Auto Calibration Checking - Probes; Purge & Calibration Check Accessories; Filter Purge Pressure Switch - Novatech 1635 User Manual

1.11 AUTO CALIBRATION CHECKING – PROBES
On-line automatic gas calibration check is not normally required. Where it is required however, the probe can be
checked for accuracy in-situ and on-line. Solenoid valves can admit a calibrated gas mixture into the probe under
microprocessor control on a timed basis. For details on installation refer Section 2.13. For details on setting up this
facility refer to Set-up steps 49 to 54 in Section 4.5.
During probe auto calibration checking, the analyser output will freeze and remain frozen for a further adjustable
period, allowing the probe time to recover and continue reading the water vapour level.
Calibration check gases may be manually admitted by pressing the 'CAL' buttons on the keyboard while in 'RUN'
mode. The analyser output is frozen during the pressing of these buttons and immediately becomes active when the
button is released.
1.12 PURGE & CALIBRATION CHECK ACCESSORIES
Due to the absolute measurement characteristics of zirconia sensors and the self-calibration features of Novatech
analysers, probe calibration checks with calibrated gas are not normally required. In some installations however,
automatic gas calibration checks are required by Environmental Protection Authorities and by engineering management
in Power Stations, Oil Refineries and similar large fuel users.
Novatech probes and analysers provide a ready method of connecting on-line calibration check gases. They provide on-
line automatic checking of probe and analyser calibration, as well as a probe purge facility.
The absolute characteristics of zirconia sensors require only one calibration check gas to properly check the probe's
performance.
Dirty flue gas applications often require the back purge facility to keep a probe filter free from blockage. (In these
applications, it is more reliable to install probes pointing vertically downwards with no filter). Purge and calibration
check solenoid valves can be operated manually or automatically from a 1635 analyser.
The external components required for automatic / manual gas calibration checking are:
•
A calibration check gas flow meter / regulator
•
A mains voltage (240 or 110 VAC) solenoid valve for calibration check gas
The external components required for automatic / manual purging are:
•
A mains voltage (240 or 110 VAC) purge solenoid valve
•
A purge pressure switch, 0 to 35 kPa (0 to 5 psi), to test for filter blockage.
The user should supply:
•
Span gas cylinder, typically 8 % oxygen in nitrogen or a similar percentage of O2 close to the normal level in
the gas stream being measured, to ensure fast recovery.
•
A 100 kPa (15 psi) clean and dry instrument air supply when filter purging is required.

1.13 FILTER PURGE PRESSURE SWITCH

To automatically sense a blocked probe filter, a pressure sensor should be connected to the 'purge' line to the probe
'cal' port. It should be adjusted so that it energises just above the purge pressure with a new or clean filter installed.
The switch contacts should be connected to terminals 12 & 13 (PURGE FL SWITCH).
If the filter is still blocked or partly blocked after an auto purge cycle, the pressure switch will energise and cause a
'Probe Filter Blocked' alarm. The contacts must be normally closed.
The pressure switch should have an adjustable range of 0 to 100 kPa (0 to 15 psi).
August 2009
1635 Water Vapour Analyser 11