KBA #1 – Troubleshooting Unstable or Fluctuating Analog PAX Panel Meter Readings

 

Product

This article applies to Red Lion PAX panel meters that accept analog process inputs, including:

• PAXP (Process Meter)
• PAXD (DC Voltage/Current Meter)
• PAX2D (Dual Input Process/DC Meter)
• PAXT (Temperature Meter for Thermocouple and RTD Inputs)

Note: The troubleshooting principles in this article can also be applied to similar analog-input panel meters where signal stability may be affected by wiring, grounding, or electrical noise.

Issue

Display readings fluctuate, jump unexpectedly, or fail to stabilize even though the process signal appears relatively constant.

Symptoms

Users may observe one or more of the following:

• Display values continuously changing despite a stable process.
• Readings intermittently jumping higher or lower than expected.
• Excessive digit movement on low-level analog signals.
• Readings becoming unstable when nearby equipment starts operating.
• Different values displayed compared to measurements taken directly with a handheld meter.

Overview

In many installations, fluctuating readings are not caused by a failed meter but by electrical noise, grounding issues, improper wiring, or interference from nearby equipment. A systematic troubleshooting approach can help identify and eliminate these sources of instability.

Determine Whether the Signal Is Actually Fluctuating

Before troubleshooting the meter, verify whether the source signal itself is stable.

Procedure

1. Measure the signal directly at the sensor or transmitter using a calibrated meter.
2. Measure the signal again at the PAX input terminals.
3. Compare the measurements.

Results

• If the signal fluctuates at both locations, the process itself may be changing.
• If the signal is stable at the sensor but unstable at the PAX, the issue is likely wiring or electrical noise.
• If the signal is stable everywhere except on the display, review meter configuration and filtering settings.

Example

A pressure transmitter consistently outputs 12.00 mA at its terminals, but the displayed value varies significantly. This typically indicates noise being introduced between the transmitter and the panel meter.

Verify Input Wiring and Signal Grounding

Incorrect wiring is one of the most common causes of unstable readings.

Inspect Input Wiring

Verify that:

• All terminal screws are tight.
• No conductors are loose or damaged.
• Stranded wire is fully contained within the terminal.
• Wiring matches the applicable connection diagram for the input type.

Confirm Proper Grounding

Review how the signal common and sensor grounds are connected.

Potential issues include:

• Multiple grounding points
• Improper signal reference connections
• Ground currents flowing through signal wiring

Ensure the installation follows a consistent grounding strategy and avoids unintended alternate current paths.

Verify Shielded Cable Installation

Why Shielding Matters

Industrial environments often contain sources of electromagnetic interference (EMI), including:

• Variable Frequency Drives (VFDs)
• Motor starters
• Contactors
• Relays
• High-current power conductors

These devices can induce unwanted voltages into low-level signal wiring, causing displayed values to fluctuate.

Proper Shield Termination

The recommended practice is to ground the cable shield at one end only.

Benefits include:

• Reducing external electrical noise pickup
• Preventing shield current from affecting the measurement signal
• Minimizing the possibility of ground loops

Additional Recommendations

• Use twisted-pair shielded instrumentation cable whenever possible.
• Keep signal cables separated from power wiring.
• Cross power conductors at 90° when crossings are unavoidable.
• Avoid routing instrumentation wiring in the same conduit as motor power cables.

Check for Ground Loops and VFD Interference

Ground Loops

A ground loop occurs when equipment is grounded at multiple locations with different electrical potentials.

Common symptoms include:

• Unstable readings
• Random fluctuations
• Increased noise during equipment operation
• Inconsistent measurement accuracy

Troubleshooting Ground Loops

Inspect the grounding scheme and look for multiple ground paths between devices. If safe and appropriate, isolate suspected grounding paths to determine whether the instability improves.

VFD Interference

Variable Frequency Drives are a common source of electrical noise in industrial systems.

Indicators of VFD-related interference include:

• Instability appearing only when motors are running
• Fluctuations changing with motor speed
• Problems appearing after installation of new drives or motor equipment

Recommended Checks

Verify that signal wiring is routed away from:

• VFD output conductors
• Motor power cables
• High-current feeders
• Contactor and relay wiring

Where practical, use separate conduit and cable trays for instrumentation wiring.

Use Input Filtering

PAX meters provide input filtering functions that can reduce the effects of electrical noise and small signal variations.

When Filtering Should Be Used

Filtering is useful when:

• The process changes slowly.
• Display stability is more important than rapid response.
• Electrical noise cannot be completely eliminated.

Typical applications include:

• Tank level monitoring
• Temperature monitoring
• Pressure indication

When Filtering Should Be Limited

Excessive filtering may not be appropriate when:

• Fast response times are required.
• Rapid process changes must be displayed immediately.
• The meter is being used for dynamic monitoring applications.

Best Practice

Increase filtering gradually while observing system response. Use only enough filtering to achieve a stable reading without introducing excessive display lag.

Step-by-Step Troubleshooting Procedure

Step 1 - Verify the Signal Source

• Measure the signal directly at the sensor or transmitter.
• Confirm the process variable is stable.

Step 2 - Check Input Wiring

• Verify terminal connections.
• Tighten all wiring connections.
• Inspect conductors for damage.

Step 3 - Inspect Cable Shielding

• Confirm shielded cable is being used where appropriate.
• Verify shield termination follows recommended practices.

Step 4 - Review Cable Routing

• Separate signal wiring from power wiring.
• Check for proximity to motors, drives, and switching devices.

Step 5 - Evaluate Grounding

• Look for multiple grounding points.
• Identify potential ground loops.

Step 6 - Apply Input Filtering

• Enable filtering if appropriate.
• Increase filtering incrementally while monitoring the display.

Step 7 - Observe System Operation

Determine whether fluctuations coincide with:

• Motor starts
• VFD operation
• Relay switching
• Activation of other electrical equipment

Step 8 - Isolate the Problem

If the issue remains:

• Disconnect nonessential external devices.
• Test with a known stable signal source.
• Temporarily replace suspect wiring.
• Compare results with an independent measuring instrument.

Resolution Summary

Most unstable PAX readings are caused by one or more of the following:

• Electrical noise on the input signal
• Improper shield termination
• Ground loops
• VFD or motor-generated interference
• Poor cable routing practices
• Insufficient input filtering

By systematically evaluating the signal source, wiring, grounding, shielding, and meter configuration, technicians can efficiently identify and correct the root cause of fluctuating readings.

Information to Collect Before Contacting Technical Support

If escalation is required, gather the following information:

• PAX model number
• Input signal type
• Wiring diagram
• Cable length and type
• Grounding method
• Nearby electrical equipment
• Photos of the installation
• Filter settings currently in use

Providing this information can significantly reduce troubleshooting time and help identify installation-related causes more quickly.

 

If you need additional support, please contact Technical Support by clicking this link: Technical Support