TECHNICAL INFORMATION
17
87-900-120-01 (A)
Protection level 1 (enabled in factory):
The backflow valve fitted in series with the sampling
line is activated preventing further pumping of the
gases rich in helium and, at the same time, "purging"
the entire sampling line with helium-free atmospheric
gases (sucked in from the exhaust point or in a helium-
free environment using a dedicated line - see note).
Functioning of the backflow valve is showed in the
figure below.
Outlet
Probe
Sensor
P
Backflow
valve
1
1
2
> Normal operation
> Protection mode
1
2
Restricting delivery of helium to the sensor, this
protection extends its lifetime, also guaranteeing
maximum sensitivity.
As it restricts the amount of helium conveyed to the
sensor, tripping of the backflow device shortens waiting
times between one measurement and the next (zeroing
time).
Protection level 2:
Temporary switching off the heating element of the
sensor drastically reduces the permeability of the
sensor to helium, increasing the total level of
protection.
NOTES:
The line dedicated to suction of the cleaning gas must
have a maximum length of 5m and consists of
4x0.75mm tube.
The position of the backflow valve
A
is shown in the
figure below.
ROUTINE MAINTENANCE OF THE PHD-4
A number of important routine maintenance operations
of the PHD-4 can be carried out in field.
The components on which routine maintenance
operations can be carried out are the Ni-MH battery,
the sampling pump and the filter system.
To access the components, remove the plastic
enclosure following the instructions given below.
Removal of the enclosure.
Make sure that the device has been switched off
following the procedure described in the “user
interface” paragraph.
1. Position the PHD-4 on a flat, rigid surface able to
support its weight, as shown in the figure below;
2. Press down and at the same time turn the
“Quicklock” release device in a counterclockwise
direction.
A click and a rotation of a ¼ of turn indicate that the
enclosure has been released
A
