Conditions that will cause calcium carbonate precipitation exist in most domestic hot water heaters and virtually all of the water injection wells used by the American oil producing industry. After discovery in 1936 of the precipitation inhibiting powers of sodium hexametaphosphate, at very low concentrations, its use exploded to many water handling applications where scale was troublesome. Very little research was undertaken to examine this phenomenon. Little research could be justified because treatment costs were so low and benefits were so great. The use of sodium hexametaphosphate, known as "threshold treatment", was originally intended to prevent internal scaling of water lines and valves. It certainly accomplished this job, but no one demonstrated whether calcium was mainly held in solution or precipitates were prevented from sticking. Bottle tests suggested that calcium remained in solution. However, the data were certainly sketchy for extrapolation to much more scale forming oil field brines held at higher temperatures under pressure. The research reported herein deals with the influence of all these variables. Also considered are the very important effects of pressure, surface roughness of the container, time, proprietary inhibitor brand (liquid organic phosphates) and concentration. To simulate a domestic hot water heater or oil field water injection well condition a new test apparatus was developed and patented. It consists of two chambers separated by a free piston fitted with an 0-ring. The test water is contained below the piston, and sealed from the pressuring gas above the piston by the 0-ring. Data are presented to show that a given water under field pressure conditions precipitates more calcium carbonate as the temperature is increased. If the test is run at atmospheric pressure, as done by all previous investigators, even more calcium carbonate precipitates. It is shown that 100 percent inhibition can be achieved. For the very scale-forming water used as our principal model only a few ppm of the chemicals provide complete inhibition under pressure in the test apparatus. At atmospheric pressure 500 ppm of inhibitor provide only 50 to 60 percent inhibition in one oil field brine. The empirical Stability Index, a measure of calcium carbonate precipitation tendency developed by previous investigators, appears to be a reliable measure of precipitation and thus inhibitor requirements. In addition, the data herein indicate a good possibility that a costeffective ranking of inhibitors would remain the same in many waters. The actual dosage of each chemical to achieve 100 percent inhibition would change, but cost rankings would remain the same.