by 6:00 PM. During our summer months, like now, it does not get dark until 10:00 PM.
I will be continuing on my project to build a dew / frost point dew heater and find a way to do the math with a uC. I have ordered an Arduino. It should be here next week. As I already have the sensors, I may have something breadboarded in a couple of weeks. Then I will make the decisions on the final temperature sensors and board modifications. I will produce PCBs (one off) once the design is settled. I expect I will have to make a further revision farther down the line with features I have overlooked.
I am using CAT-5 cable with RJ-45s to connect the heaters / temperature sensors. Using 3 conductors each for the power and ground to the heater and using 1 conductor for the sensor power and 1 conductor for the sensor data. The sensor ground is connected to the heater ground. Each RJ-45 contact is rated at 1A and the CAT-5 is already twisted pairs. The heaters are not drawing anywhere near what three conductors within the cable can handle.
Tom
|