Some dip rods mark every millimetre; others mark each centimetre with four bars between, each bar 2 mm. On a bar rod, 58 cm and 4 bars is 58.8 cm. If it is written 58.4 and read as a decimal, the dip is 4 mm low: about 50 litres in a 20 KL tank near half full.
Two kinds of marking
Millimetre rods mark every millimetre between the centimetre numbers. A wet line three marks past 58 is 58.3 cm or 583 mm. Simple.
Bar rods mark each centimetre with four bars, splitting it into five steps of 2 mm. The fifth step reaches the next centimetre. A wet line at the fourth bar past 58 is:
58 cm + 4 bars × 0.2 cm = 58.8 cmThe decimal trap
On a bar rod people often write “58.4” meaning 58 cm and 4 bars. It looks like a decimal, and anyone who types it into a chart as 58.4 cm is 0.4 cm low. In a 20 KL tank near half full, 4 mm is around 50 litres. On a tanker reconciliation with two dips, the error can double.
| Written | Means | Correct dip |
|---|---|---|
| 58.1 | 58 cm + 1 bar | 58.2 cm |
| 58.2 | 58 cm + 2 bars | 58.4 cm |
| 58.3 | 58 cm + 3 bars | 58.6 cm |
| 58.4 | 58 cm + 4 bars | 58.8 cm |
A figure after the point of 5 or more cannot come from a bar rod, so if you see 58.7 in a bar-rod register, someone has mixed the two.
Make it one way
Agree with everyone who dips: write cm and mm always, or write bars as 58 + 4 so nobody mistakes it for a decimal. The dip to litres calculator has a cm.bars setting that reads 58.4 correctly as 58.8 cm.
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
How do I know which kind of rod I have?
Count the marks between two centimetre numbers. Nine small marks make ten 1 mm spaces. Four marks make five 2 mm spaces: a bar rod.
Does the app take bars?
The app takes cm or mm. Convert bars first: centimetres plus bars × 0.2. The calculators on this site have a cm.bars option that does it for you.