An altitude test is a pressure test. The input to the chamber is a pressure, in pascals or kilopascals, even when the requirement is written in metres or feet. The first job is therefore to convert the altitude the standard names into the pressure the chamber has to hold.
Altitude–pressure conversion
Sea-level standard atmosphere is about 101.3 kPa and pressure falls with height. Reference values from the International Standard Atmosphere:
| Altitude | Standard pressure |
|---|---|
| 0 m (sea level) | 101.3 kPa |
| 1,000 m | 90 kPa |
| 2,400 m | 75 kPa |
| 4,550 m | 57 kPa |
| 7,620 m (25,000 ft) | 38 kPa |
| 10,000 m | 26 kPa |
| 12,200 m | 19 kPa |
| 15,250 m | 12 kPa |
| 22,860 m (75,000 ft) | 3.7 kPa |
| 30,500 m | 1.3 kPa |
When a test method gives an altitude, convert it to pressure before writing the chamber requirement, and check whether the standard defines the condition by pressure, by altitude, or both.
Which standards govern low-pressure testing
- IEC 60068-2-13, Test M: Low Air Pressure — the IEC low-pressure method. The fifth edition, published in 2021, replaced the 1983 edition and added guidance on selecting exposure duration.
- MIL-STD-810 Method 500 — Low Pressure.
- RTCA DO-160 Section 4 — Temperature and Altitude, for airborne equipment.
- Customer or programme specifications, which often combine temperature, humidity and pressure.
IEC is clear on the point that matters here: an altitude value may describe the environmental condition, but the test system has to establish and control the specified pressure. That is why the chamber is specified in pressure, not altitude.
Depressurisation rate and pressure tolerance
On the EQT series the working range is normal pressure to 500 Pa. Depressurisation reaches 15,250 m (11.6 kPa) from site level in 10 minutes, 22,860 m (4.0 kPa) in 20 minutes and 30,500 m (1.0 kPa) in 30 minutes. Optional rapid decompression runs 75.2 kPa to 18.8 kPa in ≤15 s, and explosive decompression at ≤0.1 s. Pressure resolution is 10 Pa.
Pressure tolerance is stated by range: ≤ ±2 kPa from normal pressure to 40 kPa, ≤ ±5 kPa from 40 kPa to 4 kPa, and ≤ ±0.1 kPa from 4 kPa to 1 kPa. The tightening tolerance at low pressure is what lets a test hold a defined high-altitude condition.
Combining pressure with temperature and humidity
The same working space can simulate temperature, humidity and low pressure together (EQT and EQTH series): temperature −70 °C to +180 °C (+200 °C optional) and humidity 10% to 98% RH, with humidity uniformity ±1.0% RH and relative humidity deviation ≤ ±3.0% RH above 75% RH. If the programme only needs pressure, a pressure-only configuration is the simpler machine.
Choosing an altitude chamber
Fix the target altitude, convert it to pressure, then decide the depressurisation rate and the pressure tolerance the standard requires. Add temperature and humidity coupling only if the test calls for it. Finally confirm the vacuum pump arrangement and the site. Contact us with the altitude, the standard and the required descent profile.