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• The alternate value for C'S is obtained from a fit to measured data found here.  This is a fit to measured data that covers much higher resistivities.  The equation is log(C's)=(log(1553)-log(ρ))/0.7465, where C'S is in g/l and ρ is in Ω-cm.

• You should use de-ionized water and copper electrodes to reduce resistance change over time.  

• Example:  Suppose you want to make a 1000 ohm resistor out of 1" inner-diameter Tygon tubing with copper electrodes inserted at each end with 1 foot between electrodes.  The calculator gives C's=19.653 g/l and C'w= 991.41 g/l.  So, to make one liter of solution, fill a jar with 991.41 g of water and 19.653 g of CuSO₄*5H₂O (or make a smaller batch keeping the same proportions).  Note:  you could just add the amount, C's, to one liter of water with negligible error (you have to do this for the alternate version of C's).  Also, if you want to make just enough then use the volume (in liters) from the calculator and multiply this by C's to get the amount of CuSO₄*5H₂O you need, then fill with water.

• CuSO₄*5H₂O is know by several names (copper sulfate, cupric sulfate, cupric sulfate pentahydrate,...) and appears as blue crystals.

• Calculator assumes solution temperature of 20°C (68°F).  Actual resistance is a function of water temperature.

• In the table below, the left three columns are data from the CRC Handbook of Chemistry and Physics.  H% is the percent of hydrate (CuSO₄*5H₂O) by weight in the solution.  D²⁰₄ is the relative density of the solution at 20°C compared to water at 4°C.  γ is the specific conductance at 20°C.  The right three columns are derived from the others for this calculator.  

• H% [by wt.]	D204	γ [mS/cm]	Cs'  [g/l]	CW'     [g/l]	ρ [Ω-m]
  1.56	1.0085	5.4	15.7	992.8	1.852
  3.13	1.0190	9.3	31.9	987.1	1.075
  4.69	1.0296	12.8	48.3	981.3	0.781
  6.26	1.0403	16.0	65.1	975.2	0.625
  7.82	1.0511	19.0	82.2	968.9	0.526
  9.39	1.0620	21.9	99.7	962.3	0.457
  10.95	1.0730	24.6	117.5	955.5	0.407
  12.52	1.0842	27.2	135.7	948.5	0.368
  14.08	1.0955	29.7	154.2	941.3	0.337
  15.64	1.1070	32.2	173.1	933.9	0.311
  17.21	1.1186	34.4	192.5	926.1	0.291
  18.77	1.1304	36.6	212.2	918.2	0.273
  20.34	1.1424	38.6	232.4	910.0	0.259
  21.90	1.1545	40.5	252.8	901.7	0.247
  23.47	1.1669	42.3	273.9	893.0	0.236
  25.03	1.1796	44.0	295.3	884.3	0.227
  26.59	1.1926	45.6	317.1	875.5	0.219
  28.16	1.2059	47.0	339.6	866.3	0.213

• Note that CRC Handbook gives values for Cs and Cw, but these count the 5H₂O as part of the solution, not the solute, which would result in a slight error if not accounted for (this is why my values have the prime symbol).

• Density of 20^oC water is actually 0.99823 [g/ml] from this table.

• If you want to use anhydrous CuSO₄ (gray powder) then multiply amount of solute by 0.639 (you need less) and use more water given by the difference.

Copyright ©2003-2021  Raymond J. Allen