Dräger Polytron 8900 UGLD Instructions For Use Manual page 10

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Description
In the example, A1 is triggered at 66 dB and A2 is triggered at
72 dB.
110 dB
50 dB
+ 12 dB
+ 6 dB
Alarm delay and analyzed time
If there is a possibility of short background noise peaks, it is
reasonable to install an alarm delay. An alarm delay helps to
avoid false alarms by ignoring spurious sound peaks not
generated by a leak. Ultrasound peaks not created by a leak
can arise e.g. by the fizz of detaching compressed air hoses,
active pressure release valves, the jingling of tools falling to
the ground or similar incidents. These sounds shall not trigger
an alarm even if the detected sound levels breaches the
alarm thresholds for a short time. Real leaks are
characterized by generating a continuous signal. An alarm
shall be triggered only if the signal is breaching the alarm
threshold for a defined alarm delay time (Delay) within a time
frame (Analyzed time) set to be analyzed.
Concept of the parameter 'Delay' in the menu
All measurement data points (one per second) within 60
seconds are sorted by value. The parameter Delay (say e.g.
20 seconds) will provide the 20
this time frame. Here, it does not matter whether the larger
data are received in one sequence or at different moments
within the 60 seconds.
Concept of the parameter 'Analyzed time' in the menu
The parameter Analyzed time defines the number of
measurement data points (length in seconds) that are used to
determine the measurement value given by the parameter
Delay. The default value of the analyzed time is 60 seconds.
This parameter can be changed by the DrägerService.
Examples
A If a normal delay is wanted, like: within the delay time the
signal shall not drop below the alarm threshold and after
the delay time the alarm shall be triggered. Here, the
parameters Delay and Analyzed time shall be set to the
same number.
Possible disadvantage: In usual industrial setups the
ultrasonic signal by a leak can vary by e.g. shadowing
effects (passing vehicles, changing environments, moving
pressure vessels, etc.). Once the signal drops below the
alarm threshold, the delay time starts over again. It is
recommended to allow a few data points to drop below
10
A2 at
72 dB
A1 at
66 dB
Background
noise level
60 dB
th
largest value measured in
Instructions for use
alarm level without losing too much time in detecting the
leak. If a delay of 20 seconds within an analyzed time of
60 seconds is used, the alarm is triggered as soon as 20
measurement data points within 60 seconds are higher
than the alarm threshold. If there is no repeated or
persistent background signal to be expected, it reacts
exactly like a normal delay. This may be a valid setup.
B If there is a recurring background signal (e.g. the UGLD is
located near a large metal punch or release valve with
regular gas deflation events) that extends repeatedly
beyond the alarm level, the delay shall be set to a longer
time, preventing to trigger an unwanted alarm by adding
up background noise events.
Possible disadvantage: the delay may be too long for a
quick detection of a leak and a safe reaction to it. In order
to react to a leak quickly, the parameter Analyzed time
shall not be chosen to be much larger than the delay.
Thus, the alarm is triggered once the moments between
the background signals that are usually below the alarm
threshold are filled by the sound of the leak.
These examples show the concept of the parameters Delay
and Analyzed time. The alarm delay and analyzed time,
which define the time where no alarm is triggered, should be
optimized for each installation. Dräger recommends alarm
delays of 10-30 seconds depending on the duration of
spurious ultrasound events. Alarm delays of up to 60 seconds
can be set by the customer. If longer delays are needed, the
DrägerService can install delays of up to 240 seconds.
Signal
Background signals
Alarm 2
Alarm 1
UGLD signal
No of datapoints >A
analyzed
time
delay
delay timing for A1
3.4.4 Placement / detection coverage
Instruments are normally installed at heights of 3-5 m (approx.
10- 16 ft.) above possible gas leak locations.
The recommendation that the instruments have to be installed
above possible gas leaks may result in installations awkward
to reach for maintenance and configuration.
The optional remote sensing head that can be installed
remotely from the transmitter allows the display to be easily
accessible for configuration. Remote sensor cables are
available in lengths of 5 m, 15 m, and 30 m (approx. 15 ft., 50
ft. and 100 ft). The remote sensing head can also be used to
avoid interferences such as vibrations or precipitation.
®
| 8900 UGLD Firmware version ≥ 4.0.x
Dräger Polytron
leak
signal
Alarm 2
Alarm 1
time
delay timing for A2
time