One Hospital, Two Pressure Scales on the Same Wall
Walk a ward with a biomedical engineer and you cross a unit boundary every few metres. The piped services behind the head wall — oxygen, medical air, nitrous oxide — are designed, tested and labelled in bar. Plug a suction regulator into the vacuum terminal a hand's width away and its dial is graduated in millimetres of mercury. Same plant room, same commissioning visit, two scales, and any figure that moves between the pipework file and the device paperwork has to be converted on the way.
The Services Behind the Head Wall
Positive-pressure services
The vacuum side
Terminal units and regulators
Cylinders as backup
Checking a Terminal Unit Against Its Specification
Whether you are witnessing a commissioning test, closing a planned maintenance job or investigating a report of weak suction, the figures come off two instruments with different scales.
Enter the pipeline figure in bar
Type what the test gauge on the terminal-unit probe shows — 4, 4.2, 6.9. The millimetres of mercury appear straight away, and a comma is accepted instead of a full stop for anyone using a European keypad.
Flip the direction for the suction side
Suction figures arrive the other way round. Press the swap arrow (↔) to run mmHg → bar and see what a 400 mmHg regulator setting represents as a differential — 0.533 bar below atmosphere.
Copy the number into the asset record
The copy button on each field puts the digits alone on the clipboard, with no unit and no spacing, ready to paste into a CMMS work order or a test certificate. Ctrl + C inside a field does the same.
Reach for kPa when the standard asks for it
Several national standards state vacuum in kilopascals and some device labels use inches of mercury. Both dropdowns are searchable and cover all 26 units, so the third scale is one selection away.
Piped Medical Gas and Vacuum Services Side by Side
Representative design and test levels for the services one estates team looks after. Nominal pipeline pressures follow common design practice; verify against the standard in force locally and the scheme's own commissioning documents before using any figure in anger.
| Service | Typical level (bar) | Same value (mmHg) | Where it appears |
|---|---|---|---|
| Oxygen pipeline, nominal | 4 bar | 3000.25 mmHg | Terminal units on wards and theatres |
| Medical air, respiratory (air 4) | 4 bar | 3000.25 mmHg | Ventilators, blenders, humidifiers |
| Nitrous oxide and N₂O / O₂ mixture | 4 bar | 3000.25 mmHg | Anaesthetic and delivery-suite outlets |
| Surgical air (air 7) | 7 bar | 5250.43 mmHg | Bone saws, drills, tourniquet supplies |
| Oxygen cylinder, full at 15 °C | 137 bar | 102 758.45 mmHg | Backup manifold and transport cylinders |
| Medical vacuum at a terminal unit | 0.4 bar below ambient | 300.02 mmHg | Pipeline performance and commissioning tests |
| Suction regulator, lower band | 0.16 bar below ambient | 120.01 mmHg | Device dial, gentle settings |
| Suction regulator, upper band | 0.533 bar below ambient | 399.78 mmHg | Device dial, near the top of a 0–400 scale |
The asymmetry is worth noticing: the pressure services live in a narrow band a few times atmospheric, while the vacuum service can never exceed one atmosphere of differential — about 760 mmHg, or roughly 1.013 bar — no matter how large the pump set is.
What the Page Offers an Estates or Biomed Team
Pipeline side and device side at once
Both boxes accept typing and update each other live, so you can walk a regulator's scale from one end to the other and watch the bar differential follow every step.
Whichever scale the paperwork starts on
The swap arrow reverses the pair, which matters because pipeline documents arrive in bar and device manuals arrive in mmHg.
Every unit a medical gas file contains
Searchable dropdowns list all 26 pressure units across eight groups, covering kPa in a national standard, inHg on an imported device and psi on a manifold gauge.
Test-sheet ready values
Results carry up to eight decimals with thousands spaced apart for legibility, and copying gives the bare figure a certificate field can take unedited.
Medical Gas and Vacuum Pipeline Questions
Does a 4 bar pipeline still read 4 bar when several outlets are flowing?
Not quite, and it is not meant to. The nominal figure is a design pressure at the terminal unit; standards specify a pressure that must be maintained while a stated flow is drawn, because that is the condition a ventilator actually sees. A pipeline that reads 4 bar (3000.25 mmHg) statically but sags badly under simultaneous demand has an undersized run, a partly closed valve or a struggling regulator, which is why flow tests are part of commissioning rather than a static gauge check.
Why are suction regulators graduated in mmHg when the plant is designed in bar?
The device scale follows the clinical literature, and mercury has been the reference for medical measurement since long before pipelines existed. The plant follows engineering practice, where bar and kPa dominate. Neither community has an incentive to change, so the boundary sits at the terminal unit: bar on the wall side, mmHg on the device side, with the engineer converting across it.
What do the low, medium and high positions on a suction regulator correspond to?
They are bands on the device dial rather than fixed numbers, and manufacturers place them differently — commonly around 100 to 125 mmHg for the low band, roughly 200 mmHg in the middle and up towards 400 mmHg at the top, which is about 0.13, 0.27 and 0.53 bar of differential. Full-scale ranges of 0–160, 0–300 and 0–760 mmHg are all sold. Check the individual model's scale during acceptance testing; which band gets used on a patient is a clinical decision.
Do oxygen and medical air run at the same pipeline pressure?
The respiratory services generally do — oxygen, medical air and nitrous oxide are commonly distributed at the same nominal 4 bar so that blenders and ventilators see balanced supplies. Surgical air is the exception: it is a separate 7 bar (about 5250 mmHg) service because pneumatic tools need the extra pressure, and it is piped and terminated separately so it can never be mistaken for the respiratory supply.
What does a vacuum quoted as −400 mmHg actually mean?
It is 400 mmHg below whatever the local atmospheric pressure happens to be — a differential of 0.533 bar, not an absolute pressure. At sea level that leaves roughly 360 mmHg of absolute pressure in the line. The distinction matters at altitude, where atmospheric pressure is lower and the same regulator setting is a larger share of the total available differential, so a site's achievable vacuum should be assessed against its own ambient pressure.
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