Gas Independent vs. Gas Dependent
Direct vs. Indirect
Questions to take into consideration
Hello, Bonjour, Hola, etc. I am the Global Product Manager for Instrumentation and Gauging and I am here to help you select a gauge technology for your application. Spec'ing in a vacuum gauge, while it might seem difficult (like black magic), isn't too shabby if you know what to look for. This section titled "How to spec in a vacuum gauge" provides a thought process for how to do just that, while also providing quick or detailed suggestions along the way. Please understand that these are only suggestions, if you are confused or need additional info, don't hesitate to reach out to email@example.com where you can talk to our product specialists or product manager.
Note 1: While I would love to lead everyone down the rabbit-role that we call pressure measurement, I know that time is precious. For those that want to travel down this rabbit-hole of excitement, I have provided many different variables and detailed explanations (as detailed as my experience allows); these will be labeled as "Rabbit-Hole". For those that do not have the time, I have provided short, precise suggestions; these will be labeled as "Short, Sweet, and to the Point".
Note 2: There are many different variables to consider when spec'ing in an appropriate vacuum gauge. This technical document is meant to be a guide to do just that (with a few quirky puns along the way); taking into consideration all of the variables that I can think up. Spec'ing in a gauge revolves around a lot of chemistry and process details, so if you have one of those wonderful scientists around the office, grabbing that person could be beneficial as well. If you are reading this, I know you are eager to gets into the details, so without further adieu, lets get this show on the road.
Before we delve into the more complex variables, we will first define a few key terms that are easy to mix-up in the pressure measurement realm (active vs passive, gas independent vs. gas dependent, and direct vs. indirect).
Active vs. PassiveShort, Sweet, and to the Point
Active = integrated electronics (transducer)
Passive = no integrated electronics (gauge)Rabbit-Hole
Passive gauges are gauges that do NOT have any integrated electronics (i.e. thermocouple gauges tubes). While you are probably used to pressure measurement devices that provide at analog or digital output (transducers), passives gauges do not have this luxury. A passive gauge will interact with the molecules in some fashion and provide a "language" that needs to be decoded (more or less). That is where you would need a controller to decipher this language, convert it to something it can read (an analog and / or digital output), and then convert that output to a pressure. These gauges are especially useful in areas of high temperature or radiation, as any electronics would be damaged if exposed to such environments. Passive gauges, as of 2018, are NOT available in a combination style setup. That means that you will NOT have a pirani / ion gauge combo in one single unit!
Active gauges are the exact opposite. These gauges, more commonly known as transducers, have some type of integrated electronics. An example of this would be the KJLC 275i, Inficon Gemini, or MKS 626C Baratron. These electronics take the "language" that the unit provides and automatically converts it to some type of output. It is more common to see these transducers as you can easily extract the output (i.e. 0 to 10 VDC). You can also acquire integrated displays with transducers, which come in handy when you do not have a lot of extra room for a separate, bench-mount display. Transducers also provide the ability to come in a combination. You will see many different types of combination gauges, where there are multiples technologies in one single unit. This tends to be VERY beneficial when there aren't many ports on the system. You also don't need to worry about switchover points between technologies, as the transducer generally does it automatically.
Anything with integrated electronics will have an analog and / or digital communication. Due to this, the transducer in question can "easily" be connected to a system or PLC. One just needs to fabricate a cable that, at the least, transmits the analog or digital signal to the system or PLC in question. The system or PLC will then use an equation to convert the output to a pressure. For example, the KJLC Pirani has a measurement range voltage of +1,9 to +10,0 VDC. Using the equation p = 10((V-c)/1.286) c=6.304 and plugging in the voltage for V, you will receive a pressure measurement.
Gas Independent vs. Gas DependentShort, Sweet, and to the Point
Gas Independent: Unit measurement is not affected by gas molecules other than what it was calibrated for
Gas Dependent: Unit measurement is affect by gas molecules other than what it was calibrated for (generally N2)Rabbit-Hole
Gas independent units, such as a piezo gauge or capacitance manometer, are not affected by the gas composition of the process. This means that if you are a mad scientist and run copious amounts of different gases at once, the unit will maintain the any accuracy (pending of course that you aren't over-pressurizing the unit or causing an explosive environment… then you would be in trouble).
Gas dependent gauges, such as the thermocouple, pirani, or ion gauge, are affected by what gases are in the system. These units are generally calibrated in nitrogen, so running anything else (except air) would cause the unit to lose accuracy. This is where you would need to apply a calibration coefficient (if possible) for the gas that you are running to ensure that you have an accurate measurement. These gas factors can generally be found in a manual or by contacting your friendly neighborhood gauging specialist through firstname.lastname@example.org.
Direct Vs. IndirektShort, Sweet, and to the Point
Direct: Measures the force of the gas molecules to measure pressure (molecules impacting a cylindrical diaphragm).
Indirect: Uses properties of the gas molecules to measure pressure (ionization, thermal conductivity)Rabbit-Hole
Direct vs. Indirect tie in with gas independent vs. gas dependent. A direct measurement is where a unit, such as a piezo or capacitance manometer, measure the force of the molecules in the system. These units don't care if the gas is simply nitrogen or a mixture of argon / oxygen / hydrochloric acid (what would be an interesting combination). The unit will maintain the accuracy while measuring the force that the molecules apply to the diaphragm within the unit (for a capacitance manometer). Accuracy is maintained as long as you are not damaging the unit.
Indirect measurement however is where a unit, such as a thermocouple, pirani, or ion gauge, measures the pressure using a characteristic of the molecules. In the instance of a thermocouple gauge or pirani gauge, the filament inside the units interact with the molecules and provide an output based of a temperature difference (thermocouple gauge) or voltage difference (pirani gauge). How these gauges work are described in more detail via the Tech Notes. An ion gauge will actually ionize the molecules in the system and measure an electrical phenomenon. All three types however are using an "indirect" form of measurement. These units are generally calibrated in nitrogen so any other gas will affect the accuracy.Technologies
- Pirani konvektionsoptimiert
- Heißes Filament
- Capacitance Manometer
Where to go from here?
Now that those terms are defined, let's get right into spec'ing in a gauge. To simply this as much as possible, we can break this thought into two (2) scenarios: a) we only need to read a base pressure or b) we are looking to control pressure. If you are looking for process control, please direct yourself the following pages and / or contact email@example.com.Only Reading Base Pressure, No Pressure Control
Since we are only looking at base pressure, we can narrow what pressure measurement device you'll through answering a few questions. We will walk through each of these questions, in which I will provide some type of recommendation. Please keep in mind that these questions will lead to a certain gauge technology suggestion. From here, you can determine what flange termination, connector type, relay status, etc. is needed for the process. Following the questions section will be a few additional pieces of information that I have come across over the years that could help you in determining a gauge.Questions:
- What is the base pressure?
- What is the temperature of the process?
- Will the gauge be exposed to any of the following: magnetism, radiation, high temperature, or explosive gases?
- What are the process gases?
- What accuracy do you need?
What is the base pressure? (Answers question 1)
To make this easy (or easy in my mind), I have split the pressure range into four (4) main sections. Within each section, there is information on what devices work, with pros / cons of each gauge. These levels will all start at atmosphere (ATM; 760 Torr) and go down to: a) 0,1 Torr, b) 1e-4 Torr, c) 1e-7 Torr, d) 1e-9 Torr (or lower).Short, Sweet, and to the Point - Pressure Range:
- 0,1 Torr to 760 Torr:
- Piezo (gas independent, best accuracy, 0,1 Torr to 760 Torr)
- Thermocouple (gas dependent, meant for industrial applications, cost effective, 1e-3 Torr to 760 Torr or 1e-3 Torr to 1 Torr)
- Pirani (gas dependent, generally preferred in Europe, 1e-4 Torr to 760 Torr)
- Convection Enhanced Pirani (gas dependent, second best accuracy, 1e-4 Torr to 760 Torr)
- 1e-4 Torr to 760 Torr:
- Pirani (gas dependent, generally preferred in Europe, 1e-4 Torr to 760 Torr)
- Convection Enhanced Pirani (gas dependent, second best accuracy, 1e-4 Torr to 760 Torr)
- 1e-7 Torr to 760 Torr:
- Combination of an Ion Gauge and Pirani / Convection Enhanced Pirani (1e-4 Torr to 760 Torr); Ion gauge can be cold cathode (8e-10 Torr to 8e-3 Torr) or hot filament (1e-9 Torr to 5e-2 Torr); please also look at wide range or combination gauges (gauges will multiple technologies within 1 housing)
- 1e-9 Torr to 760 Torr:
- Combination of an Ion Gauge and Pirani / Convection Enhanced Pirani; Ion gauge can be cold cathode (8e-10 Torr to 8e-3 Torr ) or hot filament (nude UHV type) (5e-11 Torr to 5e-2 Torr); please also look at wide range or combination gauges (gauges will multiple technologies within 1 housing)
- 0,1 Torr to 760 Torr: Piezo, Thermocouple, Pirani, or Convection Enhanced Pirani
- A Piezo (gas independent 0,1 Torr to 760 Torr): This is a gas independent gauge, which means the different gases within the system do not effect accuracy. Think of this as the baby brother (or baby Groot for all of you Guardians of the Galaxy fans) to the capacitance manometer. If you need an accurate gauge, the Piezo is the most cost effective, gas independent option and will read from 0,1 Torr to 760 Torr. This gauge has integrated electronics, so one thing to consider is high temperature, radiation, or magnetism. Any of these three will affect the electronics.
- Thermocouple gauge (gas dependent, 1e-3 Torr to 760 Torr or 1 Torr to 1e-3 Torr): This is a passive gauge, meaning it does not have built-in electronics. Since it does not have built-in electronics, it can be used in processes with high temperature, radiation, or magnetism. This gauge needs to be accompanied by a separate, remote controller, but is cost effective when having to replace the gauge tube itself. If this type of gauge is contaminated or broken, you could literally toss this around and get another one. Please note, there are many types of thermocouple gauges. Some read from 1e-4 Torr to 760 Torr, while others read from 1e-3 Torr to 1 Torr. Due to the design of this gauge however, the accuracy from 10 Torr to 760 Torr is not the best (+/-50%). If you are looking for a higher accuracy in that region, please look at a Piezo or Convection Enhanced Pirani.
- Pirani gauge (gas dependent, 1e-4 Torr to 760 Torr): This is similar in to a thermocouple gauge in regards to accuracy, but reads from 1e-4 Torr to 760 Torr. These gauges generally have integrated electronics, so processes with high temperature, radiation, and magnetism will not work for this gauge (unless you can separate the electronics). Generally speaking, this type of gauge is exceptionally popular in Europe.
- Convection Enhanced Pirani gauge (gas dependent, 1e-4 Torr to 760 Torr): This gauge reads from 1e-4 Torr to 760 Torr and is the bigger brother to the Pirani gauge. The "Convection Enhanced" portion is in regards to an upgraded design that allows an even flow of molecules, minimizing the probability that these molecules will "sit" on the filament (which causes an inaccurate reading). This type of gauge provides the best accuracy out of the thermocouple, pirani, or convection enhanced pirani gang. Unlike the standard Pirani gauge, the Convection Enhanced Pirani is generally popular in the US.
- 1e-4 Torr to 760 Torr: Pirani gauge (gas dependent, 1e-4 Torr to 760 Torr) or Convection Enhanced Pirani gauge (gas dependent, 1e-4 Torr to 760 Torr):
- In a similar fashion to the above, you have either a Pirani or Convection Enhanced Pirani. For a similar price point, I generally suggest the Convection Enhanced Pirani, as it is an "upgraded" version of the pirani. With that said, this is where you "get in the weeds" as you need to consider whether you need an integrated display or a remote display. The units with integrated displays generally cost more up front and over time (unless you can replace just the sensor if it becomes contaminated). The units with remote displays have a lower cost of ownership (usually) as you only have to replace the sensor.
- 1e-7 Torr to 760 Torr: Combination of an Ion Gauge and Pirani / Convection Enhanced Pirani (1e-4 Torr to 760 Torr); Ion gauge can be cold cathode (8e-10 Torr to 8e-3 Torr) or hot filament (1e-9 Torr to 5e-2 Torr) (glass, nude, or with electronics)
- This is where we add in another type of technology; the ion gauge. For this selection, we will exclude the roughing gauge as that was discussed in the previous selection. When selecting an ion gauge, there are two options: a) cold cathode or b) hot filament. Also note that you can have either two separate gauges (roughing and high vacuum) or a combination (aka wide range) gauge that incorporates multiple technologies into one housing.
- Hot Filament (1e-9 Torr to 5e-2 Torr) : Back in the day, the Bayard-Alpert (B-A for short) glass tube hot filament gauge (yes, that a mouth-full) was the go-to gauge. Side note: Any of the glass or nude hot filament gauges (you may also hear Kovar, Nonex, or Pyrex) are passive gauges so you need an external controller. This type of ion gauge had far better accuracy over the cold cathode and was used for almost everything! Since then, the hot filament ion gauge has seen an upgrade, now having units with integrated electronics (tranducers) and optional displays. These gauges can be activated at any pressure, but it is suggested to not activate until at least below 5e-5 Torr. If you start any earlier, you will damage the filament. With a tungsten filament, it will go "POOF" and instantly oxidize (at atmosphere).. Which means you need to buy another filament. If you use an iridium filament, you can get away with exposing it to atmosphere a few times, but you damage the filament each time. The more you expose the iridium filament to high pressures, the more likely you are to have to replace the filament. With this said, you are able to replace the sensors if the filaments either burn out or become contaminated. You can also "degas" a hot filament ion gauge. This is where you send a high current through the unit and literally bake-out the inside of the gauge. Say goodbye to most organic materials if that is your area of contamination. Degas too much however will also cause the filaments to burn out quickly if not performed properly, so it is only suggested to degas for a max of 5 minutes.
- Cold Cathode (8e-10 Torr to 8e-3 Torr): While this was the gauge to stay clear of back in the day, modern technology has really enhanced this mamma-jamma! The accuracy for standard units is similar to the hot filament gauge (+/- 25%). The main difference here is that the cold cathode does NOT have a filament to burn out. No filament to burn out; what craziness is this?!? The inner-workings of the cold cathode allow for electron release by the use of magnets. I won't get into the crazy details here (if you are interested in how the cold cathode actually works, check out youtube video). I mention this for two reasons. The first reason is that the filament inside the hot filament gauge is the source of the electrons to ionize the molecules. Since the cold cathode does have a filament, it is harder to active / ignite the unit because the molecules have to interact with the magnets to do so. This causes an issue when you try to activate / ignite a cold cathode at pressure lower than 1e-3 Torr. If you try to ignite at 1e-3 Torr, it might take a few hours. If you try to ignite at 1e-5 Torr, I would personally either go home or get a cot to sleep because you will be waiting a long time (potentially a week or so). You could also pull an intern and have them watch the pressure to see when the cold cathode activates / ignites (they might not come back after that though..). The second reason, which is a positive one, is that you can physically clean the cold cathode. You can usually remove the ionization chamber of the cold cathode and scrub (yes I said scrub) the inside of the unit. You would usually do this with a clean cloth to slightly polish the inside. This adds that "rugged charm" to the cold cathode. If you have a dirtier application where you are running more than just N2, a cold cathode might be the way to go.
- 1e-9 Torr to 760 Torr Combination of an Ion Gauge and Pirani / Convection Enhanced Pirani; Ion gauge can be cold cathode (8e-10 Torr to 8e-3 Torr ) or hot filament (nude UHV type) (5e-11 Torr to 5e-2 Torr): Combination of an Ion Gauge and Pirani / Convection Enhanced Pirani; Ion gauge can be cold cathode or hot filament (nude UHV type)
- Similar to the above, you have the option of either a cold cathode or hot filament. Most cold cathodes are designed to reach 1e-9 Torr with ease; some even reaching into the 10-10 Torr range. Since these gauges are designed differently, they aren't as effected by what is called the "X-Ray Limitation", like the hot filament gauges are. At a certain pressure, emitted electrons will impact the grid and collector on a hot filament gauge. This will cause X-Rays to be produced. If these X-rays strike the collected, the molecules already stuck to the collector will emit more electroncs, which induce another electron current. This throws the whole unit out of whack. All of this is just a fancy way of saying, "don't use a standard hot filament gauge below 1e-9 Torr as it doesn't work well". Only the Nude UHV hot filament gauges should be used as these units have a different design to help push the X-Ray Limitation back to 1e-11 Torr. I have found that in this case of UHV (1e-9 Torr), it comes down to the dealer's choice. At this limit, most scientists have a specific technology that they prefer and they tend to stick with that. If you are working with a new scientist or are setting up your own lab, both technologies have very similar specifications.
- Also note that you can have two separate gauges (roughing and high vacuum) or a combination (aka wide range) gauge that incorporates multiple technologies into one housing.
Recommended Environments (Answers questions 2 and 3)
Now that you have selected a technology, we must think about the environments that the gauges will be exposed to. While I might have referenced some environments in the above pressure section, I wanted to clearly split these apart (just in case someone was specifically searching for this type of info).
Magnetism:Short, Sweet, and to the Point
Suggested: passive gauge, not a cold cathodeRabbit-Hole
Magnetism tends to play an interesting role in synchrotrons where the design of the systems uses magnets to bend or push the electron beam such that in travels in a circular pattern (Please note that this is a VERY, VERY simplistic explanation.). In this process however, you do need to measure the vacuum level of that electron beam. In areas of high magnetism, transducers (units with intergraded electronics), cannot be used as this magnetic field with interfere with the electronics. There are versions of all the different technologies out there that are specifically designed for this type of application. You will generally not have a combination gauge (a single unit that with multiple technologies) that is passive. Also, since cold cathode gauges have magnets in the internal volume, it is recommended to look to a hot filament if possible.
Radiation:Short, Sweet, and to the Point
Suggested: passive gaugeRabbit-Hole
Radiation is another interesting player.. What happens if you were measuring pressure near or in a nuclear reactor (pending the appropriate shielding is used as well)? No, your gauge is not going to grow arms and run around terrorizing people.. It will however erode and degrade over time when exposed to radiation. Again, this is where transducers should not be used.. Radiation will EASILY affect the electronics, causing the gauge to fail even faster than expected. There are gauges out there that can be used in radiation environments. They all tend to be passive gauges and have a gray (Gy) rating, but there are a few transducers that have a very high radiation resistance.
Temperatur:Short, Sweet, and to the Point
Suggested: if above 50°C, passive gaugeRabbit-Hole
This environment is more forgiving than the first two. All passive gauges will generally have a higher temperature threshold (some rated up to a 400°C bakeout). We have to understand the difference between the process temperature and what the gauge will ACTUALLY be exposed to. There have been times where I have had customers say that the process temperature is close to 500°C (sometimes even higher). Gauges will not usually withstand that kind of temperature. So, where is the temperature located? The middle of the chamber might be 500°C but the distance between the process and the gauge does provide a substantial temperature difference. Also, during a process, you would usually pump the chamber down to a base pressure (let's say 1e-7 Torr for this scenario). Once pumped down, you would then start the furnace. Since there aren't a plethora of molecules to absorb and transfer heat, your gauge more than likely won't be exposed to that high of a temperature (no conductive or convective heating, only the potential for radiate heating).
Process Gases (Answers question 4)
ExplosiveShort, Sweet, and to the Point
Suggested: Piezo, capacitance manometer, cold cathode (nothing with a hot filament)Rabbit-Hole
If you are using gases such as hydrogen, oxygen methane, etc., you could be exposing yourself to an explosive environment. While I love fireworks, I would prefer not to see a system go off like a nuclear bomb (anything over 4% hydrogen can easily go BOOM). If using these gases (yes, there are many more), it is suggested to NOT provide an ignition source, such as a hot filament. Unfortunately, this is EXACTLY what the thermocouple, pirani, convection enhanced pirani, and hot filament ion gauge have. You also need to take into account a molecules auto-ignition temperature. While all of the above mentioned gauges have a hot filament, a thermocouple gauge filament can reach upwards of 450°C in lower vacuum. A hot filament ion gauge can reach upwards of 900°C + during a degas cycle. Keep this in mind when choosing a unit for pressure measurement.
KorrosivShort, Sweet, and to the Point
Suggested: Piezo, capacitance manometer, Hot filament ion gauge (tungsten filament only), cold cathodeRabbit-Hole
So, we all know that corrosive gases aren't fun (HCl, HF, NH3, etc.). They tend to be used in a plethora of processes, but also tend to corrode certain materials. This should always, always be considered when choosing a gauge.
This scenario is better when using an example. I am looking for a gauge that will read down to 10 Torr, but I am using nitric acid. I have a choice between a pirani gauge with integrated display ($350) or a capacitance manometer ($1750 for unit and display). Yeah, that $1750 looks terrible, so I go with the Pirani. After a month of using it, the pirani fails and I send it in for an evaluation, only to find out that the nitric acid corroded the filament. I have to buy a new gauge. I buy another pirani, moving the unit farther from the process (thinking this will solve the issue). 2 months later, the pirani fails again (due to the same issue). I am already $700 in the hole.. what do I do? Can I buy a trap to protect the gauge? That might help, but the trap also costs another $400, on top of the $350 for a third pirani gauge. If I buy the trap and third gauge, I am now down $1450, not accounting for the time I have lost sending the units in for evaluation. All the while, I could have bought a capacitance manometer and display that isn't affected by the nitric acid. If you have to go into higher vacuum than the capacitance manometer will allow, you have the choice of either ion gauge. Cold Cathodes tend to be more rugged since you can physically clean the inside. The tungsten filament of a hot filament ion gauge are slightly resistive to chemical attack, so this is another possibility.
Moral of the story: the upfront cost may be appealing, but also look at the cost of ownership long term.
CondensableShort, Sweet, and to the Point
Suggested: piezo, capacitance manometer, Hot filament ion gauge (tungsten filament only), cold cathodeRabbit-Hole
Condensable material interrupts the method in which a gas dependent gauge provides feedback from the pressure it is exposed to. For instance, if material condenses on a hot filament, that material will absorb heat, rendering the measurement inaccurate. For this instance, it is best to use a direct type of measurement, such as a piezo or capacitance manometer. While condensable material isn't suggested for any gauges, the direct types will still provide an accurate measurement. With this said, direct measurements only go so low, 1e-6 in fact. In you have to measure lower than that, a hot filament ion gauge with tungsten filaments or a cold cathode is suggested. The tungsten filaments are less susceptible to chemical reactivity and a cold cathode tends to be more rugged, allowing the user to physical clean the inside of the gauge.
Accuracy (Answers question 5)
Short, Sweet, and to the Point
Suggested: Piezo, Capacitance Manometer, special ion gaugesRabbit-Hole
With great accuracy comes an even greater price tag..
Before we delve into a higher accuracy, lets take a look at standard accuracy and know what this actually means. Most ion gauges will state an accuracy of +/- 25 of the reading. If you are a 5e-6 Torr, that means your pressure range could be anywhere between 6.25e-6 Torr to 3.75e-6 Torr. If this works for you, look no further as a standard gauge will work.
If you process requires the highest accuracy possible however, any gas independent gauge is going to be the first and best option (if the pressure range allows). Anything below 1e-5 Torr however (1e-6 Torr for a heated capacitance manometer) is gas dependent however. There isn't enough physical force applied to the diaphragm of a capacitance manometer to render a stable or accurate pressure below this range. There are a few ion gauge models however that provide a better accuracy than the standard unit (+/- 20%). How does this work you ask? These units provide calibration factors for different gases that help maintain the accuracy. This part is something you could theoretically perform yourself. The design of these units however are where things differ. Both units have a more rigid and detailed design that increase the probability of ionization and accurate measurements. I can tell that you are wondering why every ion gauge isn't made like this. Well, it comes down to money. Due to the intricate design and additional calibration coefficients, the price can double what you would pay for a similar model with standard integrated electronics. It just all depends on how accurate you really need to be.
Troubleshooting TidbitsCleaning certain vacuum gauges:
When you have nasty gases flowing through a gas dependent gauge, the measurement tends to be thrown off as these gases like to condense. Depending on the gauge type, you can attempt to "clean" the filament. This cleaning pertains to thermocouple tubes, pirani's, and convection enhanced pirani gauges. This method does not guarantee to allows solve the problem, but it could help remove the condensables (depending on what they are and what solvent you use). This method required you to take the tube and simply pour a small amount of solvent down the flange termination. Remember to ALWAYS read the SDS (Safety Datasheets) for the solvent and the condensable material before mixing them!! Once inside, you can swirl the tube around (do not shake like a maraca) so the solvent reaches the entire internal volume. After about 30 seconds, you can pour out the remnants (per your state and local regulations). If the condensable gas was soluble in the solvent, then your gauge should be ok. Please note that the residual solvent could still be inside the gauge and will need time to evaporate. You can also power the gauge, which will provide heat to quicken the evaporation process. The measurement will not be accurate until all solvent is gone.Monitoring the Fillament Current and Voltage of an Ion Gauge to determine lifespan:
For all hot filament ion gauges, the user has an option of a few emission currents during operation. These are typically 4 mA and 100uA. When activated, the gauge needs a specific amount of power to reach that specific emission current. In most hot filament gauges or transducers, you can view the Filament Voltage and Current to see just how much "juice" is required from the R&D screen. As you continue to use the gauge, you may see both values start to increase. If these values do increase, that is an indication that there is contamination on the filaments, whether from the process or oxidation. Monitoring and plotting these values over time, starting when you first receive the gauge, provides valuable information on how your process if affecting the filaments of the ion gauge, which correlates to its lifespan. An example of a unit that showing these values is the KJLC 392. The manual provides clear instructions on how to easily view these values.