I am going to quote the exact sentence they used so folks understand exactly what they said.
"This was done by taking into account the water content that is always
present in KOH flakes and by verifying the sap values of our oils by
making single-oil liq. soap batches with each of our oils."
This is why we don't have to click a 90% purity box on that site like you do on Soapcalc. I still get consistently smaller amounts of KOH on SBM than SoapCalc with the 90% purity box checked.
Thanks, Susie. I took the time today to dig into this further. To add to Susie's findings, I also saw this on the Summer Bee Meadow (SBM) calc's front page: "...Calculations take into account the typical impurities percentage found in NaOH and KOH supplies...."
So if I'm understanding all this, the SBM calc does not factor in a lye excess at zero superfat as I was thinking. Instead they're doing a correction for KOH purity. For a recipe created with the SBM calc at to zero superfat and using KOH that meets SBM's (unstated) purity, the result should theoretically be a soap with zero lye excess, zero fat excess. I wish they'd just come out and state the amount of correction they're using so people can make an informed decision.
***
Although I can't make that update to the SBM calc, I can reverse-engineer what they are doing, and here is what I found:
I entered the Carrie-Irish Lass LS recipe of 10% castor, 25% CO, and 65% olive into three soap recipe calcs -- SBM, Soapcalc, and my personal calc. I can easily change the lye purity in my calc, so it's possible for me to reconstruct another person's recipe and really understand what they're doing. When I compare results from my calc with the other two, I can replicate their numbers with only a small error.
Recipe:
Castor 10%
Coconut oil 25%
Olive 65%
Total wt of fats 1000 g
0% superfat
KOH (liquid) soap recipes:
SoapCalc:
KOH = 205.5 grams (or ounces) in my calc when I set KOH purity = 100%
KOH = 205.75 grams (or ounces) in SoapCalc, if the 90% purity box is NOT checked.
Conclusion:
If you do NOT check the 90% box in Soapcalc, the answer for KOH is based on 100% purity.
KOH = 228.33 grams (or ounces) in my calc when I set KOH purity = 90%
KOH = 228.61 grams (or ounces) in SoapCalc, if the 90% purity box IS checked.
Conclusion:
If you DO check the 90% box in Soapcalc, the answer for KOH is based on KOH purity = 90%. No surprise there, but it's a double check that results from my calc are similar to Soapcalc.
Summer Bee Meadow:
Using the same recipe in SBM at its unknown KOH purity, I got this answer: KOH = 218.8 ounces.
I then plugged different KOH purity values into my calc and adjusted the purity to get about the same 218.8 ounces of KOH.
When I got to KOH purity = 94%, my calc returned KOH = 218.62 ounces.
Conclusion: The
SBM calc is based on a KOH purity = 94%. This result is true for both the SMB basic calc and the SBM advanced calc.
So, yes, SBM and Soapcalc are going to give different answers because they're based on distinctly different starting assumptions. There's more discrepancy between the two calcs from the % purity than from any variation in their sap values, at least for olive, castor, and CO. The variable results that people are seeing in their liquid soaping adventures is partly due to the two calc's quite different assumptions about KOH purity and because LS is so very sensitive to any variations in superfat.
My suggestion is to check the purity of your KOH as supplied and choose the calc that is based on the KOH purity closest to what you're actually using. And, if using Soapcalc, change the lye solution concentration to 25% OR change the water:lye ratio to 3:1, whichever you prefer. Either one means the same thing.
Essential Depot KOH: 90% purity
Source:
http://www.essentialdepot.com/msds/
Lye Guy KOH: 96% purity
Source:
http://www.thelyeguy.com/store.php
NaOH (bar) soap recipes:
Here are the results for SBM and Soapcalc using the same recipe shown above, just set for NaOH now:
SoapCalc: NaOH = 146.7 at Soapcalc's unknown purity setting
SBM: NaOH = 146.52 at SBM unknown purity setting
When I reverse engineered these results in my calc, I got NaOH = 146.47 at NaOH purity = 100%.
Conclusion: Both
SBM and Soapcalc are based on an NaOH purity = 100%
Essential Depot NaOH: 95-96% purity
Lye Guy NaOH: 96% purity
Sources: See above in this post
Due to the hygroscopic nature of NaOH (absorbs water very easily from the air), the purity of NaOH drops with time. These calcs are basically building in a hidden superfat of at least 4% when using NaOH from common suppliers.
I hope this helps....
***
7 March 2015 update:
I checked several more online
soap recipe calculators to find out what lye purity levels they were using. The attached PDF shows my findings. The summary is this:
All of the recipe calcs are set to about 100% purity for NaOH. This is building in a hidden superfat to almost all bar-soap recipes, since most NaOH that soapers use is not 100% pure.
Most of the calcs are set for 100% KOH purity as well, with
three exceptions: SoapCalc (90% or 100%), Brambleberry (95%), and Summer Bee Meadow (94%).
My suggestion for making liquid soap is to check the purity of the KOH you are using and choose the calc that is based on the KOH purity closest to what you're actually using. By choosing the "wrong" calc for your KOH, you could end up with liquid soap that is lye heavy or fat heavy --
Example 1: You use Soapcalc set at 90%, set the superfat at 3%, and use 95% pure KOH. Your soap could have about 2% too much lye (lye heavy).
Example 2: You use a calc set for 100% KOH purity, set the superfat at 3%, and use 95% pure KOH. Your soap could have an actual superfat of 8%. This excess fat may make the liquid soap cloudy and may even separate from the soap after dilution.
If someone wants me to check a calc not on the list, please send me a PM (private message) and I will look into it.
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9 June 2015 update
Sometimes your KOH purity may be quite different than what any of the calcs are based on. So another way to correct the KOH purity is to use the calc you like best and adjust the KOH weight to correct for the purity of the KOH you are using --
KOH you need = (KOH based on calc's purity) X (Calc's KOH purity) / (Actual KOH purity)
Example 3: You use SBM calc which is set for 94% purity. Your actual KOH is 85%. The calc says you need 145 g of KOH at the calc's default purity. The weight of KOH that you need at your actual purity of 85% is this:
KOH you need = (145 g) X 94 / 85 = 160 g
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View attachment Dee on Lye purity for online calcs.pdf