Dwyer Instruments MTL7700 Series, Instruction Manual

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12 INM7700-6 Jan 2010
8.2.2 Multimeter tests
See tables 4 to 10. The use of a digital multimeter for testing barriers
is described in section 7. The tables assume that the multimeter is set
to a suitable
Ω
range for the end-to-end resistance tests (except for
diode return channels) and for the continuity tests; and that the
multimeter diode test function is used for the diode tests, channel
isolation tests and for the end-to-end resistance tests for diode return
channels.
8.2.3 Constant-current tests
For these tests (last column of tables 4 to 10) a constant-current
generator capable of supplying 10µA, 20mA and 40mA currents
from a 30V source is needed. Ideally, a purpose-built current
generator should be used but, alternatively, a set-up based on a
laboratory power supply and two multimeters (figure 23) will serve.
The current is measured by one multimeter and trimmed by adjusting
the output voltage of the power supply indicated on the second
multimeter.
When using a constant-current generator for testing MTL7700 Series
barriers, note the following points:
a) The current must be limited to 50mA to avoid damaging the
barriers.
b) The accuracy of the current is not critical and can vary by ±5%.
c) Test leads must be connected securely to the barrier terminals;
however, the changeover switch and current selector reduce the
need for connection changes.
d) Using a high resistance in series with the barrier gives more
stable results and makes it easier to set the required current.
8.3 Tests for Active Barriers
8.3.1 Tests for the
Comprehensive testing requires specialised equipment, beyond the
scope of on-site tests. However, an effective test which will confirm
that the unit is operating correctly is shown in figure 24. Ammeter 1
measures the transmitter simulator current of between 4 and 20mA
flowing from terminal 4 and the safe-area load current flowing to
terminal 4 simultaneously. Since these two currents are equal and
opposite, the resulting reading on the ammeter should be virtually
zero. Ammeter 2 verifies the presence of the 4–20mA transmitter
signal.
1
2
3
4
5 7
Bridge supply
Measurement
Safe-area
equipment
MTL7756ac
Variable
resistance
Measurement
Variable
resistance
y y x
x
Figure 22: Calibrating an RTD barrier circuit
To barrier on test
Digital
multimter
(current)
Digital
multimter
(voltage)
current
selector
current limited
to 30mA
2-channel laboratory power supply
0 - 30V
variable
0 - 30V
variable
+ - + -
Changeover
switch
40mA
20mA
10 m A
3M W
1.5k W
750 W
Figure 23: Constant-current test circuit using a power supply
and digital multimeters
1
2
3
4
A1
A2
Transmitter
simulator
4-20mA
250 W
+20 to
35Vdc
+
+
_
_
Figure 24: test circuit
8.3.2 Tests for the
Since this unit incorporates a built-in protection circuit, it has to be
tested in a different manner to an ordinary shunt-diode barrier.
Referring to figure 25, set the transmitter simulator to 4mA and to
20mA, and check that the ammeter reads approximately the same
value. Then, having set the simulator to 20mA, check that the voltage
between terminals 1 and 3 is less than 8.5V and between terminals
2 and 4 is less than 1.6V.
Figure 25: test circuit
1
2
3
4
A
Transmitter
simulator
4-20mA
+20 to
35Vdc
250 W
+
_