Factors to consider when choosing vacuum gauges

August 25, 2020


When it's time to select a new vacuum gauge to complement your vacuum pump or system, it's important to take a thoughtful approach. At first glance, it might seem like a simple task, but there are several factors you need to consider. 

Various elements affect a vacuum gauge’s overall performance and govern the decision making process. Each gauge has its own operating range, accuracy level, and offers its own sets of benefits… and limitations. Looking at these key areas we can help to define the right choice of gauge for your vacuum system.

  •  Operating vacuum level and accuracy

  • Process or application impact on gauge  

  • Interface to your system

  • Cost and maintenance

Vacuum gauge options

Before diving into these main options, lets investigate gauge types. There are two overarching categories of vacuum gauges: Direct Gauges and Indirect Gauges. 

Direct gauges

 A direct gauge will give you the pressure of the vacuum system, independent of gas type (therefore also known as gas independent gauges). These will only effectively work at pressures above 10-4 mbar. This is because they rely on the molecules in the system having a physical effect, and at lower pressures there are not enough of them to create a measurable effect. They include:

  • Piezo – accurate measure of vacuum and overpressure down to around 1 mbar, at which point the accuracy drifts.

  • Mechanical Dial – giving a visual indication of the pressure, so helpful in applications where having a local display is useful.

  • Capacitance – highly accurate, capable of measuring from atmosphere to 10-4 mbar. 

Indirect gauges

Indirect gauges often rely on two methods, thermal transfer or ionisation. Both of these are dependent on the gas type, and therefore knowing what gas you are measuring is important if you want to know the true pressure. By knowing the gas type, you can apply a gas correction factor. These gauges fall into three main types:

  • Pirani – thermal gauge – used in rough to medium vacuum, often as a cheap alternative to Piezo/Capacitance gauges where accuracy is less important.

  • Cold cathode – ionisation gauge – used in medium to high vacuum for indication of pressure.

  • Hot Cathode – ionisation gauge – used in medium to ultra-high vacuum – higher accuracy than the cold cathode and with a wider range. The downside is higher power, and a susceptibility to vibration/shock air inrush.

With indirect gauges there is another sub section which is Active and Passive. Most gauges you see on the market are Active, with the electronics to do the signal processing to give a simple 0-10 V or other output on board. However, in environments with large amounts of radiation, this is simply not workable as the electronics would break. In these circumstances, you can get versions of the gauges with the electronics moved into a controller which can be positioned away from the radiated area. But this is a topic for another time!

Related: Find out more about vacuum gauges and how sensitive they are to a range of external influences in this blog post by Vacuum Science World,  Seven Factors Affecting the Sensitivity of Vacuum Gauges.

Vacuum level and accuracy

Vacuum level is perhaps the most critical consideration. 

It affects each of the other factors, as you obviously need a gauge that measures at your working pressure. Typical classifications and ranges include: 

  • Rough vacuum: from above atmosphere to 1 mbar 

  • Medium vacuum: 1 mbar to 10 -3 mbar 

  • High Vacuum: 10 -3 mbar to 10 -9 mbar 

  • Ultra-high vacuum: 10 -9 mbar to 10 -12 mbar 

Different Gauge technologies are required to achieve different vacuum pressure ranges. Depending on the target vacuum level, a mix of Gauges might be necessary to get measurement across the entire pumping range.  

Rough/Medium vacuum gauges — there are many options, including both Direct and Indirect gauges, so knowing the process requirements is vital for choosing the right gauge. 

High and ultra-high vacuum gauges — slightly fewer options, with the area dominated by either Hot Cathode or Cold Cathode type gauges, both of which are Indirect gauges. 

Pressure gauge graph

Generally speaking, gauges operating in rough to medium vacuum have higher accuracy than those operating in medium to ultra-high vacuum. Direct gauges are also more accurate than indirect. Direct gauges usually have an accuracy of between 0.2 to 2%, with their accuracy decreasing as the pressure drops. Indirect gauges generally vary between 10% and 30% depending on the product type, but have consistent accuracy across their ranges.

Therefore, depending on what you are intending to use the gauge for, you need to choose the right balance between desired accuracy and operating pressure. 
Related: Explore the Leybold range of  precision vacuum gauges and controllers on our product page. 

Process or application impacts on gauge

The way processes can impact gauges plays a significant role in selection. The impact of the application on the selected gauges technology needs to be evaluated carefully, as well as the potential impact of:

  • Dust or debris from the process

  • Corrosive gases like chlorine or acidic mixtures in the presence of water vapor

  • Frequent venting

  • Vibrations

  • Particle or X-ray radiation

  • Magnetic and electric fields

As an example, Pirani Gauges are well-suited for many Rough and medium vacuum applications, such as:

  • Research and development

  • Analytical instruments

  • Industrial and coating activities

  • Freeze drying

  • Process engineering 

On the other hand, Capacitance Gauges provide high accuracy by electrically measuring the small deflections on a diaphragm membrane caused by fluctuations in vacuum levels. However, with great accuracy comes an increased cost, and also a more focused measuring range. That’s why it is important to know what you require from your measuring system. 

Interface into your system

Every vacuum gauge will output a signal. However, with the proliferation of communication interfaces it can be a minefield determining what signal is best. The majority of applications utilise a simple 0-10 V signal which can be converted into a pressure using a simple equation. This is the method of setting up a system which requires the least effort, although some industrial applications still use a 4-20 mA output.

However, with the increased need for data collection, digital communication protocols are becoming common directly on the gauge head itself, instead of via a hub converter. These include RS232/RS485, EtherCAT, Profibus, Ethernet IP, and the list goes on.

Cost and maintenance

As mentioned earlier, knowing what pressure you want to measure is important, but everyone has a budget. You may ideally want 0.1% accuracy, but if you can get 1% accuracy for half the cost it may be important to consider what you actually require in order to operate and monitor your system.

The main points that affect cost are the accuracy and pressure range:

  • Accuracy – as you’d expect, high accuracy usually drives higher cost. This is especially true in Rough/Medium vacuum with  Capacitance gauges being significantly more expensive than Piezo gauges.

  • Pressure range – often two gauge types could be used together or integrated into one to measure across a wider pressure range, leading to a higher cost for the integrated gauge

Final thoughts

Making the right gauge choice for effective vacuum generation requires an understanding of your vacuum levels, and your data needs. Vacuum level needs, cost, and maintenance also impact vacuum gauge choice. 

Whilst it may seem daunting, the options can be easily narrowed down to the right gauge by carefully looking at your system requirements, of which we have a very wide range to suit every process need.

If you've got questions that require a little more detailed attention, reach out and chat to the team! Click the button below, and we'd be glad to set up a meeting and find efficient, cutting-edge solutions to your vacuum needs.

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