Cambridge Sensotec Rapidox 1100ZR-H-PFC-E O
2
Analyser Instruction Manual
Version 1.2
12
Last printed 22/07/2015 18:08:00
2.
LOG (ppm):
This setting produces an output that is scaled logarithmically
between 0V and 10V (or 4 and 20mA). So, for example, if the scale is set to 0V
= 0.0001% (1 ppm) and 10V = 100% (1000000 ppm) then 5V would indicate
an oxygen reading of 0.1% (1000 ppm). This scale is more suitable for large
ranges of oxygen down to very low ppm levels. Note that because the scaling
is logarithmic 0V cannot be set to 0% oxygen since log(0) is -
∞
! The lowest
permitted setting is 0V = 10
-20
ppm (or 10
-24
%) O
2
. The scaling of this example
is shown in the blue line in following plot, from which other intermediate
values can also be read. For comparison, the red line shows the limitations of
using the linear oxygen output mode over the same range.
Figure 5: Graph showing the logarithmic output mode option for oxygen (blue) compared with
linear mode (red).
The following general formulae can be used to calculate the oxygen from the
voltage or current outputs when set to the log output mode:
Output type
Formula for calculating log
10
(O
2
) from analogue output signal
0-10V
log
10
(O
2
) = V
out
/ 10 * log
10
(O
2H
/ O
2L
) + log
10
(O
2L
)
4-20mA
log
10
(O
2
) = (mA
out
– 4) / 16 * log
10
(O
2H
/ O
2L
) + log
10
(O
2L
)
Where:
O
2L
= user setting of oxygen for 0V or 4mA output
2
O
2H
= user setting of oxygen for 10V or 20mA output
2
V
out
= the measured voltage output in volts
mA
out
= the measured current output in mA
2
O
2
, O
2L
and O
2H
must all be in the same units. Note: O
2
= 10
log(O2)
"Log(ppm)": Log output mode
0
1.25
2.5
3.75
5
6.25
7.5
8.75
10
0.0001
0.001
0.01
0.1
1
10
100
O
2
(%)
O
2
V
o
lt
s
o
u
tp
u
t
4
6
8
10
12
14
16
18
20
-4
-3
-2
-1
0
1
2
log(O
2
(%))
O
2
m
A
o
u
tp
u
t