At 07:20 AM 1/4/01 -0500, Eric wrote: > > At 07:49 AM 1/2/01 -0500, you wrote: > > >Hello, Robert, > > > > > >How do you know all these saturation vs humidity values?? It is great > > >if they are accurate and repeatable. I live in the Toronto area and > > >have a green house in the back yard. I have always been trying to > > >control the temperature and humidity inside it so I can do hydroponics. > > >As you probably know there is no simple ways we can tell those from > > >Walmart if they are any good in humidity measurement! I therefore wish > > >to obtain further info or calibration methods at different % points from > > >you. Also, if there are points and tips I need to know to increase > > >calibration accuracy please tell me. Equilibrium relative humidity (i.e., water activity) of saturated salt solutions @ 25 C: NaCl 75.28% KNO3 92.48% KCl 84.26% KBr 80.71% (NH4)2SO4 79.97% NH4Cl 77.10% NH4NO3 61.83% LiCl.H2O 11.05% NaCl is the most convenient to work with, since it's cheap and has a low tempco (less than 1% change from 15 C to 37 C). Unsaturated solutions of NaCl will provide RH values between 75 and 100%: w/w % RH or Aw @ 15 C RH or Aw @ 35 C 23.50% 79.5% 79.5% 22.50% 80.8% 80.8% 21.50% 82.1% 82.1% 20.50% 83.3% 83.3% 19.02% 85.2% 85.0% 18.38% 85.9% 85.7% 17.70% 86.7% 86.4% 17.01% 87.4% 87.2% 15.25% 89.1% 88.9% 13.42% 90.8% 90.6% 11.98% 92.1% 92.0% 10.71% 93.0% 92.9% 8.25% 94.9% 94.8% 6.18% 96.3% 96.2% 2.44% 98.6% 98.6% For lower RH, use solutions of H2SO4, NaOH or CaCl2. There are a lot of difficulties in measurement of Aw or RH. Even the best measurements have perhaps a true accuracy of +/- 0.5%. Rapid measurement sensors exhibit severe hysteresis and are generally accurate only to +/- 2% even when calibrated. The most accurate measurements of RH are based on a thermopile chilled mirror and a dew point sensor. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.