First identify what the instrument measures, its reference and the measurement point. Gauge pressure, absolute vacuum and pipe temperature answer different questions. A pressure reading alone cannot establish refrigerant charge, and a small display increment does not establish measurement accuracy.
Source-based explanation
Why does a pressure reading need a stated reference?
Gauge pressure is relative to the surrounding atmosphere; absolute pressure is referenced to zero pressure (a perfect vacuum). At the same place and time, absolute pressure equals gauge pressure plus local atmospheric pressure. Weather reports commonly quote a value adjusted to sea level. That is not necessarily the atmospheric pressure at your instrument.
| Quantity | Useful record | Common mistake |
|---|---|---|
| Gauge pressure | bar(g), kPa(g) or psi(g), plus measurement point | Treating zero gauge pressure as zero absolute pressure |
| Absolute vacuum | Microns of mercury, Pa absolute or mbar absolute | Treating a manifold’s negative-pressure display as a deep-vacuum measurement |
| Pipe temperature | °C, probe position, surface condition and stabilisation | Treating poor clamp contact as the refrigerant’s true temperature |
| Temperature difference | K or °C difference, with both source values | Confusing a calculated difference with a directly measured property |
Does converting to a smaller vacuum unit make the instrument more capable?
One micron of mercury is 0.001 Torr, approximately 0.133322 Pa. This is a pressure unit, not a length measurement in this context. Conversion changes the unit, not the sensor’s capability. The negative end of a refrigerant manifold’s pressure range may be too coarse for the evacuation criterion specified for the equipment.
Record where the vacuum gauge is connected. Pressure at the pump and pressure elsewhere in a connected system can differ while gas is flowing. A pump-side reading alone does not demonstrate the condition throughout a system.
Why do temperature and the refrigerant table belong together?
Superheat is measured vapour temperature minus saturation temperature at the corresponding pressure. Subcooling is saturation temperature minus measured liquid temperature. For blends with temperature glide, dew-point data are normally relevant to superheat and bubble-point data to subcooling; follow the equipment procedure for the actual refrigerant.
An unexpected negative superheat value is a reason to check the refrigerant selection, pressure reference, probe location, clamp contact and connection before diagnosing the system. A calculation can be numerically correct with the wrong inputs.
What is the difference between resolution and accuracy?
Resolution describes the smallest displayed step; accuracy describes the stated relationship between a reading and the measured quantity. A fine display cannot remove sensor error, uncertainty or unsuitable measurement conditions. Compare the full accuracy expression, including fixed terms and operating limits, before deciding whether an instrument can support the required decision.
| Specification | What it describes | What remains unresolved |
|---|---|---|
| Resolution | The smallest displayed step | The uncertainty or error of the reading |
| ±% of reading | An error term that scales with the value | Additional fixed terms and operating conditions |
| ±% full scale (FS) | An error term based on a specified scale | Which full-scale definition and temperature conditions the manufacturer uses |
| Calibration result | Comparison with a reference at stated points | Performance outside those points or after damage/contamination |
Arithmetic illustration, not a test: applying a specification of ±(10% of reading + 10 microns) at 500 microns gives ±60 microns. The display resolution must be considered separately from that accuracy specification. The illustration is not an evacuation acceptance target.
How should the reading be used in context?
Keep the model, measurement point, unit, operating state, time and probe condition beside the reading. Repeat observations under comparable conditions before attributing a change to the system. Use the equipment manufacturer’s acceptance criteria; this page supplies no universal pressure, temperature or vacuum pass value.
Which sources support this reference, and what are their limits?
These sources support the terminology, declared capabilities and attributed observations discussed in this reference. Manufacturer declarations and other authors’ observations are not our physical test results. Check the exact model and the source’s stated conditions before applying a claim; the references were consulted on .
- HVAC School: negative superheat and tool checks ↗
US technical explanation; not UK regulatory guidance.
- AC Service Tech: why pressure alone does not establish charge ↗
US technical education; system-specific commissioning instructions remain necessary.
- HVAC School: barometric pressure and calibration ↗
Explains local absolute pressure versus weather pressure corrected to sea level.
- ElitechEU: VGW-mini product specifications ↗
Manufacturer declaration; options and customer comments are not a verified kit inventory.
What should stay with the reading?
A useful measurement record preserves the conditions, instruments, connection details and original reading, then adds any repeated observation or retest. Use the blank field record to keep those details together. See how we assess evidence and our commercial relationship when judging the scope of a published conclusion.
