SpectraPure
®
SpectraPure
®
Inc.
480.894.5437 Call us toll-free 1.800.685.2783
2167 East Fifth St, Tempe, Arizona 85281
18
CALCULATING EXPECTED OUTPUT FROM THE MEMBRANE
Membranes produce the rated gallons per day (GPD) at 60 psi (4.1 bars) operating pressure, 77°F (25°C) operating temperature and
500 ppm total dissolved solids.
Membrane output gallons per day (GPD) depends on operating pressure, water temperature and the ppm TDS in the feed water.
Expected GPD = Rated GPD × PCF × TCF × OCF
PCF is the pressure correction factor, TCF is the temperature correction factor and OCF is the osmotic correction factor.
1. Calculation of PCF:The output GPD from the membrane are directly proportional to the applied pressure. Note: The membrane
is rated to produce the rated GPD at 60 psi. For any pressure other than 60 psi the output GPD is multipled by the PCF (Pressure
correction factor).
PCF = Pressure available (in psi) ÷ 60
2. Calculation of TCF: The output GPD also decreases with decrease in temperature. This is because the water viscosity increases with
decrease in water temperature.
The GPD increases by approximately 3% for every °C rise in temperature. (Refer to the following table for TCF values).
3. Calculation of OCF: The output GPD of the system decreases with an increase in ppm TDS of tap water. This is because the osmotic
pressure increases with an increase in ppm TDS, and the increased osmotic pressure reduces the net driving force for pure water
passage through the membrane. The osmotic pressure is approximately 1 psi for every 100 ppm TDS.
NOTE:
Generally speaking, the effect of osmotic pressure can be neglected for most tap water supplies and the OCF may be
assumed to be 1.
Example:
What is the expected GPD from a 90 GPD system at 40 psi pressure and 60°F water temperature?
PCF = 40 ÷ 60 = 0.666
TCF = 0.754 (from Table 1)
OCF = 1
Expected GPD = 90 x 0.666 x 0.754 = 45.2 GPD
°F (°C)
TCF
°F (°C)
TCF
°F (°C)
TCF
41.0 (5)
0.521 59.0 (15) 0.730
77.0 (25)
1.000
42.8 (6)
0.540 60.8 (16) 0.754
78.8 (26)
1.031
44.6 (7)
0.560 62.6 (17) 0.779
80.6 (27)
1.063
46.4 (8)
0.578 64.4 (18) 0.804
82.4 (28)
1.094
48.2 (9)
0.598 66.2 (19) 0.830
84.2 (29)
1.127
50.0 (10)
0.620 68.0 (20) 0.857
86.0 (30)
1.161
51.8 (11)
0.640 69.8 (21) 0.884
87.8 (31)
1.196
53.6 (12)
0.661 71.6 (22) 0.912
89.6 (32)
1.232
55.4 (13)
0.684 73.4 (23) 0.941
91.4 (33)
1.267
57.2 (14)
0.707 75.2 (24) 0.970
93.2 (34)
1.304
Table 1: Temperature Correction Factor
(TCF)
