Installation 7
2.2.2 Hazardous area wiring
Intrinsically safe wiring including using the reed switch pulse output as simple
apparatus, wiring to an Intrinsically Safe Instrument or wiring to the Exd explosionproof option( Exd IIB T4/T6 )
wiring techniques must be undertaken in accordance with the rules, regulations and requirements applying to the
territory in which the meter is being installed. The meters should only be connected by qualified staff, the qualified
staff must have knowledge of protection classes, regulations & provisions for the apparatus in hazardous areas.
If the flowmeter is fitted with an intrinsically safe instrument refer to the appropriate manual & I.S. supplement for
wiring of the instrument inputs and outputs.
Earthing lugs are located within the terminal housing cover and on the meter body.
Use a separate earth within the
cable making sure that the earth conductor does not come in contact with the cable shield / screen.
Use only high temperature cable at the flowmeter when the process temperature exceeds 85
º
C.
2.3 Pulse Output selection for pulse meters
Two types of output are available on each meter, open collector
from Hall Effect sensors or reed switch contact. Each output type is linearly proportional to volumetric flow and each
pulse is representative of an equal volume of liquid.
2.3.1 Hall Effect Sensor Pulse Output
The Hall Effect Sensor is a solid state 3 wire device providing a NPN
open collector output. It requires a dc voltage between 5~24Vdc to operate and is the recommended pulse output
for powered installations such as local or remote batching.
The pulse output between signal and -0V is a voltage square wave with the high level being the dc voltage
available at the open collector and the low level being -0V.
The receiving instrument must incorporate a pull up resistor
( typically greater than 10K ohms in most instruments )
which ties the
open collector to the available dc voltage level when the Hall sensor is not energized. When
energized the
open collector output is pulled to ground through the emitter (-0V).
2.3.2 Reed Switch Pulse Output
The reed switch output is a two wire normally open SPST voltage free contact
ideal for installations without power or for use in hazardous area locations when Intrinsically Safe (I.S.) philosophy
is adopted.
Note:
when using the reed switch output the liquid temperature must not change at a rate greater than
10ºC per minute (50ºF per minute).
In general the reed switch life will exceed 2 billion actuations when switching
less than 5Vdc @10mA as is the case when combined with the RT, EB or BT instruments.
8 Installation
2.3.3 Quadrature (QUAD) Pulse Output
The diagrams below apply when the meter is fitted with the
Quadrature pulse output option
(two Hall Effect sensors arranged to give separate outputs out of phase with
one another).
The Quadrature output is typically suited to custody transfer applications where signal integrity verification is
required, it is also used for metering bi-directional flow.
2.3.4
Signal integrity verification
Many fiscal transactions require the primary measuring device
(flowmeter)
to have Quadrature outputs in order to detect any difference in the number of pulses from each input
(
from
& ) during delivery.
2.3.5 Bi-directional flow
Combining the Quadrature feature and model PD1 pulse discriminator module
produces forward & reverse outputs both of which may be integrated to provide a “net” reading. The RT12 flow
rate totaliser will take both output & will perform the “net” flow function.
1
2
phase shift between
outputs
Quadrature
Pulse Output
1
2
common
to
1 & 2
VDC
+
0V
-
QUAD
NPN HALL EFFECT
REED Sw.
VDC
+
0V
-
QUAD
NPN HALL EFFECT
REED Sw.
Hall Effect NPN
open collector
Reed Switch
contact closure
VDC
+
0V
-
QUAD
NPN HALL EFFECT
REED Sw.
PD1 Pulse
Discriminator
2
1
10
9
7
6
5
4
2
1
8~24Vdc
+
0V ground
-
Reverse flow
Forward flow