49
48
13. Appendix 2: Calculating TDS Conversion Factors
You can calibrate your meter using TDS calibration standard solutions. The calibra-
tion standard only needs to give the TDS value at a standard temperature such as
25°C. To determine 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 calibrated 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.
12. 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 r
epr
esents the conductivity to ppm r
elationship, on average, for naturally occurring fr
esh
water
.
3.
TDS Y
our Material
—These columns ar
e for you to write in your application-specific conductivity-to-ppm values and conversion
factors for futur
e r
efer
ence.
Factor = actual TDS ÷
Actual Conductivity @ 25°C
C
onductivity
TDS KCl
TDS NaCl
TDS 442
2
TDS Y
our Material
3
at 25°C
ppm Factor
1
ppm Factor
1
ppm Factor
1
ppm
Factor
V
alue
V
alue
V
alue
V
alue
84 µ
S
40.38
0.5048
38.04
0.4755
50.50
0.6563
447 µ
S
225.6
0.5047
215.5
0.4822
300.0
0.6712
1413 µ
S
744.7
0.5270
702.1
0.4969
1000
0.7078
1500 µ
S
757.1
0.5047
737.1
0.4914
1050
0.7000
8974 µ
S
5101
0.5685
4487
0.5000
7608
0.8478
12880 µ
S
7447
0.5782
7230
0.5613
11,367
0.8825
15000 µ
S
8759
0.5839
8532
0.5688
13,455
0.8970
80 mS
52,168
0.6521
48,384
0.6048
79,688
0.9961
