44
Calculating TDS Conversion Factors
You can calibrate your meter using TDS calibration standard
solutions. The calibration standard only needs to give the
TDS value at a standard temperature such as 25°C. To deter-
mine to the conductivity-to-TDS conversion factor use the
following formula:
Factor = Actual TDS ÷ Actual Conductivity @ 25°C
Definitions:
Actual TDS:
Value from the solution bottle label or as a
standard you make using high purity water and precisely
weighed salts.
Actual Conductivity:
Value measured using a properly cali-
brated Conductivity/ Temperature meter.
Both the Actual TDS and the Actual Conductivity values
must be in the same magnitude of units. For example, if the
TDS value is in ppm the conductivity value must be in µS; if
the TDS value is in ppt the conductivity value must be in mS.
Check your factor by multiplying the conductivity reading by
the factor in the above formula. The result should be the
TDS value.
Appendix 1: Conductivity to TDS Conversion
Factors
1. Factor
—the conductivity to ppm TDS conversion factor.
Multiply conductivity by this factor to get ppm TDS for the
type of TDS reading needed.
2. 442
—a formulation that most closely represents the con-
ductivity to ppm relationship, on average, for naturally
occurring fresh water.
3. TDS Your Material
—These columns are for you to write
in your application-specific conductivity-to-ppm values and
conversion factors for future reference.
Factor = actual TDS ÷ Actual Conductivity @ 25°C
Conductivity
TDS KCl
TDS NaCl
TDS 442
TDS Your Material 3
at 25°C
ppm
Factor 1
ppm
Factor 1
ppm
Factor 1
ppm
Factor
Value
Value
Value
Value
84 µS
40.38
0.5048
38.04
0.4755
50.50
0.6563
447 µS
225.6
0.5047
215.5
0.4822
300.0
0.6712
1413 µS
744.7
0.5270
702.1
0.4969
1000
0.7078
1500 µS
757.1
0.5047
737.1
0.4914
1050
0.7000
8974 µS
5101
0.5685
4487
0.5000
7608
0.8478
12880 µS
7447
0.5782
7230
0.5613
11,367
0.8825
15000 µS
8759
0.5839
8532
0.5688
13,455
0.8970
80 mS
52,168
0.6521
48,384
0.6048
79,688
0.9961
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