5 tips and tricks for choosing the right vacuum gauges

January 27, 2023

7 MIN READ

When it's time to select a new or replacement vacuum gauge for 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 must consider before choosing a gauge.

Various factors affect a vacuum gauge’s overall performance/reliability and must be part of the decision process. Each gauge has its own operating range and accuracy level and offers its own set of benefits and limitations. Looking at the following tips will help you make the right choice of gauge for your unique application.

Operating vacuum level and accuracy
Process or application impact on the gauge
Ambient conditions
Interface to your system
Cost and maintenance

Vacuum gauge options

Before diving into these main options, let’s investigate gauge types. There are two main types of vacuum gauges: direct gauges and indirect gauges.

Direct gauges

Direct gauges will give you the pressure of the vacuum system, independent of gas type (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 molecules to have a measurable effect. They include:

Piezo – accurate measure of vacuum and overpressure down to approximately 1 mbar, below 1 mbar accuracy tends to drift
Mechanical Dial – visual indication of the pressure, useful in applications for local display
Capacitance – highly accurate, capable of measuring from atmosphere to 10^-5 Torr (Measures 4 decades with extreme accuracy within top 2 decades)

Indirect gauges

Indirect gauges often rely on two measurement technologies: thermal conductance and ionization. Both measurement technologies are dependent on the gas type. 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 typically fall into three main types:

Pirani (thermal gauge) – used in rough to medium vacuum, often as a low-cost alternative to Piezo/Capacitance gauges where accuracy is less important.
Cold cathode (ionization gauge) – used in medium to high vacuum for indication of pressure - robust technology for high particulate applications
Hot cathode (ionization gauge) – used in medium to ultra-high vacuum, higher accuracy compared to a cold cathode, and with a wider range, the downside is its higher power and susceptibility to vibration/shock air inrush – good for clean high vacuum applications

With indirect gauges, there is another sub-section: active and passive. Most gauges you see on the market are active with embedded electronics to do the signal processing to give a 0-10 V or other analog output proportional to the pressure. However, in environments with large amounts of radiation, the active sensors are affected and not reliable. In these circumstances, you can get versions of the gauges with the electronics removed and installed in the controller, which can be positioned away from the radiated area for optimum performance.

Vacuum level and accuracy

Vacuum level is perhaps the most critical consideration; it affects each of the other factors. Typically, you need a gauge that measures within your operating pressure range. Typical classifications and ranges include:

Rough vacuum:  from above atmosphere to 1 mbar
Medium vacuum:  1 mbar to 10^-3mbar
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 combination of gauge technologies might be necessary to get measurements across the entire pumping range. 

Rough/medium vacuum gauges — many options, with both direct and indirect options available, 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.

Typically, gauges operating in rough to medium vacuum have higher accuracy than those operating in medium to ultra-high vacuum. Direct gauges are 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 50%, depending on the gauge technology, but have consistent accuracy across their ranges.

Depending on what you are intending to use the gauge for, you need to choose the right gauge to cover your operating pressure and desired accuracy.

Reference: Explore the Leybold range of precision vacuum gauges and controllers on our product page.

Process or application impacts on gauge

The way process gases can impact gauges plays a significant role in selection. The impact of the application on the selected gauge 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 and hydrocarbons
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 in a specific range of vacuum by electrically measuring the small deflections on a diaphragm membrane caused by fluctuations in vacuum levels. However, with great accuracy comes an increased cost. Therefore, it is very important to know what you require from your measuring system.

Ambient conditions

The general environment for the installation of a particular gauge can impact the decision of gauge equipment. If the gauge is being installed in a high-temperature environment and there is the risk of dirt, dust, and/or potential of ingress of water, you may consider using additional protection. This leads to the need for additional protection of the gauge and possibly an enclosure or a modified environment. You must also consider the potential effects of radiation and output signal loss with the need to control a gauge from long distances.

Interface into your system

Every vacuum gauge will output a signal. However, with the proliferation of communication interfaces, it can be a minefield knowing what signal is best. The most common of applications utilize a simple 0-10 V analog signal that can be converted into a pressure using a simple equation. This is the lowest effort way of setting up a system, although some industrial applications still use a 4-20 mA output.

With the increasing need for data collection, digital communication protocols are becoming common as a direct output from the gauge head itself, instead of via a hub converter. The requirements include RS232/RS485, EtherCAT, Profibus, Ethernet IP (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 want 0.1% accuracy, but if you can get 1% accuracy for half the cost, it may be important to consider. That’s why it’s so important to understand the actual requirements needed to monitor and control your system effectively.

Main points that influence costs are the accuracy and pressure range:

Accuracy – As you’d expect, high accuracy usually drives higher costs. This is especially true when in rough/medium vacuum with capacitance gauges being significantly more expensive than Piezo gauges.
Pressure range – To measure across a wider pressure range, two gauge types are often used together or integrated into one (combination gauge), leading to a higher cost.

Tricks for installation to improve performance and reliability

If possible, it’s recommended to install your gauge in the vertical orientation with a 90-degree elbow as close to the chamber as possible to increase the overall performance and to keep unwanted particulate, dust, dirt, and moisture from entering the gauge. It’s also important to install your gauge 90 degrees from any inlet purge gas or vent stream (see sketches below):

Recommended:

Not Recommended:

Final thoughts

I hope these tips and tricks for choosing the right vacuum gauge will help you make the right decision when choosing an effective gauge for your application. A better understanding of your vacuum pump and/or system requirements, like pressure range, accuracy, control requirements, and cost impacts, will help you make the right decision.