Bath life is a money decision disguised as a chemistry decision. Top-up too little and you pay in rejects; dump too often and you pay in water, heat and wastewater fees. Here is where the line sits, with the arithmetic.
4-6 weeks
Bath life
0.5-1.0 kg/day
Top-up
25-40%
Water/waste saving
Decision rules for a 600 L tank, single shift
| Situation | Indicator | Do this | Typical cost impact |
|---|---|---|---|
| Normal week | Free alkali 6.5-8.5% | Top up 0.5-1.0 kg/day | Baseline |
| Heavy soil | Oil > 2% by volume | Skim, then partial 20% dump | + EUR 0.9 per 1,000 parts |
| Drifting | Alkalinity falls 1.5 pts/day | Investigate rinsing and wax type first | Rejects cost 10x the chemical |
| End of life | Tank rebuild needed weekly | Full rebuild + inspect heaters | EUR 40-70 per rebuild |
Cost per 1,000 parts (illustrative, single shift)
| Item | Top-up only | Weekly partial dump | Full rebuild weekly |
|---|---|---|---|
| Detergent | EUR 3.10 | EUR 4.20 | EUR 6.80 |
| Water + heat | EUR 1.40 | EUR 2.60 | EUR 5.10 |
| Wastewater | EUR 0.60 | EUR 1.30 | EUR 2.90 |
| Rework (at 2%) | EUR 0.80 | EUR 0.80 | EUR 3.20 |
| Total | EUR 5.90 | EUR 8.90 | EUR 18.00 |
Three signals that recycling beats dumping
| Signal | Threshold | Recycling option |
|---|---|---|
| Oil layer visible | > 2% volume | Coalescer / skimmer, reuse 60-80% of the bath |
| Alkalinity recovered on top-up | < 15% drop per week | Ultrafiltration loop, extend to 8-10 weeks |
| Wastewater cost | > EUR 2 per 1,000 parts | Batch treatment with pH correction and reuse |
Watch out
Never dump a hot bath to the drain. Cool it, pH-correct it and log the volume - wastewater rules are where a cheap habit becomes an expensive incident.
Common mistakes and the fix
| ✗ Top up with cold water only | ✓ Top up with warm water plus the right chemical ratio |
| ✗ Measure cost per litre instead of per 1,000 parts | ✓ Per-litre maths hides rework cost completely |
| ✗ Let the oil layer ride on top | ✓ Skim daily; the oil is what blocks the rinse |
| ✗ Rebuild the tank when the heater has failed | ✓ Check heaters every rebuild - that is often the real cause |
What each role watches
| Operator | "Skim the surface before the first load, not at the end of the day." |
| Chemist | "Track oil volume weekly and pair it with alkalinity trend." |
| QC | "Tie every bath action to a batch and a reject count." |
| Manager | "Compare cost per 1,000 parts monthly; it is the only number that matters." |
From the shop floor
Case: a shop cutting bath changes from weekly to monthly saved EUR 2.10 per 1,000 parts in chemicals and water, but lost 1.4% to adhesion until the alkalinity trend was automated. The final answer was a monthly rebuild plus daily top-up - the cheapest column above.
FAQ
How do I measure oil loading quickly?
Take a 100 ml sample in a clear jar, let it stand 30 minutes and read the oil band. One millimetre on a 100 ml sample is roughly 1% by volume.
Can I reuse the rinse water?
Yes, if you add a cascade and a final DI stage. Counter-flow rinsing typically cuts rinse water by 60% without losing quality.
When is a rebuild cheaper than a top-up?
When a top-up holds for less than a day. That means the bath is saturated with soil and chemistry cannot fix it.
Where to go next
- Level guide for this trade: Dewaxing & Ultrasonic Cleaning — technician levels
- Other levels: Junior, Senior, Management
- Owner view: Process Overview for owners
- Supplier view: Chemistry & Supply · Find content by trade + level
Related reading in this trade
FAQ - Dewaxing Bath Life: When to Top Up, Dump or Recycle - With Cost Data
Straight answers from the shop floor. Ask us on WhatsApp if your case is different.
What problem does this article address?
Bath life is a money decision disguised as a chemistry decision. Top-up too little and you pay in rejects; dump too often and you pay in water, heat and wastewater fees. Here is where the line sits, with the arithmetic.
What does the data say about normal week?
Normal week Free alkali 6.5-8.5% Top up 0.5-1.0 kg/day
What are the practical steps or checks?
Operator Load spacing again - parts stacked flat keep wax in the blind holes. Chemist Free alkalinity below 6% and the ultrasonic only pushes wax around the tank. QC Water-break must hold 10 seconds; if the film closes, the load goes back. Manager On
What should the workshop do differently?
Supplier view: Chemistry & Supply · Find content by trade + level