A continuous reel-to-reel plating line experienced a frustrating problem: the tank temperature would occasionally drop 5–8°C below setpoint for no apparent reason, then recover on its own. The heater current was normal when checked. The controller showed no alarms. The heater was pulled and tested-resistance within tolerance, insulation resistance above 200 MΩ, sheath intact. The problem was intermittent, and intermittent faults are the hardest to diagnose because they disappear when the maintenance team arrives.
Intermittent heating on a continuous line can originate from the heater itself, the supply circuit, the controller, or the process conditions. The diagnostic challenge is to catch the fault in the act. A systematic approach-checking the most likely causes first and using data logging where necessary-narrows the search efficiently.
Start With the Heater Circuit
The heater circuit includes the element, the internal connections, the terminal block, and the supply wiring. An intermittent open circuit in any of these components causes the heater to stop heating temporarily.
A broken element that makes intermittent contact is rare but possible. More common is a loose terminal connection. Thermal cycling loosens terminal screws over time. The connection may be tight when the heater is cold and lose contact when the heater is hot, or vice versa.
A failing contactor or relay in the control panel can also cause intermittent heating. Pitted contacts may conduct intermittently, especially when carrying high current. The contactor may pull in but fail to make solid contact on all poles.
| Circuit Component | Intermittent Fault | Symptom Pattern | Diagnostic Method |
|---|---|---|---|
| Element | Partial break | Random dropouts | Resistance test when cold and hot |
| Terminal connection | Loose screw | Dropouts during thermal cycling | Torque check, thermal imaging |
| Contactor | Pitted contacts | Dropouts under load | Voltage drop across contacts |
| Breaker | Weak trip mechanism | Random trips | Replace and retest |
| Wiring | Broken strand | Dropouts when moved | Continuity test with flexing |
Intermittent fault sources in the heater circuit
Check the Controller and Sensor
An intermittent sensor or controller fault can cause the heater to stop receiving power even though the heater itself is healthy. A thermocouple with a loose connection may read erratically, causing the controller to reduce output when the bath is actually below setpoint.
A controller with a failing output relay or SCR may stop delivering power intermittently. The display may show normal operation while the output has failed. Checking the voltage at the heater terminals when the fault occurs is the definitive test.
Sensor placement can also cause apparent intermittent heating. A sensor positioned in a dead zone may read the local temperature, which can differ from the bulk temperature. When flow patterns change-due to a pump cycling or a strip moving-the sensor reading changes, and the controller adjusts output.
Monitor Voltage and Current Over Time
The most effective diagnostic for intermittent heating is to log voltage and current at the heater over a period of days. A data logger or a power monitor connected to the heater circuit records the actual electrical behavior, catching the fault when it occurs.
If the current drops to zero while voltage remains present, the fault is in the heater or its connections. If the voltage drops to zero while the controller is calling for heat, the fault is in the supply circuit or the controller output. If both voltage and current remain normal but the bath temperature drops, the problem is in the process-flow, sensor placement, or heat loss.
| Logged Behavior | Fault Location | Next Step |
|---|---|---|
| Current drops, voltage normal | Heater or terminal connection | Inspect and torque terminals |
| Voltage drops, current normal | Supply circuit | Check breaker, contactor, wiring |
| Both drop to zero | Controller output or supply | Check controller output relay |
| Both normal, temp drops | Process or sensor | Check flow, sensor placement |
Interpreting logged voltage and current data for intermittent heating
Inspect for Process-Related Intermittence
Not all intermittent heating is electrical. Process conditions can cause the bath temperature to drop even when the heater is operating normally.
A pump that cycles on and off changes the flow pattern in the tank. When the pump is off, the heater may be in a low-flow zone and overheat locally, causing the controller to reduce output. When the pump is on, the heater cools and the controller increases output.
A tank level that fluctuates can expose the top of the heater intermittently. The exposed section overheats, and the controller may reduce output to protect the process, even though the bulk fluid is below setpoint.
Strip movement can also cause temperature drops. A cold strip entering the bath absorbs heat faster than the heater can replace it. The temperature recovers once the strip warms up.
Practical Diagnostic Sequence
Start with the simplest checks: torque the heater terminals, inspect the contactor contacts, and verify the sensor calibration. These steps resolve a significant share of intermittent heating problems.
If the problem persists, install a data logger on the heater circuit. Log voltage, current, and bath temperature for at least 48 hours, including periods when the fault occurs.
Review the logged data against the process schedule. If the fault correlates with a specific operation-pump cycling, strip entry, or shift change-the cause is likely process-related rather than electrical.
If the fault is electrical and the logged data points to the heater, replace the heater. Intermittent element faults cannot be reliably repaired.
When Intermittent Heating Indicates a Specification Problem
Repeated intermittent heating on a continuous line, even after electrical repairs, indicates that the heater or control system is not suited to the process. A heater that operates near its thermal limit, a controller that is too slow for the load changes, or a sensor that is poorly positioned can cause chronic intermittency. A custom PTFE immersion heater configuration with lower watt density, better flow access, and integrated sheath sensing addresses the root cause of intermittent heating. The specification process requires the bath chemistry, operating temperature, tank dimensions, load profile, and the history of intermittent faults.
