KROHNE VFM 3100 EN - Manual (Page 28)

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If the voltages are not within specifications, disconnect the
red and orange wires to the preamp and measure the voltages
again. If they do not return to ±2.6, replace the electronic module.
(See “Electronic Module Replacement” on page 33.) If they do
return to normal, replace the preamplifier.
d. If the voltage remains low, the housing/field terminal wiring is
bad. Replace the housing or return the VFM 3100 to Krohne for
repair.
• Checking the 4-20 mA output loop.
a. The 4-20 mA loop may be monitored via the test jacks in the
field output terminal board. The signal produced will be 0.1-0.5
volts, corresponding to 4-20 mA. Be sure the VFM 3100 is not
configured for the multidrop mode by verifying that the polling
ddress is zero. The output is fixed at a constant 4 mA in the
multidrop mode.
b. Increase the flow to be sure that the lack of response is not
caused by operation below the Low Flow Cut-in.
c. If there is no response to increasing flow, perform one of the
following tests:
• “Module Test Procedure”
• “Preamplifier Test Procedure”
• “Sensor Test Procedure” on page 29.
3.3. Module Test Procedure
The module may be tested for input frequency with a frequency ge-
nerator. Connect the frequency generator to the outside terminals of
the four-position terminal block. Connect the positive to the brown
terminal and the negative to the yellow terminal. Be sure that loop
power is connected. Increase the frequency until a flow rate is read.
Do not exceed 3000 Hz. If no flow measurement is indicated, check
that the module is configured correctly.
3.4. Preamplifier Test Procedure
3.4.1. Extended Temperature Range Sensor
Integral Mount Electronics
• Check slide switch mounted on the preamplifier to verify that the
switch is in the EXT position.
• Check the electronic module to be sure it can provide the
required power for the preamplifier. Loosen the mounting
screws and remove the module from the housing.
The 4-position terminal block on the back of the module
provides power for the preamplifier assembly mounted near
the sensor neck. The voltage with the preamp connected
should read:
Red to Yellow: +2.6 ±0.2 Volts dc
Orange to Yellow: - 2.6 ±0.2 Volts dc
If it does not, disconnect the preamp and measure again. If the
voltage returns to normal, replace the preamplifier.
• If the voltage in Step 1 is satisfactory, use the module to power
the preamplifier. Disconnect the sensor leads from the pream-
plifier.
• Connect a 32 pF ±5% 50 V dc NPO ceramic capacitor to the
brown terminal of the sensor terminal block. Connect a sine
wave generator across the input by connecting the positive lead
to the capacitor and the negative lead to the yellow terminal.
Use a 50-ohm terminator on the output of the signal generator.
Use coaxial cable between the signal generator and the sensor
input board.
• The preamplifier must be shielded to prevent 60 Hz interference.
An aluminum foil tent grounded to the electronic module hou-
sing may be necessary.
• Set the generator for 500 Hz and 0.5 Volts peak to peak.
The preamplifier output, brown to yellow leads, should be
500 Hz at 0.700 Volts (0.650 to 0.750 Vpp).
• Increase the frequency to 4200 Hz. The output should be
between 0.444 and 0.540 Volts peak to peak.
• Decrease the frequency to 7.5 Hz. The output should also be
between 0.444 and 0.540 Volts peak to peak.
• If the output is not within the correct values, replace the
preamplifier.
Remote Mounted Electronics
The preamplifier is located in the junction box mounted on the flow
tube. Execute test Steps 1 through 8 above.
3.4.2. Standard Temperature Range Sensor
Remote Mounted Electronics
• Check slide switch mounted on the preamplifier to verify
that the switch is in the STD position.
• The preamp is located in the junction box. Check the electronic
module to be sure it can provide the required power for the
preamplifier. Loosen the mounting screws and remove the
electronic module from the housing. The 4-position terminal
block on the back of the electronic module provides power for
the preamplifier assembly mounted near the sensor neck.
The voltage with the preamp connected should read:
Red to Yellow: +2.6 ±0.2 Volts
Orange to Yellow: -2.6 ±0.2 Volts
If it does not, disconnect the preamp and measure again. If the
voltage returns to normal, replace the preamplifier.
• If the voltage in Step 1 is satisfactory, use the electronic module
to power the preamplifier. Connect the red, yellow, and orange
leads to the electronic module and disconnect the brown lead.
Disconnect the brown and yellow sensor leads.
• Connect a 3300 pF ±5% 50 V dc NPO ceramic capacitor to
the brown terminal of the sensor terminal block. Connect a
sine wave generator across the input. Connect the positive lead
to the other capacitor lead and the negative lead to the yellow
terminal. Use a 50-ohm terminator on the output of the signal
generator. Use coaxial cable between the signal generator and
the sensor input board.
• The preamplifier must be shielded to prevent 60 Hz interference.
An aluminum foil tent grounded to the electronic module hou-
sing may be necessary.
• Set the generator for 500 Hz and 0.5 Volts peak to peak.
The preamplifier output, brown to yellow leads, should be
500 Hz at 0.475 Volts (0.425 to 0.525 Volts peak to peak).
• Increase the frequency to 3200 Hz. The output should be
between 0.275 and 0.375 Volts peak to peak.
• Decrease the frequency to 0.1 Hz. The output should be
between 0.375 and 0.475 Volts peak to peak.
• If the output is not within the correct values, replace the
preamplifier.
For this test, the preamplifier should be mounted in the housing in
order to achieve the best shielding. Do not attempt this test with the
preamplifier on the bench. It is very difficult to shield it from 50 or
60 Hz interference from fluorescent lighting.
Note that separate power supplies may be used to provide power
a place of the electronic module.