RRDgraph uses Reverse Polish Notation (RPN) for calculations. Obtaining dew point data from stored temperature and relative humidity is simple once the RPN is worked out. Below is the code for using stored Celcius data in the Closer Approximation formula found on the dew point Wiki page.
where
v0 equals Temperature in C
v1 equals Relative Humidity in percent
CDEF:aTRH0=17.271,v0,*,237.7,v0,+,/,v1,100,/,LOG,+ \
CDEF:TdC0=237.7,aTRH0,*,17.271,aTRH0,-,/ \
To display dewpoint in degrees F from RRDs "A" (temp in degrees F) and "B" (%RH), using just one RPN calculation, I use:
ReplyDelete237.7,17.271,A,32,-,1.8,/,*,237.7,A,32,-,1.8,/,+,/,B,100,/,LOG,+,*,17.271,17.271,A,32,-,1.8,/,*,237.7,A,32,-,1.8,/,+,/,B,100,/,LOG,+,-,/,1.8,*,32,+
This calc would be shorter if using multiple CDEFs.