I read Dom Ruggeri's comments about diluent water for watermix metalworking fluids with interest.  It is good to see someone highlighting the significant effect that water quality has on the performance the fluid as this is often overlooked by the fluid user. Hell, following dilution the water can be as much as 97% of the usable fluid.   In relation to the testing of water hardness and to allow relatively easy on site testing it is possible to purchase a dispensable test paper for this purpose.  Similar in type to pH type test papers, these can monitor approximately water hardness up to 600 ppm as calcium carbonate. For further information contact Tektrak in the UK on fax 44 161 877 1303. I have found you email magazine extremely interesting,  POLARTECH (http://www.polartech.co.uk/)