Eco Physics CLD 780 TR - Introduction

E C O P H Y S I C S Calibration
CLD 780 TR / July 2000 82
6.1 Introduction
Appropriate to greater demands for high accuracy, a carefully performed
calibration of the analyzer is essential. As a calibration method for this
kind of high sensitivity analyzer, only the dilution method is suitable. Since
large dilution ratios are necessary it is also very important to work with
very pure gases and air. The linearity and stability of the flow controllers
is equally crucial. When calibrating normally single point or multi point
calibrations are possible. If a one point calibration is done it is called a
”Span Check“. For a complete calibration the linearity of the analyzer has
to be checked over the selected measurement range. This can only be ac-
complished by checking several concentrations within that range. Finally
the correct NO
2
and NO
x
concentrations depend on the accurate deter-
mination of the converter efficiency (W
K
). It is known that with increasing
operating hours the converter efficiency is reduced. When involved in
ambient air monitoring the EPA (U.S. Environmental Protection Agency)
demands that the converter be exchanged when its conversion efficiency
drops below W
K
<96 % in order to prevent an excessive dependability on
correction factors.
If the CLD 780 TR is used, combined with the PLC 762 photolytic con-
verter, a totally new aproach is possible:
The analyzer can automatically determine W
K
(see also section 5.2.2.7.2
Calibration of the PLC 762). In addition the analyzer automatically stores
the corresponding correction factor and performs a measurement value
adjustment for NO
2
and NO
x
on a continuous basis.
The only reliable method, in the opinion of the manufacturer, for determin-
ing the conversion efficiency W
K
is gas phase titration (GPT). In GPT, NO-
calibration gas is titrated with ozone. A portion of the NO is oxidized to
NO
2
. Then the calibration gas is diluted with zero air to the desired con-
centration. All other known methods such as permeation tubes and NO
2
calibration gas can produce errors that lead to unacceptable inaccura-
cies. Therefore these methods are considered inferior to the GPT method.
Appendix 3 contains a detailed description of the calibration procedure
using GPT as accepted by EPA. Paragraph 6.2.4 is a short description of
the performance of a GPT utilizing a special calibration instrument espe-
cially suited to the task.
Periodic adjustment of the zero point is not necessary for the CLD 780 TR
as long as the normal ”MEAS“ mode is used for the continuous monitoring
function. Should the analyzer not exactly respond with 0 when sampling
zero air , there could be several reasons:
– the zero air is not absolutely free of NO or NO
2
– the inside walls of the sample tubing desorb captured NO and NO
2
resp.