Vacuum gauge selection for industrial processes
January 19, 2021
9 MIN READ
When the time comes to select a new vacuum gauge to complement your vacuum pump or system, it's important to take a thoughtful approach. This is especially true when integrating into an industrial process where the duty can be harsher than normal. Taking this into account, we will look at the main gauge types used in industrial processes, their pros and cons and how they are best used.
Vacuum gauge options
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 1e-4mbar. This is because they rely on the molecules in the system having a physical affect, and at lower pressures there are not enough of them to create a measurable effect. They include:
CTRN (Capacitance) – the gold standard of accuracy and reliability due to its Inconel diaphragm. This gauge is used as a reference gauge around the world and across the harshest applications. Capacitive gauges are highly resistant to pressure bursts/cycling, corrosive gasses, and with heated options, able to cope with various condensables over a measuring range of atmosphere to 1e-4mbar. This best in class performance obviously comes with a cost, but considering this gauge won’t need servicing or replacing, it can often pay itself off over the lifetime of a system.
DI/DU (Piezo/Capacitive) – One step down from the CTRN range you have the DI/DU range, utilizing similar technology, but in a condensed, IP54 rated package. Whilst the CTRN range is targeted at a broad range of the market, the DI/DU gauge is firmly targeted towards the industrial market, and as such comes in both standard 0-10v outputs and 4-20mA, with ranges covering 2000 to 0.1 mbar.
DIAVAC/Capsule/BOURDONVAC (Mechanical Dial) – when most people think of a gauge this is what they imagine. A simple mechanism physically moves a dial against a background giving a clear indication of pressure without the need for a separate display. As such, they are often used as a back-up so the users of a system can see what is going on.
Indirect gauges
Indirect gauges often rely on two methods, thermal transfer or ionisation. Thermal transfer typically covers atmosphere to medium vacuum, and ionisation typically covers medium to high or ultra-high vacuum. Often these two technologies are combined into a single sensor offering full range measurement.
TTRN/TTRR (Pirani - Thermal) - used in rough to medium vacuum, often as a cheap alternative to Piezo/Capacitance gauges where accuracy is less important. These come in two main types; MEMS (TTRN) and Filament (TTRR). Whilst MEMS technology has increased range and accuracy, the sensor itself is less able to cope with particle and other contamination and therefore requires protection if being used. Filament technology has been used for many years in industrial processes, and with its high temperature filament is not only able to cope with 99% of process requirements but is more cost effective and therefore is more commonly chosen in industrial applications.
PTRN (Cold cathode – ionisation gauge) – when seeking to measure below 1e-4, Cold cathode gauging is the most popular choice. Its simple anode/cathode construction is robust against pressure bursts, and with its small size and low magnetic stray field it has little to no impact on its surrounding environment. Because of its simple construction it is easily and cheaply serviceable by the user, reducing the overall cost of ownership
ITRN (Hot Cathode – ionisation gauge) – where cold cathode gauging is more robust, hot cathode gauging instead offer higher levels of accuracy and a wider measuring range. So this type of gauge can be used where needed for industrial applications, but will often need protection from the process by being fitted on an elbow or with some form of baffle.
With indirect gauges there is another sub section of Active and Passive. Most gauges on the market are Active, with electronics to do the signal processing to give a simple 0-10v or other digital output on board. However, in environments with large amounts of radiation, this is simply not possible 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. These are our TR (pirani), PR (cold cathode) and IE (hot cathode) ranges.
Vacuum level
So now you know a bit about the various gauge types and their strengths and weaknesses, it’s time to narrow down the choice. Vacuum level is a good place to start, as the sensor has to be able to measure at your process pressure. Typical classifications and ranges include:
Rough vacuum: from above atmosphere to 1 mbar – if this is all you need to measure, then typically a DI/DU gauge would be used, although if you need an increased level of accuracy then upgrading to the CTRN will ensure accurate process tracking.
Medium vacuum: 1 mbar to 10 -3 mbar – once you are measuring down to this level, the choice is between using multiple CTRN gauges to cover the entire pump down range, or a TTRN which will cover the range with a single gauge, but with a lower accuracy.
High Vacuum: 10 -3 mbar to 10 -9 mbar – The options for high vacuum are PTRN and ITRN gauging. For ease of use, lifecycle cost, and instillation flexibility, most applications will suit the PTRN, with only a few specific cases requiring the increased accuracy of the ITRN.
Ultra-high vacuum: 10 -9 mbar to 10 -12 mbar – when measuring into UHV, the only choice is hot cathode gauging, so either our active ITRN line or our passive IE line. Both of these are highly accurate but will benefit from not being directly in the process line of sight.
Process impacts on gauges
The way processes 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 – if dealing with this on a regular basis, then the best gauge types are the CTRN and the DI/DU. If measuring a lower level of vacuum is needed, then the TTR91R with its hot filament and integrated filter will also offer good performance.
Corrosive gases – again the CTRN and DI/DU ranges are perfect as the materials in contact with vacuum are all highly corrosion resistant. When measuring to lower levels, you will need to assess the specific gasses with our materials of construction to ensure a long lifetime, something our dedicated applications team can help you with.
Frequent venting – Here, the CTRN is perfect as it is designed for fast load lock style applications. Alongside this, the TTR101 combines a small capacitive cell with a filament pirani, so if working across a wide range it can accurately track the pressure bursts and return reliable figures. Again, if measuring into lower vacuum levels, the PTRN is a suitable choice as its rugged anode/cathode is resistant to shock air inrush.
Vibrations – vibration can affect the stability of readings where gauges utilize filament technology (such as the TTR91R) or hot cathode (ITR). So, in both cases the gauge should be isolated if possible, or use the cold cathode (PTRN) which is not affected.
Particle or X-ray radiation – Here you are restricted to passive gauging as radiation will interfere with active gauging electronics.
Whilst the CTRN and DI/DU gauges get mentioned a lot as being suitable for harsh applications, their Achilles heel will always be their range and cost. As such, the most commonly used gauge type is the pirani gauge, specifically the filament type as it offers a very well-balanced cost vs performance.
Interface into your system
The majority of applications utilize a simple 0-10v signal which can be converted into a pressure using a simple equation. This is available on all our active gauges. However, some industrial industries still favor the 4-20mA output, therefore the DI/DU gauge, which is firmly aimed at heavy industrial applications, has a 4-20mA variant.
However, with the increase of data collection, more 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 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 what you actually require in order to operate and monitor your system.
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 when in rough/medium vacuum with capacitance gauges being significantly more expensive than Piezo gauges.
Pressure range – often to measure across a wider pressure range two gauge types are integrated into one, leading to a higher cost for the integrated gauge, but then you can save by only having one flange on your system.
Maintenance is also important to consider when looking at the lifecycle of the product. As with pumps, a proper maintenance schedule can vastly increase the lifespan of the gauge and ensure performance to spec when operating in harsh environments. On some gauges the maintenance is simply swapping out the measuring cell (such as on the TTR91R) or it can be removing the measuring cell and manually cleaning it (such as on the PTRN series). Then at the higher end, gauges like the CTRN are classed as being “maintenance free” as they are able to cope with the harshest environments. The only input needed by the customer would be to zero the gauge, but even this is only needed in the toughest of processes.
Final thoughts
Making the right Gauge choice for effective vacuum generation requires an understanding of your vacuum levels, and your process needs. Whilst it may seem a daunting choice, by carefully looking at your system requirements, the options can be easily narrowed down so that you implement the perfect gauge for your system.
The final point to consider is whether you have a requirement for a dedicated local display/controller for your gauge. This obviously enables you to see the pressure of your system away from any dedicated HMI, but also offers the ability to have one integration point for your system of gauges, rather than multiple, making said integration easier. We have two main ranges, the basic DISPLAY models, which are focused on simply providing a local display, and the more advanced GRAPHIX models which offer greater connectivity options.
