Oregon Scientific GR101

 Hacking the GR101 revisited (see [0]) or: 
 How to convert the raw impedance reading from a GR101 to percentage body fat and percentage body water - by Ap15e
 
 
 Definitions:
 
 raw_impedance: data from GR101
 BD           : Body Density [kg/m^3]
 FFM          : Fat Free Mass [kg]
 Wt           : Weight [kg]
 Ht           : Height [cm]
 Z            : Impedance [ohm]
 %fat         : percentage body fat [%]
 %water       : percentage body water [%]
 
 
 Constants (best fit to formulas for data from a spreadsheet publicly available, see [1])
 
 impedance_factor = 39.198
 hydration        = 0.728895
 
 
 Formulas for males and females
 
 Z = raw_impedance / impedance_factor
 %water = ( Wt - %fat / 100 x Wt ) x hydration / Wt x 100
 
 
 Formulas for males
 
 BD = 1.100696 - 0.107903 x Wt x Z/Ht^2 + 0.00017 x Z [Tanita Corporation, see [2]]
 %fat = ( 4.57 / BD - 4.142 ) x 100                   [Brozek, see [3]]
 
 Additional formulas for percent body fat:
 
 %fat = ( 4.95  / BD - 4.5   ) x 100                  [Siri, see [3]]
 %fat = ( 4.374 / BD - 3.928 ) x 100                  [Schutte; for African-Americans, see [3]]
 %fat = ( 4.86  / BD – 4.39  ) x 100                  [Wagner]
 
 
 Formulas for females
 
 FFM = 13.96674 + 0.348613 x Ht^2/Z + 0.168998 x Wt   [Tanita Corporation, see [2]]
 %fat = ( Wt - FFM ) / Wt x 100
 
 
 Notes
 
 The formulas for males (Brozek) match the values on the remote display to a high degree 
 (error is < 0.5% for %fat and < 0.3% for %water for the data from the spreadsheet).
 
 The formulas for females yield higher errors. There is insufficient data in the spreadsheet 
 for further analysis.
 
 
 References
 
 [0] http://www.fabienletort.com/blog/2008/domotic-help-us-hack-the-gr101.html
 
 [1] http://spreadsheets.google.com/ccc?key=pQy-b-0ho4QdU4ecNy0TLLg&hl=fr
 
 [2] Jebb SA, Cole TJ, Doman D, Murgatroyd PR, Prentice AM.
     Evaluation of the novel Tanita body-fat analyser to measure body composition by comparison with a four-compartment model.
     Br J Nutr. 2000 Feb;83(2):115-22.
     PMID:10743490
     Abstract: http://www.ncbi.nlm.nih.gov/pubmed/10743490
     Full text: http://journals.cambridge.org/abstract_S0007114500000155
     NOTE: The unit of measure for the height in the formula for body density is cm, not m.
 
 [3] The Process of Physical Fitness Standards Development 
     Full text: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA495349

Variables to configure:
 
ScaleSensorID            device id of RFX Weight WT device to watch
TriggerOnWeightChange    1: watch for weight changes instead of impedance changes
WeightMin	         only watch for weights in [ WeightMin .. WeightMax ]
WeightMax		 only watch for weights in [ WeightMin .. WeightMax ]
Height		         your height
Sex		         your sex (0: female, 1: male)
Asian		         1: Asian (affects BMI_HUF only)
AfricanAmerican		 1: AfricanAmerican (if set to 1, results may differ from the values on the remote display)
BirthYear		 your year of birth (YYYY)
BirthMonth		 your month of birth (1-12)
BirthDay		 your day of birth (1-31)
 
Output variables:
 
LastUpdate		 last update of variables (Unix timestamp)
Weight		         measurement
BMI		         Body Mass Index
BMI_HUF		         Body Mass Index Human Understandable Format (HUF) (adults only)
Fat		         percentage body fat
Fat_HUF		         percentage body fat HUF (adults only)
Water		         percentage body water
Water_HUF                percentage body water HUF