Coolant Batch Reconciliation in the Machine-Shop Store
The store at a district ITI holds one 20-litre drum of soluble cutting-oil concentrate, bought for ₹420. The wall chart specifies 1 part concentrate to 20 parts water for turning and 1 part to 40 for surface grinding. On Monday a trainee prepares 60 litres of turning coolant by pouring 3 litres of concentrate into 57 litres of water, and reports the batch as “1:20”. On Tuesday a second trainee needs 200 litres of grinding coolant and intends to draw from the same drum. The instructor is due for an audit and wants every batch recorded both as a ratio and as a percentage, with the chemical cost per 1,000 litres of mixed coolant stated. The storekeeper also produces a quotation for 25-litre drums at ₹480 and asks whether the larger pack is the better purchase. Records show that the drum is opened repeatedly over six months.
1. Verify the Monday batch. Calculate the exact concentrate and water required for 60 L at 1:20, compare with the 3 L actually used, and express both the specified and the actual mix as a percentage of concentrate and as a ratio. What corrective action, if any, is needed?
Read the worked answer
1:20 means 1 part concentrate plus 20 parts water, i.e. 21 parts total.
- Correct concentrate = 60 × 1/21 = 2.857 L; water = 60 × 20/21 = 57.143 L.
- Specified concentration = 1/21 × 100 = 4.762%, ratio 1:20.
- Actual: 3 L in 60 L = 5.000%; ratio 3:57 = 1:19.
- Error = 3.000 − 2.857 = 0.143 L excess, i.e. +0.238 percentage points, about 5% relatively high. Correction: concentration is based on total volume, so add water, not drain oil. The total volume needed to bring 3 L down to 4.762% is 3/0.04762 = 63.0 L; add 63.0 − 60 = 3.0 L of water. The batch then becomes 3 L in 63 L = 4.76%, ratio 1:20. Judgement: a 5% over-strength mix is not a scrapping offence, but it wastes concentrate, increases mist, foam and skin risk, and the log entry '1:20' is factually wrong. Correct the entry before signing and record the true final volume of 63 L, not 60 L.
2. For Tuesday, calculate the concentrate needed for 200 L at 1:40, check it against the drum balance, and compute the chemical cost per 1,000 L of mixed coolant at each ratio.
Read the worked answer
1:40 = 41 total parts; concentrate = 200/41 = 4.878 L; water = 195.122 L. Drum balance: 20 − 3 = 17 L remaining after Monday (correcting Monday by adding water consumes no extra concentrate). Since 4.878 L < 17 L, the grinding batch can be drawn from the same drum, leaving about 12.1 L. Cost of concentrate = ₹420/20 L = ₹21.00 per litre. At 1:20: concentrate per 1,000 L = 1000/21 = 47.62 L → 47.62 × 21 = ₹1,000.00, i.e. ₹1.00 per litre of mix. At 1:40: 1000/41 = 24.39 L → 24.39 × 21 = ₹512.20, i.e. ₹0.512 per litre. Cross-check: 1,000 L at 1:40 uses half the oil of 1:20, so the cost should be about half - ₹512 is consistent. Judgement: the grinding mix is cheaper per litre but is not interchangeable with the turning mix. Label the sumps by ratio and date; topping up a 1:20 sump with 1:40 mix silently reduces concentration and leads to rust and poor finish.
3. The storekeeper proposes the 25 L drum at ₹480 instead of the 20 L drum at ₹420. Compare unit costs and advise, considering the six-month open-drum history.
Read the worked answer
Unit cost: 20 L pack = 420/20 = ₹21.00/L. 25 L pack = 480/25 = ₹19.20/L. Saving = ₹1.80/L, or 8.57%. Per 1,000 L of mix: at 1:20 the concentrate is 47.62 L, saving 47.62 × 1.80 = ₹85.71. At 1:40 the concentrate is 24.39 L, saving 24.39 × 1.80 = ₹43.90. Judgement: the 25 L drum is genuinely cheaper per litre, but the saving is small per 1,000 L and the drum is opened repeatedly over six months. A larger pack stays open longer, can absorb moisture, stratify or pick up tramp oil and swarf, and a 25 L container may not fit the existing dispensing tap. If consumption is below roughly one 20 L drum every two months, the ₹86 per 1,000 L saving can easily be lost in rejected coolant and disposal cost. Advise a trial of one 25 L drum only if it will be used within its shelf life and decanted into a clean, lidded, labelled container; otherwise stay with the 20 L pack.