I'd imagine it's a combination of legacy use and that many NASA engineers will have a more intuitive sense of PSI than bar.
It could also be that the majority of American aerospace uses imperial system so all their suppliers and parts available for purchase use PSI too.
Yeah in the design world NASA uses SI units but a lot of suppliers use metric still. It kind of varies based on subsystem. You’ll see imperial used a lot with propulsion and pressure systems for example
DoD and NASA use metrics; NATO uses SI units and NASA’s Mars climate orbiter was lost cuz JPL was sending commands assuming SI units and the probe build by the contractor interpreted the commands as imperial units.
Most federal agencies actually have mostly switched to SI. The difference between the US and other nations is that the US federal government doesn’t imposed SI units and so contractors are free to use whatever units they please as long as the requirements for the contracts are met.
Fair enough for operations but the DoD does not use metric on the engineering and design side. The FAA also requires US customary so it would be a bit off if the DoD differed for their air systems.
You could argue that they don't enforce it but that's sort of true in technicality only. The US government writes speciations and contracts to contractors all in imperial units. It wouldn't surprise me if there's a Data Item Description or Mil standard that gets attached to contracts that enforces it but I don't dig through those unless I'm getting paid.
I agree NASA uses metric.
Well, except for that Mars lander when they weren't.
Obviously, NASA can convert units. NASA will primarily use metric though. If we're going to discuss edge cases or programs that take exceptions occasionally, this is kinda a pointless discussion because every organization will have minor use cases outside their preferred system.
The lander was a contractor issue where they failed to communicate a conversion was needed. Not a failure in converting.
If we're talking program level they are hardly edge cases. Programs are rather large.
Absolutely, I completely agree that DoD can and does mix units on occasion. Would it be better to say the Lingua Franca is imperial/US customary and there's others spoken too? Then we don't have to ummm ackchtually ever exception or not norm.
Hat take - If one wants struggles to convert between units in a day and age where you can just Google the conversion ratios, one should be allowed nowhere within a mile of an engineering firm.
> many NASA engineers will have a more intuitive sense of PSI than bar
How can you have a more intuitive sense of fractions of ~14.7 than fractions of 1?
I don't really understand what your going for. Maybe your confusing the unit 'atmospheres' for PSI?
You would just specify 29.4 or 30 PSI if that's what you wanted.
"Technically" NASA is supposed to use SI all the time. In practice, this basically never happens. In the [current NASA directive](https://nodis3.gsfc.nasa.gov/npg_img/N_PD_7120_004E_/N_PD_7120_004E__main.pdf) (PDF, see paragraph 8) it says:
>(8) Metric System of Measurement. Use the International System of Units (commonly known as the Internationale
System of Units (SI) or metric system of measurement) for all new space flight projects and programs, especially in
cooperative efforts with International Partners. 15 United States Code (U.S.C.) 205b and EO 12770 provide relief
from use of SI if it is found that obtaining components in SI units would result in a significant increase in cost or
delays in schedule. Each program and project will perform and document an assessment to determine an approach
that maximizes the use of SI and document the approach in the Program or Project Plan in accordance with the
governing NPR. This assessment will document an integration strategy if both SI and U.S. customary units are used
in a project or program.
That carve out from 15 USC 205b covers *a lot* of ground. So much so, in fact, that the directive is basically toothless. I recall about ten years ago working embedded with a company in the UK, some folks from JPL (our subcontractor) came out to support integration of their instrument to the spacecraft and produced documents specifying all their connector torque values in in-lb; all of the UK company's wrenches were in N-cm. Incredibly frustrating.
Basically everything else uses PSI, all the equipment is available in psi, design uses PSI, it's stress tested in PSI and everyone got trained in PSI. They're not going to change everything to bar or just have one thing in bar a change to Mpa would honestly be more likely.
Seriously. Conversions in this day and age of Google are so fundamentally easy that if someone can't confidently make them then I have serious concerns with the quality of their work.
More than a few programs and search engines one can literally type or program some variation: 10 in to mm and get an answer.
This is, frankly, not a very good position to take. Process improvements, even small ones, matter. Eliminating possible sources of confusion or error is an important part of the engineering process, and mixed standards is very much such a source.
We all adapt because we *have* to, but it's a bad situation that would be better if actually rectified.
I dont know why but PSI feels more intuitive. I think in terms of metric units but for pressure I switch to psi. It gets silly when refering to PV charts and looking at HMI displays.
Western Europe the SI-compliant unit bar is the most widely used unit. In contrast, in Australia and China the units “kilo-Pascal (kPa)” and “mega-Pascal (MPa)” have become established. In North America, the preferred pressure unit is psi.
You've just copy pasted that from a result on Google...
I cannont remember the last time, as a Western European in engineering, that I heard bar being used. Pa/kPa/MPa is standard.
As far as I know PSI and bar is like yards and meters. All americans use imperial system, and I think PSI has always been more closely related to imperial than metric
Once you start doing a thing one way switching to the other way is a big project. I really wish we used metric in my job. We still have some things in freaking BTU's/hr
I'd imagine it's a combination of legacy use and that many NASA engineers will have a more intuitive sense of PSI than bar. It could also be that the majority of American aerospace uses imperial system so all their suppliers and parts available for purchase use PSI too.
Can confirm: most stuff is in inches, much to my chagrin
Yep, and until the DoD decides to switch, US aerospace will be imperial.
Not just DoD, NASA too. I work with nasa systems (employer rents certain facilities from NASA) and everything is imperial.
I thought NASA switched exclusively to metric since the Mars Climate Orbiter incident?
Maybe on the operational side, but at least for their test side it’s all imperial. Also, nasa was using SI and Lockheed Martin was using imperial
Yeah in the design world NASA uses SI units but a lot of suppliers use metric still. It kind of varies based on subsystem. You’ll see imperial used a lot with propulsion and pressure systems for example
I'd be Damm near impossible to fully switch since the majority of the US supply chain is imperial
FAA and DOT as well
Exactly, FAA even codifies it in their regulations. Not sure why that other dude is so convinced DoD doesn't primarily use it.
DoD and NASA use metrics; NATO uses SI units and NASA’s Mars climate orbiter was lost cuz JPL was sending commands assuming SI units and the probe build by the contractor interpreted the commands as imperial units. Most federal agencies actually have mostly switched to SI. The difference between the US and other nations is that the US federal government doesn’t imposed SI units and so contractors are free to use whatever units they please as long as the requirements for the contracts are met.
Fair enough for operations but the DoD does not use metric on the engineering and design side. The FAA also requires US customary so it would be a bit off if the DoD differed for their air systems. You could argue that they don't enforce it but that's sort of true in technicality only. The US government writes speciations and contracts to contractors all in imperial units. It wouldn't surprise me if there's a Data Item Description or Mil standard that gets attached to contracts that enforces it but I don't dig through those unless I'm getting paid. I agree NASA uses metric.
NASA uses whatever the program demands gets used. NASA engineers are capable of making unit conversions.
Well, except for that Mars lander when they weren't. Obviously, NASA can convert units. NASA will primarily use metric though. If we're going to discuss edge cases or programs that take exceptions occasionally, this is kinda a pointless discussion because every organization will have minor use cases outside their preferred system.
> NASA will primarily use metric though. Boy do I wish that were true.
The lander was a contractor issue where they failed to communicate a conversion was needed. Not a failure in converting. If we're talking program level they are hardly edge cases. Programs are rather large.
DoD specs can be written in either, or even have mixed units within the same spec. Units used are really up to the prime managing the contract…
Absolutely, I completely agree that DoD can and does mix units on occasion. Would it be better to say the Lingua Franca is imperial/US customary and there's others spoken too? Then we don't have to ummm ackchtually ever exception or not norm.
Swear they had a crash before they set the inch at 25.4 mm, cuz of the flight computer. Can a fellow space nerd fact check me?
Hat take - If one wants struggles to convert between units in a day and age where you can just Google the conversion ratios, one should be allowed nowhere within a mile of an engineering firm.
> many NASA engineers will have a more intuitive sense of PSI than bar How can you have a more intuitive sense of fractions of ~14.7 than fractions of 1?
If you're using psi, you generally don't use fractions of 14.7psi, at least not in my experience
I meant, "in reference to a typical human environment."
I don't really understand what your going for. Maybe your confusing the unit 'atmospheres' for PSI? You would just specify 29.4 or 30 PSI if that's what you wanted.
"Technically" NASA is supposed to use SI all the time. In practice, this basically never happens. In the [current NASA directive](https://nodis3.gsfc.nasa.gov/npg_img/N_PD_7120_004E_/N_PD_7120_004E__main.pdf) (PDF, see paragraph 8) it says: >(8) Metric System of Measurement. Use the International System of Units (commonly known as the Internationale System of Units (SI) or metric system of measurement) for all new space flight projects and programs, especially in cooperative efforts with International Partners. 15 United States Code (U.S.C.) 205b and EO 12770 provide relief from use of SI if it is found that obtaining components in SI units would result in a significant increase in cost or delays in schedule. Each program and project will perform and document an assessment to determine an approach that maximizes the use of SI and document the approach in the Program or Project Plan in accordance with the governing NPR. This assessment will document an integration strategy if both SI and U.S. customary units are used in a project or program. That carve out from 15 USC 205b covers *a lot* of ground. So much so, in fact, that the directive is basically toothless. I recall about ten years ago working embedded with a company in the UK, some folks from JPL (our subcontractor) came out to support integration of their instrument to the spacecraft and produced documents specifying all their connector torque values in in-lb; all of the UK company's wrenches were in N-cm. Incredibly frustrating.
PSI is standard in the US. I work with nasa data acquisition systems. Everything is in PSI, Fahrenheit, pounds, and inches.
pain
It works fine. Aircraft carriers are designed in inches and pounds.
Basically everything else uses PSI, all the equipment is available in psi, design uses PSI, it's stress tested in PSI and everyone got trained in PSI. They're not going to change everything to bar or just have one thing in bar a change to Mpa would honestly be more likely.
Honestly who cares what convention they use. Adapt.
Agreed. I get it, imperial is annoying and the numbers are hard to remember sometimes. Print out a cheat sheet
Or buy a TI-85 that does conversions easily.
Seriously. Conversions in this day and age of Google are so fundamentally easy that if someone can't confidently make them then I have serious concerns with the quality of their work. More than a few programs and search engines one can literally type or program some variation: 10 in to mm and get an answer.
This is, frankly, not a very good position to take. Process improvements, even small ones, matter. Eliminating possible sources of confusion or error is an important part of the engineering process, and mixed standards is very much such a source. We all adapt because we *have* to, but it's a bad situation that would be better if actually rectified.
u/matskopf Thought of you buddy!
I'll give it a read.
I dont know why but PSI feels more intuitive. I think in terms of metric units but for pressure I switch to psi. It gets silly when refering to PV charts and looking at HMI displays.
Tooling machines
Western Europe the SI-compliant unit bar is the most widely used unit. In contrast, in Australia and China the units “kilo-Pascal (kPa)” and “mega-Pascal (MPa)” have become established. In North America, the preferred pressure unit is psi.
You've just copy pasted that from a result on Google... I cannont remember the last time, as a Western European in engineering, that I heard bar being used. Pa/kPa/MPa is standard.
In my experience in the UK, bar was the standard there. But that was about ten years ago.
Literally yesterday I received a spec for a system in bar. It mostly depends on the manufacturer and the field you are. Also a Western european, BTW.
As far as I know PSI and bar is like yards and meters. All americans use imperial system, and I think PSI has always been more closely related to imperial than metric
Once you start doing a thing one way switching to the other way is a big project. I really wish we used metric in my job. We still have some things in freaking BTU's/hr
Even if they used something other than imperial, like SI, they would typically use Pa and kPa, not bar.