"Ultimate vacuum" is the deepest vacuum a pump can hold against a perfectly sealed, leak-free inlet — the point where its pumping speed drops to zero and it can remove no more gas. It gets quoted in mbar absolute, mmHg (millimetres of mercury) and torr, and those three units describe exactly the same physics. Mixing them up, or confusing absolute with gauge, is the single most common vacuum spec error. This page fixes that in a few minutes.
Absolute vs gauge — the distinction that matters most
There are two ways to state a vacuum, and they run in opposite directions:
- Absolute counts up from perfect vacuum (zero). Atmosphere is about 1013 mbar absolute. A deep vacuum is a small absolute number — the closer to zero, the deeper.
- Gauge counts down from atmosphere. Zero gauge = atmospheric pressure; a deep vacuum is a large negative number, e.g. −850 mbar.
So the same vacuum can read as a big negative gauge number or a small positive absolute number. −850 mbar gauge and ~163 mbar absolute are the same vacuum. Datasheets mix conventions freely, which is where the confusion starts — always know which one you're reading.
| Convention | Atmosphere | Deep vacuum | Direction |
|---|---|---|---|
| Absolute | 1013 mbar abs | small number (→0) | up from zero |
| Gauge | 0 mbar g | large negative (−850) | down from atmosphere |
Rule of thumb: absolute → smaller is deeper; gauge → more negative is deeper.
The three units are the same thing
mbar, mmHg and torr are just different rulers for the same pressure:
- mmHg — millimetres of mercury; how high the pressure lifts a mercury column. Atmosphere ≈ 760 mmHg.
- torr — defined as 1/760 of a standard atmosphere. For all practical purposes 1 torr = 1 mmHg exactly enough for pump work.
- mbar — millibar, an SI-friendly unit. Atmosphere ≈ 1013 mbar.
Conversions you'll actually use
| From | To | Multiply by |
|---|---|---|
| mmHg (= torr) | mbar | 1.333 |
| mbar | mmHg (= torr) | 0.75 |
| Standard atmosphere | — | 760 mmHg = 760 torr = 1013 mbar |
So 1 mbar ≈ 0.75 torr, and 1 torr ≈ 1.333 mbar. Keep the atmosphere reference (760 mmHg / 1013 mbar) in your head and you can sanity-check any figure instantly.
Reading the Yash datasheets correctly
Here's how the vacuum figures on our own range translate — a good calibration exercise:
| Series | Datasheet vacuum | What it means |
|---|---|---|
| ACME | 750 mmHg | Gauge. ≈ 10 mmHg absolute ≈ 13 mbar abs — deep |
| YELV / YELVD | 600 mmHg | Gauge. ≈ 160 mmHg abs ≈ 213 mbar abs — moderate |
| YBDVP | −850 mbar | Gauge. ≈ 163 mbar absolute — deep |
| DSVP | ≤6×10⁻² Pa / 759.95 mmHg | Fine vacuum — see note below |
Two worked reads:
- ACME at 750 mmHg gauge. Atmosphere is ~760 mmHg, so 750 mmHg of vacuum leaves ~10 mmHg of gas — about 13 mbar absolute. That's a genuinely deep single-stage vacuum.
- YBDVP at −850 mbar gauge. Atmosphere ~1013 mbar abs, minus 850 = ~163 mbar absolute remaining. Deep enough for packaging, conveying and gripping.
Notice ACME's 13 mbar abs is deeper than YBDVP's 163 mbar abs — the oil seal pulls harder than the dry clearance, exactly as the dry vs oil comparison explains.
Fine vacuum and scientific notation
The two-stage DSVP quotes ≤6×10⁻² Pa — that's 0.06 pascal, or about 6×10⁻⁴ mbar absolute (roughly 4.5×10⁻⁴ torr). This is fine vacuum, orders of magnitude deeper than the single-stage pumps, which is why two stages are needed and why it belongs in labs and drying rather than general handling. The full duty is in double-stage oil vacuum pumps for labs. The DSVP sheet also prints 759.95 mmHg gauge — essentially the full 760 mmHg of atmosphere removed, the gauge way of saying the same near-total vacuum.
Why this matters for buying a pump
Two practical consequences:
- Compare like with like. Never compare a gauge figure on one pump with an absolute figure on another — convert both to the same convention first. Two pumps quoted "600" and "−600" may or may not be the same; check the unit and convention.
- Ultimate vacuum sets the floor, not the working point. You don't operate at ultimate — pumping speed there is zero. You pick a pump whose ultimate is comfortably deeper than your operating vacuum, then size flow at the operating point. That's the whole basis of choosing a pump by CFM and vacuum level.
The vacuum ranges, and why the words matter
Vacuum work is loosely divided into ranges, and knowing where your duty sits tells you what kind of pump you need before you look at a single model.
| Range | Rough span (absolute) | Typical duties | Pump type |
|---|---|---|---|
| Rough / low vacuum | 1013 to ~1 mbar | Gripping, conveying, packaging, filtration | Single-stage rotary vane, dry |
| Fine / medium vacuum | ~1 to 10⁻³ mbar | Drying, freeze-drying, degassing, distillation | Two-stage rotary vane |
| High vacuum | below 10⁻³ mbar | Coating, analytical, research | Turbomolecular / diffusion (backed) |
Almost all industrial handling lives in the rough range — which is why single-stage pumps like the ACME and dry YBDVP cover the bulk of factory work, from vacuum conveying to packaging. Cross into fine vacuum for drying and lab duty and you need a two-stage pump like the DSVP. True high vacuum needs specialised pumps backed by a rotary-vane fore-pump — a different world again.
The practical point: the range names map to pump types. When a supplier quotes a duty as "rough vacuum" or "fine vacuum," they are already telling you which pump family to shortlist, and converting the number to absolute confirms the range instantly.
A common trap: "-1 bar" is impossible
You will sometimes see a spec claim a vacuum of "−1 bar" or "−1000 mbar." That would be a perfect vacuum — zero absolute pressure — which no real pump reaches; there is always residual gas. A genuine deep industrial pump reaches perhaps −850 to −990 mbar gauge, never a true −1 bar. Treat any "−1 bar" claim as marketing rounding, convert to absolute, and compare the real residual pressure instead.
Quick reference card
- Atmosphere = 760 mmHg = 760 torr = 1013 mbar abs.
- 1 mbar = 0.75 mmHg = 0.75 torr; 1 mmHg = 1 torr = 1.333 mbar.
- Absolute: smaller = deeper. Gauge: more negative = deeper.
- Convert to one convention before comparing pumps.
- Ultimate vacuum is the floor; operate above it with margin.
Summary
Ultimate vacuum is the deepest a pump can hold at a sealed inlet, quoted in interchangeable units — mbar, mmHg and torr — under two opposite conventions, absolute (small = deep) and gauge (negative = deep). Hold the atmosphere reference (760 mmHg / 1013 mbar) in mind, convert everything to one convention before comparing, and remember you size a pump so its ultimate sits comfortably below your operating point. Do that and the datasheets on the ACME, YBDVP and DSVP read plainly.
Want us to translate a competitor's vacuum spec into plain numbers and match it? WhatsApp +91 9311693322 or email sales@yashblowers.org.