Dräger SE Ex HT M, Инструкция по эксплуатации, Страница: 34 / 60

Intended Use
34
3. Long-term gassing with methane at very low temperatures
If the sensing heads Polytron SE Ex PR M or SE Ex HT M are operated at very low
temperatures and methane/air-mixtures are applied, the measuring signal at long-
term exposition may decrease after alarm activation and may lead to misinterpre-
tation.
A comprehensive study of this effect showed that the measuring signal during
long-term gassing with 40 %LEL methane at –50
o
C may decrease to 20 %LEL
after 90 minutes.
1)
Consequently, if a gas alarm occurs, countermeasures need to be taken immedia-
tely. The decrease of the measuring signal shall not lead to the assumption that
the gas concentration has decreased.
We recommend to have latching alarms on the associated central controllers and
not to reset these alarms without having ensured a safe condition by means of an
independent gas concentration measurement.
Important Remark
Although the sensing head has been factory-tested before delivery, the commission-
ing after installation must include the zero- and span-adjustment.
The commissioning has to be terminated by a function test of the complete gas
detection system.
1)
At temperatures above –50
o
C this effect is less, e.g. the measuring signal at –10
o
C
decreases by 8 %LEL max. after 90 minutes gassing with 40 %LEL methane. For other gases
the signal decrease at long-term exposure at low temperatures is considerably lower.