10
The following table shows the switching thresholds
for the various transducers, plus the maximum
operating frequency and the input terminal
numbers. For reliable counting the input signal
must fall below the lower threshold and rise above
the upper threshold.
Switching
thresholds
Freq
max
Input
terminals
Switch
100
Ω
1000
Ω
100Hz
5 & 6
Proximity
detector
1.2mA
2.1mA
5 kHz
5 & 6
Open
collector
2k
Ω
10k
Ω
5kHz
5 & 6
Magnetic
pick-off
0mV
20mV
peak
5kHz
3 & 4
Voltage
pulse
1.0V
3.0V
5kHz
3 & 4
4.3.3 Switch contact input
Any mechanically activated switch contact located
in the hazardous area may be directly connected
to terminals 5 & 6 providing it can withstand a
500V rms insulation test to earth for one minute.
This includes most magnetically activated reed
relays used in turbine flowmeters. The BA338C
contains filtering to prevent contact bounce being
counted which limits the maximum operating
frequency for a switch contact to 100Hz.
4.3.4 2-wire proximity detector input
Any certified intrinsically safe 2-wire proximity
detector complying with NAMUR switching
thresholds may be used, providing the input safety
parameters are greater than the output safety
parameters of terminals 5 & 6. The system
certificates list some of the acceptable devices.
When programmed to operate with a proximity
detector, the BA338C maximum input frequency is
5kHz.
4.3.5 Voltage pulse input
Voltage pulse sources should be connected to
terminals 3 and 4. These terminals comply with
the requirements for simple apparatus. They may
be directly connected to any certified intrinsically
safe voltage source within the hazardous area,
providing it can withstand a 500Vrms insulation
test to earth for one minute and has output
parameters equal to or less than:
Uo
=
28V dc
Io
=
100mA dc
Po
=
0.7W
This enables the BA338C to be connected directly
to most flowmeters incorporating a certified
intrinsically safe magnetic pick-off, or a certified
intrinsically safe amplifier producing a high level
pulse output. The BA338C has two programme
selectable voltage switching thresholds, ‘COIL’ for
magnetic pick-offs and ‘UOLtS’ for higher voltage
pulses.
Maximum voltage pulse counting frequency is
5kHz.
4.3.6 Remote reset
The BA338C total display is reset to zero when
terminals 7 & 8 are connected together for more
than one second. Permanent interconnection
inhibits totalisation. Remote resetting may be
accomplished by any mechanically operated
switch located in the hazardous area providing it
can withstand a 500Vrms insulation test to earth
for one minute. No galvanic isolator is required.
The BA338C may be reset to zero from the safe
area. Any switch may be used but a galvanic
isolator is required to transfer the contact closure
into the hazardous area. Fig 4 illustrates how the
BA338C may be reset from both the safe and the
hazardous area.
Note:
The BA338C can be programmed to reset
the total display to zero when the Up and Down
push-buttons are operated simultaneously for more
than two seconds – see 6.12.
