January 5, 2021
2 MIN READ
At Leybold, we understand that measurement accuracy is fundamental to getting good results. Unfortunately, accuracy is not always simple to guarantee. Several factors may influence the sensitivity of vacuum gauges and thus impact their accuracy and reliability.
The top concerns are:
Vacuum and General Environment
Operating pressure range
Level of accuracy
To dial in the sensitivity of a vacuum gauge to the greatest degree possible, it is necessary to understand each of these potential mitigating factors and how they may impact your equipment.
Not all gauges are suited to all uses.
There are two main types of gauges used in measuring vacuum — ionization and thermal gauges. Both are considered indirect means of measurement, but ionization gauges are generally considered less accurate. They are better employed for low-pressure measurement.
The vacuum and ambient environment conditions can subject the gauge to high temperatures, stray magnetic fields, and cause unwanted shock and vibrations. The vacuum media being pumped may react with the gauge.
High background temperature and risk of water or dust ingress will also impact the gauge performance.
Gauges are usually factory-calibrated with nitrogen gas, so when using nitrogen gas, your correction factor is one. However, when using a different gas, the gauge must be calibrated for that substance. Failing to do so can affect gauge accuracy. Thermal and ionization gauges have separate equations used for determining the gas correction factor.
Not all gauges are capable of accurately measuring all vacuum pressure ranges. Vacuum levels (between 10 mbar and atmospheric pressure) can use Bourdon tubes, bellows, active strain gauges, and capacitance sensors. Mid-range vacuums (10-1 to 10-3) should use capacitance manometer, thermocouple, or Pirani-type gauges. High-level vacuums (10-3 to 10-9) should use cold cathode or Bayard-Alpert hot cathode gauges.
Extreme temperatures can contribute to false readings from vacuum gauges. This is often due to those temperatures breaking down vital gauge components. Extreme temperatures may also interact with water, producing frost or condensation that can obscure readings.
A gauge will generally arrive from the manufacturer with a “rough” calibration, where no correction factor has been applied. While applying a gas correction factor can greatly improve calibration, other steps should be taken when high accuracy is essential. In such situations, calibration should be conducted across the entire pressure range and for each gas type where possible.
If the work being performed causes regular and repetitive overpressure spikes, this can damage the gauge. This damage results in marked accuracy issues.
Vibrations from heavy machinery, motors, and other equipment can damage a gauge over time. This damage is especially common in the pointer mechanism, due to its being frequently shaken off of zero. Additionally, readings taken while vibrations are affecting the gauge are generally prone to inaccuracy.
Do you have additional questions about gauges in your environment?
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