# 1.5 Calculating acceptable hardness at various temperatures (Water for coffee)

As one can see from the LI formula, rising temperature raises
the LI and the scaling potential. So it's useful to determine the
maximum hardness and alkalinity levels that don't cause scaling for a
given operating temperature. This requires solution of the following
rewriten LI* equation:

Max Hardness Allowed = Alog{44.01 - 14.58*log(T+273) - 2.739*log(A)}
or at higher alk. Alog{39.61 - 13.12*log(T+273) - 2.365*log(A)}

The following table gives some results for espresso boiler
temperatures (115C - 130C) and coffee brewing temperatures (90C -
95C):

```MAXIMUM NON-SCALING HARDNESS BY TEMPERATURE AND ALKALINITY
Alkalinity
C  Bar     33  37  40  43  47  50 53 57 60 63 67 70
-----------------------------------------------------
90   --    342 250 202 166 130 110 93 77 67 59 51 46
95   --    280 205 166 136 106  90 77 63 55 49 42 38
-----------------------------------------------------
115  0.67   130  95  77  63  49  42 37 31 27 24 21 19
120  1.00   108  79  63  52  41  36 31 26 23 21 18 16
125  1.33    89  65  53  43  35  30 26 22 20 18 15 14
130  1.67    75  55  44  37  30  26 22 19 17 15 13 12
-----------------------------------------------------
```

The Bar column gives the approximate boiler pressure that
equates to the stated temperature.
This table amplifies the standard recommendations of 90mg/l
hardness at brewing temperature and 50mg/l at boiler temperature. A
slightly better rule of thumb, whenever the hardness and alkalalinity
are roughly at the same magnitude, is to add the two together: then
coffee temperature water should not exceed 130 total, boiler water 80
total.

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