Hach BioTector B3500s, User Manual

Page: 48 / 166

Page 48
4.2.4.3 VOC (POC) Analysis
BioTector Volatile Organic Carbon (VOC) analysis type is a combination of TC analysis followed by a TIC &
TOC (NPOC) analysis. The VOC result obtained from the VOC analysis type represents the Purgeable
Organic Carbon (POC) content of the sample. When both TC and TIC & TOC analysis are complete, the
flowing data is available:
•
TC result, as measured and displayed from the TC analysis.
•
TIC result, as measured and displayed from the TIC & TOC analysis. TOC result obtained from the
TIC & TOC analysis represents the NPOC.
•
TOC result including the VOC, which is calculated from the difference between the TC and TIC:
TOC
v
= TC
– TIC
The TOC result displayed in VOC analysis type includes the purgeable organic carbon (POC) present
in the sample. In other words the TOC result, obtained in VOC analysis type, is the sum of NPOC and
POC:
TOC
v
= NPOC + POC
•
VOC (POC) result, which is calculated from the difference between the measured TC (from the TC
analysis), and the sum of measured TIC and measured NPOC (from the TIC & TOC analysis):
VOC (POC) = TC
– (TIC + NPOC)
The NPOC result, as measured from the TIC & TOC analysis, is not displayed, it is only used to
calculate the VOC (POC) element in the sample. All displayed results can be programmed in the
system and sent as 4-20mA output signals to an external device.
•
In VOC analysis mode, it is possible to program BioTector to carry out scheduled TIC (TIC
s
) analysis
and measure TOC, which includes NPOC and POC, for samples which may contain volatile organic
carbon and the TIC concentration of the sample remains constant or changes slowly. With this
measurement, the TIC value is measured on a scheduled basis depending on the TC CHECK
concentration level programmed in Stream Program menu (see section
8.2.3 Stream Program for
details). In VOC analysis mode with TIC
s
, TOC result including the volatiles is calculated from the
difference between the TC and TIC
s
:
TOC
v
= TC
– TIC
s
•
Scheduled TIC (TIC
s
) measurement is unique to BioTector and is developed for applications such as
condensate return monitoring in the petrochemical sector where rapid response is required in the
event of a breakthrough of a volatile material. The traditional approach is to measure TC only,
however this can result in inaccurate process decisions as TIC can often be present in the samples.
In other words, any high TC response due to an increase in TIC levels may create unnecessary
alarms. Additionally, the TIC result is a valuable measurement in its own right as it may highlight other
specific or undesirable conditions within the sample such as the condensate return.
BioTector analysis types are optional features. If the BioTector is built as a TIC &
TOC system only, modifications in the system configuration will be required for VOC
analysis to be possible.
BioTectors built with the VOC analysis option can be programmed on site to operate
with the TIC & TOC only or TC only analysis types.