Babbitt Liquid Level Gauge - Manual (Page 16)

USING FLOAT IN LEVEL CHAMBER (cont.)
4. Place magnet (float) at 100% (or 20mA mark) on sensor tube. Adjust P3 to read 20mA
on 24 VDC current Loop.
5. Repeat Steps 3 and 4 if necessary until transmitter is reading 4mA to 20mA.
6. Check 50% (12mA mark).
Clamp the sensor back on the gage chamber loosely and slightly higher than it should be.
Make sure the float in the chamber is in the zero position. Slowly adjust the sensor tube
downward until the output goes to 4mA. Tighten the clamps and the transmitter is calibrated. If
there is a question about the output, compare it to the visual indication on the gage. One can
also place a magnet against the sensor tube at a point above the gage float and the transmitter
should give an output corresponding to that point, i.e.: if the gage is indicating 2 feet and a
magnet is placed against the sensor at the 4 foot level then the output of the transmitter should
jump to indicate 4 feet.
CALIBRATION SWITCHES
The LT-1 is equipped with an onboard calibration circuit that is factory set to simulate a level
represented by a current loop output. The calibration switches can be set to produce a 4mA,
12mA and a 20mA output. The proper settings for the switches are displayed on page 20 and in
figure 1-0. The LT-1 calibration circuit was designed for simple adjustments of the zero and
span without having to shut down the process. It should be noted that the calibration switches
bank of four DIP switches are adjusted to produce the appropriate current output based upon
the length of the LT-1 sensor tubes, and are therefore not interchangeable with sensor tube of
different lengths.
CALIBRATION USING A VOLTMETER
Figure 1-0 shows a standard setup using a series resistor (250 ohms) to measure a voltage
range of 1 to 5 volts. Connect a voltmeter in parallel with the 250-ohm resistor. Set the
calibration switches to the 4mA position and adjust P2 on the LT-1 circuit board until a reading
of volt is displayed on the voltmeter. To adjust the span set the calibration switches to the 20mA
position and adjust P3 until a reading of 5 volts is displayed on the voltmeter. To verify that the
midpoint is accurate and linear, place the calibration switches in the 12mA position. The
voltmeter should now display 3 volts.
CALIBRATION USING AN AMMETER
Figure 1-1 shows a typical set up for measuring the output of a transmitter within the current
loop. This particular procedure does not require the use of a resistor. Connect an ammeter in
series with the power supply and the LT-1 transmitter. Set the range on the ammeter to read a
level of 20mA maximum. Set the calibration switches to the 4mA position and adjust P2 on the
LT-1 circuit board until a reading of 4mA is displayed on the ammeter. Set the calibration
switches to the 20mA position and adjust P3 until a reading of 20mA is displayed on the
ammeter. Verify the midpoint by setting the calibration switches to the 12mA position. The
ammeter should display 12mA.