Working From a Dial That Is Still Marked kgf/cm²
Walk the floor of an older plant and the gauge boards tell you when the machines were bought. Presses, injection moulding machines and hydraulic power units built before the SI changeover carry dials graduated in kgf/cm², often written kg/cm² and occasionally kp/cm². The nameplate, the setup sheet and the original manual all speak that language, while the replacement transmitter in the stores, the new controller screen and every current catalogue are in bar. Translating between the two is a daily chore for anyone keeping that equipment alive.
Why the Old Unit Refuses to Retire
Dial faces outlive the machine
Paperwork drifts apart
The unit suits how fitters think
Three spellings, one quantity
Matching a New Instrument to an Old Face
The usual sequence starts with a number somebody wrote down decades ago and ends with a setting you have to enter on current equipment.
Enter the bar value you are working to
Put the pump setting, the transmitter span or the catalogue rating in the left field. The kg/cm² equivalent appears as you type, and spaces inside a pasted number are ignored rather than rejected.
Place it against the old scale
Hold the result up to the graduations actually printed on the dial. A value that lands awkwardly between two marks is a strong hint that the design figure was round in the other unit.
Move the figure into the record
Each field has a copy button that puts the plain number on the clipboard with no unit attached, which is what a maintenance log entry or a calibration certificate field wants. Ctrl + C inside a field behaves identically.
Reverse it when the nameplate leads
Press the swap arrows (↔) to run kg/cm² → bar, the direction you need when a plate quotes 210 kg/cm² and you have to order an instrument. Worked by hand the multiplier is 0.980665, giving 205.94 bar.
Spec-Plate Values You Meet on Legacy Machines
These are the sorts of entries stamped on nameplates, printed in old process sheets and painted on gauge boards. The right-hand column is what the same pressure reads on modern instrumentation.
| Nameplate or document entry | Marked value (kg/cm²) | Same pressure (bar) |
|---|---|---|
| Small ram gauge, full scale | 25 kg/cm² | 24.52 bar |
| Hose assembly, stamped working pressure | 60 kg/cm² | 58.84 bar |
| General plant gauge, full scale | 100 kg/cm² | 98.07 bar |
| Hydraulic press, rated system pressure | 140 kg/cm² | 137.29 bar |
| Injection moulding machine, hydraulic maximum | 175 kg/cm² | 171.62 bar |
| Proof figure on an old test certificate | 210 kg/cm² | 205.94 bar |
| Moulding process sheet, filling pressure | 350 kg/cm² | 343.23 bar |
| Large gauge board, full scale | 400 kg/cm² | 392.27 bar |
Read the other way the pattern reverses: a modern 250 bar power unit corresponds to 254.93 kg/cm², and a flat 100 bar lands at 101.97 kg/cm². Round numbers in one unit are almost never round in the other, which is itself a useful clue to which system a machine was originally designed against.
What Helps at the Gauge Board
Type into whichever unit the paperwork uses
Neither box is locked, so a nameplate figure and a controller setpoint can be checked against one another without clearing anything in between.
One press for the nameplate direction
The swap arrows move kg/cm² to the input side, which is where it belongs while you transcribe an old test certificate into a current record.
The other retired units are listed too
Searchable dropdowns on both sides cover 26 pressure units, including the technical atmosphere and metre of water column that appear on the same vintage of drawings.
Enough decimal places for a certificate
Results carry out to eight decimals with thousands spaced for legibility, and the copy button returns the bare figure for the record sheet.
What Maintenance Crews Ask About kgf/cm²
Why is one bar 1.0197 kg/cm² instead of exactly one?
The two units were built on different foundations. A bar is defined outright as 100 000 Pa, a round figure chosen for convenience. A kilogram-force per square centimetre is a force divided by an area: one kilogram pulled by standard gravity, 9.80665 N, spread over one square centimetre, which comes to 98 066.5 Pa. Divide 100 000 by 98 066.5 and you land on 1.019716213. The near-miss is an accident of history, and it is why every legacy conversion drags that stubborn two-percent tail behind it.
Is kgf/cm² the same thing as the technical atmosphere, at?
Yes. They are two names for 98 066.5 Pa, so a value in kg/cm² reads straight across as the same number of at, and this page applies an identical factor to both. The trap is the other atmosphere: the standard atmosphere, atm, is 101 325 Pa, about 3.3 % larger. A German or Eastern European drawing marked "at" means the technical one, while "atm" or "ata" on the same sheet does not. Confusing them puts you out by roughly a third of a bar for every 10 at on the page.
The dial says kg/cm²g — what does that little g change?
It marks the reading as measured above the surrounding air, which is what an ordinary Bourdon gauge does: shut the machine down and vent it, and the needle rests on zero even though real air is still pressing on everything. A face marked kg/cm²a starts from a true vacuum instead, so at rest it sits near 1.033 kg/cm². The conversion factor is unchanged either way at 1.0197 per bar, but never compare a kg/cm²g number off a press with a kg/cm²a figure from an instrument datasheet until you have added that offset.
Can I fit a bar gauge where a kgf/cm² gauge used to sit?
Mechanically it is usually a straight swap once thread, case size and accuracy class match, because both dials measure the same physical quantity. The difficulty is human: the new face reads about 2 % lower than the operator expects, so a limit written as 140 in the setup sheet becomes 137.29 on the replacement. Either restate the operating limits in bar right through the paperwork, or specify a dual-scale face carrying both graduations, and note the changeover date so nobody spends a shift chasing a pressure loss that never happened.
The plate says kg/cm² but the manual says kp/cm² — are those different units?
No. Kilopond was the older name for kilogram-force, standard in German-language engineering into the 1970s, so kp/cm² and kgf/cm² are one unit written by two generations of draughtsmen. Plain kg/cm² is shorthand for the same thing, even though a kilogram is strictly a mass rather than a force. Treat all three as 98 066.5 Pa. The only genuine variant is a rare instrument calibrated against local gravity rather than the standard 9.80665 m/s², and those are labelled as such on the dial.
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