WIKA PXA - Manual (Page 2)

INSTRUCTION MANUAL
NI-290E
Rev. 0 01/05
1 - GENERAL
1.1 FOREWORD
The wrong choice of a series or a model, as well as the incorrect
installation, lead to malfunction and reduce instrument life. Failure
to follow the indications given in this manual can cause damage
to the instrument, the environment and persons.
1.2 ALLOWED OVERRANGES
Pressures exceeding the working range can be occasionally
tolerated provided they remain within the limits stated for the
instrument (proof pressure). Continuous pressures exceeding
the working range can be applied to the instrument, provided they
are clearly stated in the instrument features.
The current and voltage values stated in the technical
specifications and data plate must not be exceeded: transitory
overranges can have a destructive effect on the switch.
1.3 TEMPERATURES
Due to the temperature of both the environment and the process
fluid, the temperature of the instrument could exceed the allowed
limits (normally from -40°C to +85°C). Therefore, in case it does,
suitable measures (protection against heat radiation, fluid
separators, cooling coils, heated lockers), aimed at limiting the
value, must be taken.
2 - OPERATING PRINCIPLE
A pressure-tight diaphragm (or an O-ring sealed piston) applies a
force to a stiff disc. This force is directly proportional to the
pressure value and is contrasted by an helicoidal spring charged
by a suitable bush. When the force balance point is exceeded, the
stiff disc shifts and, by means of a rigid rod, actuates one or two
simultaneous release electric microswitches. The
microswitches are of the snap acting type with automatic reset.
When the pressure moves away from the set values, returning
towards the normal values, the switch is reset.
3 - NAMEPLATE AND MARKINGS
The instrument is fitted with a metal plate bearing all its functional
characteristics and – in case of explosionproof execution (Series
PXA and PXN) – also the markings prescribed by standard EN
50014 and EN 50281-1-1. Fig. 1 shows the nameplate mounted
on explosionproof instruments.
Fig. 1. Nameplate, explosionproof instruments
1 CE marking and identification number of the notified body
responsible for production surveillance.
2 Apparatus classification according to ATEX 94/9 CE directive.
3 Notified body that issued the type certificate and number of said
certificate.
4 Mode of protection and operating ambient temperature limits.
4 - SPECIAL CONDITIONS FOR SAFE USE (X)
Explosionproof instruments (Series PXA and PXN) installed
without a junction box require an electric connection suitable for
the mode of protection choosen at the free end of the cable (see
Point 8). Should the instrument be installed on a dangerous
process fluid (Zone 0), check wetted materials compatibility.
5 - SET POINT ADJUSTMENT
The charge of the helicoidal spring can be regulated by means of
the bush (for adjustment) in such a way that the switch is released
when the pressure reaches (either increasing or decreasing) the
desired value (set point). The instrument is usually supplied with
the switches set at the setting range value nearest to zero
(factory calibration). The instrument is supplied with an adhesive
label showing the set point calibration value. With factory
calibration the values are not indicated on the label as these are
temporary and will be modified with the definitive values. Prior to
installation the instrument must be calibrated and the definitive
calibration values written on the label using a suitable indelible ink
pen.
If the instrument has been ordered with a specific calibration, it
is a good rule to check the calibration values marked on the
relevant adhesive label, prior to installation.
The position of the adjusting bush is given in Fig. 2.
Fig. 2. Adjusting bush and plumbing
c
Adjustment
slot cover
d
Stainless steel
wire
e
Sealing lead
f
Adjusting
bush
The effect of the sense of rotation of the adjusting bush is shown
on the slot cover. Rotate the bush by inserting a 3mm dia. rod or
drill into the holes on the bush itself.
6 - SET POINT CALIBRATION
In order to proceed with the calibration and the periodical
functional verification of the instrument a suitable calibration
circuit (Fig. 4) and an adequate pressure source are required.
Wiring is to be carried out as shown in Fig. 3 and on the slot
cover.
Fig. 3. Electric wiring
GDN - Ground internal : leadwire yellow/green
C - Common : leadwire brown
NA - Normally open : leadwire blue
NC - Normally closed : leadwire black
Micro 1
C - Common : leadwire grey
NA - Normally open : leadwire red
NC - Normally closed : leadwire white
Micro 2
Microswitch internal wiring and contacts status at atmospheric
pressure
C - Common
NA - Normally open
NC - Normally closed