4 correction of temperature errors, 5 output processing, Calibration – Campbell Scientific 247-L Conductivity and Temperature User Manual
Page 9: 1 conversion factors, 2 typical ranges, 3 factory calibration
247 CONDUCTIVITY AND TEMPERATURE PROBES
5
25:
P34
Z=X+F
01:
3
X Loc Ct
02:
-.00378 F
03:
3
Z Loc [:Ct ]
26:
P94
Else
27:
P55
Polynomial
01:
1
Rep
02:
3
X Loc Ct
03:
3
F(X) Loc [:Ct ]
04:
-.02889 C0
05:
.98614 C1
06:
.02846 C2
07:
0.0000
C3
08:
0.0000
C4
09:
0.0000
C5
28:
P95
End
5.4 CORRECTION OF TEMPERATURE ERRORS
The effect of temperature on the sample
solution can cause large errors in the EC
measurement. A simple method of correcting
for this effect is to assume a linear relationship
between temperature and EC. This method
generally produces values to within 2% to 3% of
a measurement made at 25°C.
The best corrections are made when the
temperature coefficient is determined at a
temperature near field conditions. See Section
9 for details on how to determine the
temperature coefficient. If determining the
temperature coefficient is not possible, use a
value of 2%
°C
-1
as a rough estimate.
The following program segment implements a
previously determined temperature coefficient
(TC) and calculates the corrected conductivity.
29:
P34
Z=X+F
01:
4
X Loc Temp
°C
02:
-25
F
03:
6
Z Loc [:A ]
30:
P37
Z=X*F
01:
3
X Loc Ct
02:
100
F
03:
8
Z Loc [:100*Ct ]
31:
P37
Z=X*F
01:
6
X Loc A
02:
nnn
F Enter TC (%
°C
-1
)
03:
9
Z Loc [:TC PROCES]
32:
P34
Z=X+F
01:
9
X Loc TC PROCES
02:
100
F
03:
9
Z Loc [:TC PROCES]
33:
P38
Z=X/F
01:
8
X Loc 100*Ct
02:
9
Y Loc TC PROCES
03:
10
Z Loc [:C25mScm-1]
5.5 OUTPUT PROCESSING
Over large ranges, EC is not linear and is best
reported as samples (70). In limited ranges,
averaging (71) measurements over time may be
acceptable. Convention requires that the
temperature at the time of the measurement be
reported.
6. CALIBRATION
6.1 CONVERSION FACTORS
1 S (Siemens) = 1 mho = 1/ohm
Although mS·cm
-1
and µS·cm
-1
are the
commonly used units of EC, the SI base unit is
S·m
-1
. The result of the example programs is
mS·cm
-1
EC measurements can be used to estimate
dissolved solids. For high accuracy, calibration
to the specific stream is required. However, for
rough estimates, values between 550 and 750
mg·l
-1
/ mS·cm
-1
are typical with the higher
values generally being associated with waters
high in sulfate concentration (USGS Water-
Supply Paper #1473, p. 99). A common
practice is to multiply the EC in mS·cm
-1
by 500
to produce ppm or mg·l
-1
.
6.2 TYPICAL RANGES
Single distilled water will have an EC of at least
0.001 mS·cm
-1
. ECs of melted snow usually
range from 0.002 to 0.042 mS·cm
-1
. ECs of
stream water usually range from 0.05 to 50.0
mS·cm
-1
, the higher value being close to the EC
of sea water (USGS Water-Supply Paper 1473,
p. 102).
6.3 FACTORY CALIBRATION
The 247 is shipped with a cell constant
calibrated in a 0.01 molal KCl solution at 25.0°C
±0.05°C. The solution has a EC of 1.408 mS
cm
-1
.