Page 20 - 6_TOC in daily practice

SCA-130-610
An automatic dilution function and the self-
monitoring of calibrations permit a largely in-
dependent operation of the analyser. From a
remote control room the TOC-4200 can be
started and calibrated. Additionally it is possi-
ble to select which sample stream is to be
measured at which time. Versatile communi-
cation interfaces allow the registration of
threshold limit values or maintenance re-
quests.
■ Sampling technique
Various sample preparation systems, which
can be matched optimally to the scope of ap-
plication, are available for the TOC-4200.
Temperature and pressure resistant stainless
steel is an ideal choice for the sampling of
cooling water. A counter-flow type sampling
mechanism is a good choice in this case:
It consists of a stainless-steel pipe bend, into
which a sampling capillary is inserted. The
TOC-4200 draws the flowing sample against
its direction of flow through the capillary. After
sampling, the capillary is rinsed backwards
with water.
Sampling
Direction of flow
Fig.:
Counter-flow sampler
for 3 streams
This way the sampling device does contain as
less moving parts or filters as possible and re-
quires low maintenance effort. When no sam-
ple is taken, the flowing sample carries parti-
cles away from the capillary so that no clog-
ging occurs.
The number of sample streams the TOC-4200
monitors, can be optionally extended to a max-
imum of six.
■ Measurement method
The TC method is often used for the monitor-
ing of cooling water. In this case the sum of
organic (TOC including POC) and inorganic
(TIC) carbon compounds is determined.
For this purpose, the sample is drawn with a
syringe pump from the continuous sample
stream and then injected into the oxidation unit
using a septum-free slider mechanism. Using
TC analysis with its low cycle times of 2-3
minutes allows for a very fast process adjust-
ment. The dilution function of the syringe
pump enables dynamic expansion of the
measurement range so that exact values are
obtained even if a heat exchanger ruptures.
No additional detectors are necessary for this.
■ TC response behavior
A TOC-4200 was calibrated to a measurement
range of 10 mg/l. A sample with a TOC content
of 0.1 mg/l is continuously analyzed until a
sudden contamination up to 6 mg/L TOC is
added to the sample.
6
5
4
TC in mg/l 3
2
1 1 6 1116 212631 364146 515661 667176 81
0
values of 0.098 mg/l for normal load and 5.78
The average cycle time is 2:38min at mean
mg/l for burst contamination.
■ Recommended analyser / Configuration
TOC-4200
Counter-flow type sampling mechanism