Emerson LTM-300 manual Magnetic Interface, Gage and Transmitter Installation

SECTION 6. Troubleshooting and Maintenance	6.4 Start-Up for Gage-Mounted
6.0 Diagnostics are Via HART	LTM Transmitters
6.1 Calibration Problems	Gage and Transmitter Installation

If the transmitter does not appear to calibrate properly, or has an erratic output, check the deadband of the sensor tube and ensure that the float is within the active region of the probe. The active region of the sensor tube is typically marked with 20mA and 4mA stickers when the unit is calibrated before shipment.

If the output is still erratic, try disconnecting the power momentarily by unplugging and re-plugging-in the terminals. If a glitch was stored in RAM memory, this will generally clear it.

6.2 Magnetic Interface

It is possible for the LTM-300 to be magnetically biased or have residual magnetic energy stored along the length of the waveguide. These magnetic anomalies can interfere with the signal-to-noise ratio and the stability of the output signal itself.

If this appears to be the case, a gage float magnet (or any magnet available) may be run along the length of the sensor tube, past the head of the transmitter, in an even motion and without stopping. This will usually clear all such magnetic anomalies.

CAUTION:

Never move a magnet in a perpendicular motion from along the sensor tube. This will always leave a residual field in the waveguide which will cause the transmitter to give an erratic output

6.3 Troubleshooting Power Supply Problems

The LTM-300 is designed to operate with a supply voltage of 15-36 vdc (at 20 mA) across its terminals without affecting the mA signal. The most common loop supply used is 24vdc.

It is found sometimes that additional resistance in the loop is necessary, either in the form of a second load resistor or higher resistance safety barrier. This will appear to limit the maximum output of the transmitter to below 20 mA. The transmitter will generally perform correctly up to this point. To resolve this problem, the voltage of the loop supply must be increased somewhat. Even a slight increase by one volt may be sufficient and many supplies have such an adjustment.

All connections must be checked for improper wiring or polarity before power is applied. The LTM-300 has series diodes preventing the reverse polarity from entering and damaging the circuitry. If power is applied and the signal is 0.0 mA, chances are good that the polarity is reversed.

TERMINAL BOARD

1.Visually inspect Mag-Gage and transmitter installation to insure transmitter is positioned with the 4 mA and 20 mA labels directly adjacent to the process connections. Make sure transmitter is securely clamped and parallel to gage chamber.

2.Remove bottom flange and install float. Each float is clearly tagged with serial number of the Mag-Gage and process parameters. Top of float is clearly marked “TOP” to insure float is in right side up.

3.Inspect bottom flange for proper gasket and spring. This spring protects float and keeps it from dropping below the bottom process connection. To determine proper spring length measure “A” dimension of gage. This is the length from the center of the bottom process connection to face of the bottom drain flange. Length of spring should be

(“A” DIMENSION) (-) Minus (Float Length + 2 inches)

EXAMPLE: “A” Dimension is 14”, Float length is 12”, + 2” = 4” Spring Length.

14 – 12 + 2 = 4” spring

4.Float is properly installed if the bottom 2-3 flippers on gage indicator have flipped.

NOTE: There is a top spring in the Mag-Gage to protect float and prevent it from passing the top process connection. DO NOT REMOVE EITHER SPRING.

Transmitter Check-out and Calibration

NOTE: The LTM Series Transmitters are 24VDC Loop powered (2- wire) and require a minimum of 15 Volts at 20 mA.

1.Using a HART compatible loop calibrator, connect the “+” terminal on the LTM to positive lead of calibrator and the “-” terminal on the LTM to negative lead of calibrator.

2.With float in the gage at 4mA, the output of the LTM should be 4mA. Connect the HART Communicator to the transmitter. Upon power up, the HART Communicator should read the LRV (lower range value) or 0 inches at 4mA.

3.With float in the gage at 20mA, the output of the LTM should be 20mA. HART should also display URV (upper range value) or span length in inches at 20mA.

4.To insure complete functionality of gage and transmitter, fill the gage chamber with liquid and drain slowly to observe transmitter and gage are tracking properly.

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