Threshold Adjustment Using An Oscilloscope - K-TEK AccuTrak AT100 Installation & Operation Manual

6.0 SAFETY, MAINTENANCE, AND TROUBLESHOOTING

6.12 Threshold Adjustment Using an Oscilloscope

Principle of Operation:
The main module in the AT housing performs 10 measurement cycles per second.
1. Start of cycle: A current pulse (Start Pulse) is applied to the sensor wire, which is under tension in the sensor
tube. This current produces a magnetic field along the sensor wire.
2. The interaction of the sensor wire magnetic field, and the magnetic field at the float, causes a small twist in the
wire at the point where the float is located.
3. The small twist is like an ultrasonic vibration, which propagates along the sensor wire from the float position, up
the wire, towards a piezo-ceramic sensor located at the top of the tube.
4. The piezo-ceramic sensor is located at the top of the tube.
5. The AT electronics‟ measures the time between the Start Pulse (of Step 1) and the Return Pulse (of step 4).
The measured time changes with the position of the float, and the level output is calculated from that.
Note: To detect the return pulse, the AT module looks for a signal amplitude exceeding a certain threshold
voltage which is set by the variable potentiometer on the bottom board of the AT. (See drawing below.)
6. The threshold voltage should be set for one half of the return signal strength.
Using a scope meter to assess transmitter operation:
Note: Before using an oscilloscope on an AT transmitter, confirm the electrical classification of the working area and take all
necessary precautions for safe operation and connection to the instrument.
Settings using a Fluke Oscilloscope 97 (50 MHz) or any other two channel scope (Min Bandwidth 10 MHz)
AT Module Faceplate
* Place a small wire in each of the 4 used pin
connector holes for contact using scope.
NOTE: The diagram to the right is what should be observed using a
dual channel oscilloscope. If the float were upside down, the Return
Pulse would be inverted. If noise is present, it would show up on the
baseline.

All AT200 floats have a correct orientation in the chamber.

Floats are scribed >>>>UP>>>>

Some AT100 Floats have an orientation.
(Refer to factory for assistance.)
AT100-0200-1 Rev L (10-2010) DCN0528
Channel A: Refer to the drawing at left for probe connections.

Connect probe tip to the Return Pulse (Move probe tip to the pin
below it to measure the threshold voltage)

Set range to 500 mV DC
Channel B:

Connect probe to the Start Pulse

Connect the probe ground to the common

Set range to 2VAC / Division
Timing and Trigger:

S e t t h e t i m e
(Note: The lower the float is, the farther the return signal is from
the start pulse. On units longer than 10 ft., the time base will need
to be set to 200uSec/Div or the delay function and increment
ahead should be used.)

Set the trigger to Channel B, Level between –.05 to –3.0 volts

Set trigger to negative pulse, mode to normal
t o 5 0 , 1 0 0 , 2 0 0 u S e c / D i v i s i o n
*
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