[sienabrewer]

In reading Palmer he recommends that calcium levels in beer should be 50-150 ppm.  What he does not expand upon is what the ideal range of calcium needs to be for optimal mash conversion.  By optimal I mean, all others being considered (pH, temp, crush, etc.), the ability to get the best conversion possible.  I have read in some places that the Ca needs to be at or above 100ppm to get a more efficient conversion.  I.e. Ca around 60 ppm will not stand a chance of converting the mash nearly as good as a mash that has over 100 ppm (again, assuming all other variables are the same). 

[mrcceo]

The factors that affect conversion the most are, crush, mash temperature, pH, water to grain ratio, time, and grain diastatic power.  Calcium should be 50ppm at a minimum which will affect  pH but is more important for making sure that there is good protein precipitation in order to get a clearer wort and less chance of haze.

[a10t2]

IME, you can get full conversion without much calcium at all. I've brewed light lagers with ~20 ppm Ca and had no problems. In a decoction mash you could probably go even lower. The Congress reference mash is done with distilled water, and it's the benchmark for efficiency.

Like mrcceo said, there are other reasons for wanting some calcium though.

[wingnut]

My recolection is that Calcium has not effect on convertion.  As long as the PH is good, you should be able to convert with 0 Ca.  However, yeast like to see around 100ppm.  The Ca encourages yeast health and floculation.  With Ca at least 50 and ideally 100, your fermentations will be much more consistant, and your beer will clear well.

So the Ca is for the fermentation, not the mash.

[beardog]

Apparently Ca protects enzymes from being denatured to some extent but I don't think it is super-critical.

[joeysmokedporter]

Another experience here that you don't need Ca +100ppm for good conversion.

Water chemistry is complicated but boils down to three basic principles:

(1) Ca at least 50 ppm for good yeast health and performance during fermentation
(2) alkalinity in the water relative to the malts you are mashing needs to balance the pH to 5.2-5.6 range - use the Palmer resources and spreadsheet on-line to estimate the needed water profile and brewing salts based on the color of the beer.  Note that this *will* impact conversion and also flavor.
(3) balance SO4/Cl ratio based on the desired hop/malt profile of the beer, based on your brewing water profile and adding gypsum and calcium chloride, keeping in mind (1) and (2).

I highly recommend the 4 part Brew Strong podcast on water chemistry, which is the best explanation of this detailed subject I am aware of.

[tschmidlin]

You MUST have calcium in your mash, otherwise the amylase enzymes won't work at all.  That being said, there may be enough calcium in the malt to supply the amylase so any in the water is a bonus.  But the calcium in the water is important for other things as well.  I usually adjust mine to at least 100+ ppm for the mash and then don't do anything to my sparge water.  But I get good conversion even when I don't do it, and my water has 15 ppm Ca according to Ward Labs.

[thcipriani]

Fix, Principles of Brewing Science, 1999, pg 5

Calcium ions also tend to afford thermal protection for mash enzymes (Comrie, 1967). IN addition, they continue to interact with malt phosphate during wort boiling, and the ongoing reaction between calcium and phosphate is the primary reason that the pH decreases in the kettle boil. Calcium ions also tend to inhibit color formation during the boil and facilitate protein coagulation. Finally, calcium ions also influence beer fermentation. For example, they favorably affect yeast flocculation and beer clarification during maturation (Harrison et al., 1963; Saltukoglu and Slaughter, 1983; Taylor, 1990)

He goes on to say (on page 6):

A widely accepted rule in brewing is to have calcium concentrations of at least 50mg/L, and values in the range of 100-150 mg/L are very common.

Anecdotally, I've done mashes with water around 20 mg/L Ca++ and achieved 85% efficiency and I've fermented meads with 0 mg/L Ca++ that have achieved final gravities putting them into the dry range from an initial gravity north of 1.100.
 
However, when it comes to brewing, if you're going to take 4-8 hours of your day to make beer and George Fix and other sources of contemporary beery wisdom have accepted as an axiom that 50 mg/L Ca++ is desirable why not just aim for that as a minimum? Seriously, I'm asking - is there any reason compelling reason, other than anecdotal evidence, that suggests the best beer is made with Calcium concentrations below 50 mg/L or above 150 mg/L?