A coolant tank level sensor that does not alarm when the tank is low is the same as having no level sensor at all — except that everyone believes the machine is protected. An uncalibrated sensor creates a false sense of security that can lead to pump damage and production interruptions.
Level Sensor Types
Sensor Comparison
| Sensor Type | Operating Principle | Accuracy | Cost | Application | Contamination Sensitivity |
|---|
| Float switch (mechanical) | Float rises and falls with coolant level — activates switch at set point | ± 5 mm | Low | Low-level alarm, fill control | Moderate — float can stick |
| Conductivity (probe) | Two electrodes — circuit completes when coolant contacts both | ± 10 mm | Low | Low-level alarm | Low — coolant must be conductive |
| Ultrasonic | Sound wave reflects from coolant surface — measures distance | ± 2 mm | Moderate | Continuous level monitoring | Moderate — foam, mist affect reading |
| Pressure (hydrostatic) | Pressure sensor at tank bottom — pressure proportional to height | ± 3 mm | Moderate | Continuous level monitoring | Low — self-cleaning |
| Capacitance | Dielectric change between sensor and coolant surface | ± 3 mm | High | Continuous — harsh environments | Low — affected by coolant type change |
| Radar | Microwave reflects from coolant surface | ± 1 mm | High | Precision continuous level | Low — unaffected by foam, mist |
Sensor Selection by Tank Type
| Tank Type | Recommended Sensor | Why |
|---|
| Chip tank (large, open) | Ultrasonic or pressure | Handles large tanks, tolerant of contamination |
| Coolant supply tank (closed) | Float switch or conductivity | Simple, reliable, low cost |
| High-pressure pump tank | Pressure or conductivity | Compact installation, reliable |
| Bypass / kidney loop tank | Float switch | Small tank, simple alarm |
| Temperature-controlled tank | Ultrasonic or pressure | Continuous monitoring for chiller control |
Calibration Procedures
Float Switch Calibration
| Step | Action | Detail |
|---|
| 1 | Identify float switch type | Normally open (NO) or normally closed (NC) |
| 2 | Verify float moves freely | No binding, no accumulated debris |
| 3 | Determine set point | Typically 20–30% of tank height for low-level alarm |
| 4 | Drain tank to set point level | Mark the level on the tank exterior |
| 5 | Adjust float switch position | Slide switch up or down on mounting bracket |
| 6 | Test at set point | Float should activate switch at marked level |
| 7 | Test above set point | Switch should de-activate |
| 8 | Test below set point | Switch should activate |
| 9 | Secure switch position | Tighten mounting bracket |
| 10 | Document calibration | Set point level, date, operator |
Conductivity Probe Calibration
| Step | Action | Detail |
|---|
| 1 | Clean probe tips | Remove any coating or residue |
| 2 | Set probe length | Cut or extend probe to reach desired set point level |
| 3 | Fill tank above probe tip | Verify circuit closes (conductivity detected) |
| 4 | Drain tank below probe tip | Verify circuit opens (no conductivity) |
| 5 | Adjust sensitivity (if electronic controller) | Set per manufacturer specification |
| 6 | Test with actual coolant | Not water — coolant has different conductivity |
| 7 | Document calibration | Probe length, set point, date |
Ultrasonic Sensor Calibration
| Step | Action | Detail |
|---|
| 1 | Clean sensor face | Remove any coolant residue or debris |
| 2 | Enter tank dimensions in controller | Tank height, shape (cylindrical or rectangular) |
| 3 | Set empty level (4 mA or 0%) | Tank empty — reference point |
| 4 | Set full level (20 mA or 100%) | Tank full — reference point |
| 5 | Set alarm thresholds | Typically 20% for low-level alarm |
| 6 | Set damping / averaging | Prevents false alarms from surface turbulence |
| 7 | Test at empty | Verify 4 mA output or 0% reading |
| 8 | Test at known level | Fill to measured height, verify sensor reading |
| 9 | Adjust if needed | Offset or span adjustment per manufacturer |
| 10 | Document calibration | Empty, full, alarm settings, date |
Pressure Sensor Calibration
| Step | Action | Detail |
|---|
| 1 | Verify sensor is at tank bottom | Clean sensor diaphragm |
| 2 | Ensure sensor port is clear | No debris blocking diaphragm |
| 3 | Set zero (tank empty) | Sensor should read 0 bar / 4 mA |
| 4 | Fill tank to known height | Measure height with tape or rod |
| 5 | Compare sensor reading to calculated pressure | Pressure (bar) = height (m) × density (kg/L) × 0.0981 |
| 6 | Adjust span if needed | Per manufacturer calibration procedure |
| 7 | Test at multiple levels | 25%, 50%, 75%, 100% |
| 8 | Set alarm thresholds | Typically 20% low-level, 95% high-level |
| 9 | Document calibration | Height vs pressure readings, date |
Installation Considerations
Sensor Location
| Sensor Type | Optimal Location | Avoid |
|---|
| Float switch | Away from fill inlet and return lines | Turbulent areas, near pump suction |
| Conductivity probe | Vertical or near vertical in still zone | Areas with foam, near agitation |
| Ultrasonic | Top of tank — clear path to coolant surface | Foam accumulation area, fill inlet |
| Pressure | Tank bottom — in stilling well if possible | Near pump suction, near return line |
| Capacitance | Side-mounted or top-mounted per type | Areas with heavy buildup |
Stilling Well Installation
| Benefit | How It Works | When to Use |
|---|
| Isolates sensor from turbulence | Vertical pipe with openings at bottom only | Ultrasonic and float sensors in agitated tanks |
| Prevents foam interference | Foam cannot enter stilling well | Ultrasonic sensors in foaming coolant |
| Protects sensor from debris | Debris settles below stilling well intake | Pressure sensors on bottom |
| Provides consistent measurement point | Fixed cross-section, known geometry | All continuous level sensors |
Common Sensor Failures
| Failure | Symptom | Likely Cause | Corrective Action |
|---|
| False low-level alarm | Alarm sounds when level is adequate | Float stuck, sensor contamination, foam triggered | Clean sensor, check for foam |
| No alarm when low | Pump runs dry — no alarm | Float sank, probe coated, sensor failed | Replace sensor, verify alarm function weekly |
| Intermittent alarm | Alarm comes and goes | Loose wiring, connection corrosion, floating debris | Check connections, clean tank |
| Erratic reading (continuous sensor) | Level jumps 10%+ randomly | Air bubbles, foam on sensor, electrical noise | Check installation, improve grounding |
| Reading drift | Level reading slowly changes | Sensor coating, temperature effect | Clean sensor, compensate for temperature |
| No signal | Controller shows no reading | Sensor failed, wiring open, controller input failed | Replace sensor, check wiring |
Troubleshooting
No Alarm When Low
| Step | Check | If Fails |
|---|
| 1 | Is the sensor powered? | Check power supply, wiring |
| 2 | Does the sensor output change when level drops? | Simulate low level — check output |
| 3 | Is the alarm relay working? | Check relay coil and contacts |
| 4 | Is the PLC input working? | Force input in PLC — verify signal reaches logic |
| 5 | Is the alarm programmed correctly? | Check PLC program logic |
False Alarm
| Step | Check | If Fails |
|---|
| 1 | Is the coolant level actually low? | Visual check of tank |
| 2 | Is the sensor contaminated? | Clean sensor |
| 3 | Is there foam on the coolant surface? | Check for foaming cause |
| 4 | Is the sensor mounting loose? | Tighten mounting |
| 5 | Is the wiring intermittent? | Check connections, wiggle test |
Maintenance Schedule
| Sensor Type | Weekly | Monthly | Quarterly | Annually |
|---|
| Float switch | Visual check of float movement | — | Functional test (drain to alarm level) | Replace if > 3 years old |
| Conductivity probe | — | Clean probe tips | Functional test | Replace if corroded |
| Ultrasonic | Clean sensor face | — | Check reading at known level | Full calibration |
| Pressure sensor | — | — | Check zero reading | Full calibration |
| Capacitance | — | Clean sensor surface | Functional test | Full calibration |
| All types | — | Check wiring connections | — | — |
FAQ
How do I calibrate a coolant level sensor on a deep hole drilling machine?
The calibration procedure depends on sensor type. For float switches: position the switch at the desired alarm level (typically 20–30% of tank height), verify it activates at that level, and secure it. For ultrasonic sensors: set empty and full references in the controller, set alarm thresholds, and verify readings at known levels. For pressure sensors: set zero at empty, verify pressure reading at a known fill height, and set alarm thresholds. Always document calibration results.
Why does my coolant level sensor give false low-level alarms?
False low-level alarms are most commonly caused by: foam on the coolant surface (ultrasonic sensors read the foam surface, which is higher than the liquid surface — then drops suddenly when foam collapses), contamination on the sensor (floating debris can trigger float switches), electrical noise (intermittent signals to the PLC), or sensor mounting that has loosened. Check the actual coolant level manually to confirm whether the alarm is real or false.
What type of level sensor works best for coolant tanks on deep hole drilling machines?
For simple low-level alarm: float switch or conductivity probe — reliable, low cost, easy to maintain. For continuous level monitoring: ultrasonic sensor (clean coolant) or pressure sensor (dirty coolant). Ultrasonic sensors are popular because they do not contact the coolant — eliminating contamination issues — but they can be fooled by foam and mist. Pressure sensors work well in dirty conditions but require periodic zero calibration.
How often should coolant level sensors be tested?
Test low-level alarms weekly: drain the coolant below the alarm set point and verify the alarm activates. This can be done by running the system in cycle while observing the alarm. For continuous level sensors, verify the reading against the actual tank level weekly. Perform a full calibration annually or whenever a sensor is replaced.
What happens if the coolant level sensor fails while drilling?
If the sensor fails to alarm when coolant level drops, the pump continues running with inadequate coolant supply. The pump may cavitate (causing noise and vibration), overheat, and damage internal components. In extreme cases, the pump runs dry and seizes. If the sensor fails in alarm state, the machine may not allow drilling to start — stopping production. Regular testing prevents both scenarios.
A coolant level sensor that has never been tested is not a safety device — it is a decoration. Test low-level alarms weekly, calibrate sensors annually, and never bypass a low-level alarm to continue production. A $100 sensor protects a $5,000 pump. This article reflects industry practice as of 2026.
