> Tiago, some time has passed since your last complaint about the same problem. We invited you to our internal meeting, listened to your concerns, and discussed how we can improve here. As you know, the answer was not found at the meeting. Additionally, you stated that you don't want to work to contribute to our proposal and repository and that you will come back with a better interface soon. More info can be found here: https://github.com/mpusz/mp-units/discussions/552. Did you manage to find a better solution to this problem? If so we are open to rediscuss your solution whenever you are ready.

Yes, as I have stated I already had an alternative that did this correctly 3 years ago. If I was the one designing it, Degrees Celsius should be defined as a different kind of unit where the arithmetic works differently and accounts for the fact that degrees Celsius are offset (and quantity_point is not the answer here). I think there’s a better way to deal with this and I think I have provided a solution.
It didn’t go anywhere because you haven’t deemed a big enough problem to warrant consideration, and I didn’t press on the issue because I didn’t believe the design would have a chance to become part of the standard.

But since it is being proposed to become part of the standard, I think it is a perfectly valid thing to ask these questions.
If this is going to be part of my toolset, how are we supposed to deal with this?
Do you agree with my observation that it was easier to do the wrong thing than to do the right thing?
Can we change the design such that this is not an issue?

Br,
Tiago


From: Mateusz Pusz <mateusz.pusz_at_[hidden]>
Sent: Tuesday, June 18, 2024 15:55
To: std-proposals_at_[hidden]
Cc: Tiago Freire <tmiguelf_at_[hidden]>; Chip Hogg <chogg_at_[hidden]>; Anthony Williams <anthony_at_[hidden]>; Johel Ernesto Guerrero Peña <johelegp_at_[hidden]>
Subject: Re: [std-proposals] On the standardization of mp-units P3045R1

Hi,

Tiago, some time has passed since your last complaint about the same problem. We invited you to our internal meeting, listened to your concerns, and discussed how we can improve here. As you know, the answer was not found at the meeting. Additionally, you stated that you don't want to work to contribute to our proposal and repository and that you will come back with a better interface soon. More info can be found here: https://github.com/mpusz/mp-units/discussions/552. Did you manage to find a better solution to this problem? If so we are open to rediscuss your solution whenever you are ready.

For all other participants of this mailing list, here is a correct solution:

#include <mp-units/ostream.h>
#include <mp-units/systems/si.h>
#include <iostream>

using namespace mp_units;

inline constexpr struct atmospheric_pressure final : named_unit<"atm", mag<101'325> * si::pascal> {} atmospheric_pressure;

int main()
{
  using namespace mp_units::si::unit_symbols;

  quantity Volume = 1.0 * m3;
  quantity_point Temperature(28.0 * deg_C);
  quantity n_ = 0.04401 * kg / mol;
  quantity R_boltzman = 8.314 * N * m / (K * mol);
  quantity mass = 40.0 * kg;
  quantity Pressure = R_boltzman * Temperature.in(K).quantity_from_zero() * mass / n_ / Volume;
  std::cout << Pressure.in(Pa) << "(" << Pressure.in(atmospheric_pressure) << ")\n";
}

https://godbolt.org/z/E8bf51hKG

Temperatures are tricky, and there is no good default here. People often mean either a point or a difference, depending on the context. In case anyone has an idea on how to improve, we are open to feedback.

Best

Mat

wt., 18 cze 2024 o 15:30 Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>> napisał(a):

How about the following scales? Are they also an issue?





 - Time (Calendar) relative to either anno domini or Unix time?
 - Position Coordinate relative to Greenwich?
 - Electric Potential relative to earth potential?
 - pH, pKa, pKb scales relative to a neutrality of 7?
 - Decibels, phon and sone relative to threshold of human hearing?
 - Pressure (hydraulic or blood) relative to atmospheric pressure?
 - Altitude relative to sea level?

   -> For pressure and altitude there are lots of other scales, e.g. used in aviation
 - Richter scale relative to detectable earthquakes?
 - Beaufort relative to calm wind instead of zero wind?
 - Borg physical exertion not starting at zero?

--
Std-Proposals mailing list
Std-Proposals_at_lists.isocpp.org<mailto:Std-Proposals_at_[hidden]>
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

wt., 18 cze 2024 o 15:44 Tiago Freire via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>> napisał(a):
I understand what the problem is, that is why I’m bringing it forward.
My concerned is that I haven’t written any code that anyone wouldn’t have written and got the wrong answer.

> An absolute value in the paper is a quantity_point, a possibly relative value is a quantity.

Which is a perspective, not convinced that it is the right thing. But That also poses the question, volume is also an absolute value, so is the mass, pressure, etc..

Which means that the right way to write it would be this:
```
quantity_point Volume {1.0 * m*m*m};
quantity_point Temperature {si::ice_point + 28.0 * deg_C};
quantity_point n_{0.04401 * kg / mol};
quantity R_boltzman = 8.314 * N * m / (K * mol);
quantity_point mass {40.0 * kg};
quantity_point P = R_boltzman * Temperature * mass / n_ / Volume;
std::cout << Pressure << std::endl;
```

But this doesn’t compile because quantity_point can’t math.
In order to get it to compile you would have to do this instead:
```
quantity_point Pressure = quantity_point{0.0*Pa} + R_boltzman * (Temperature - mp_units::si::absolute_zero) * (mass - quantity_point{0.0*kg}) / (n_ - quantity_point{0.0*kg / mol}) / (Volume - quantity_point{0.0* m*m*m});
```
Which doesn’t even module the problem properly because the values in PV=nRT are supposed to be absolute values, not deltas.

Hence it raises the question, doing what it seems obvious is the wrong thing (thus questionably safe), and doing the right thing is kind of hard (thus questionably user-friendly). But that what is expected as the correct way to use it.


From: Std-Proposals <std-proposals-bounces_at_[hidden]<mailto:std-proposals-bounces_at_[hidden]>> On Behalf Of Sebastian Wittmeier via Std-Proposals
Sent: Tuesday, June 18, 2024 15:03
To: std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>
Cc: Sebastian Wittmeier <wittmeier_at_[hidden]<mailto:wittmeier_at_[hidden]>>
Subject: Re: [std-proposals] On the standardization of mp-units P3045R1


You are specifically talking about



https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3045r1.html#potential-surprises-while-working-with-temperatures





Discussing the difficulty, when to use a difference in temperature or an absolute temperature.



An absolute value in the paper is a quantity_point, a possibly relative value is a quantity.





If I understand correctly, in the current paper to initialize and use absolute temperatures



quantity_point qp2 = (isq::Celsius_temperature(28.0 * deg_C)).in(K)

and

qp2.quantity_from_zero()





would have to be used instead of



quantity Temperature = (28.0 * deg_C).in(K);



The paper also says



"We have added the Celsius temperature quantity type for completeness and to gain more experience with it. Still, maybe a good decision would be to skip it in the standardization process not to confuse users."










-----Ursprüngliche Nachricht-----
Von: Sebastian Wittmeier <wittmeier_at_projectalpha.org<mailto:wittmeier_at_[hidden]>>
Gesendet: Di 18.06.2024 14:42
Betreff: AW: [std-proposals] On the standardization of mp-units P3045R1
An: std-proposals_at_lists.isocpp.org<mailto:std-proposals_at_[hidden]>;

Hi Tiago,

where does this difference of 11x come from?

The temperature with 28°C vs. 301K?

-----Ursprüngliche Nachricht-----
Von: Tiago Freire via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>>
Gesendet: Di 18.06.2024 14:28
Betreff: [std-proposals] On the standardization of mp-units P3045R1
An: std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>;
CC: Tiago Freire <tmiguelf_at_[hidden]m<mailto:tmiguelf_at_[hidden]>>;

Hi, I will be participating in St. Louis.

And one of the papers that interested me was P3045R1, unfortunately I may or may not be on time to participate in this particular session.



There’s this question that I would like an answer too, and I wonder if there is anyone who will be attending St. Louis who would be willing to make this question on my behalf:







A lab worker puts in 40Kg of dry ice into a 1 cubic meter pressure tank rated for 10atm, they then vacuum the tank and seal it.

As the CO2 warms up to room temperature (which at a specific date was 28°C) it evaporates, and eventually following the ideal gas law:

PV=nRT



Is this setup dangerous?



Using mp-units (with the exact same design as the one being proposed for standardization) to solve this problem:



```

quantity Volume = 1.0 * m*m*m;

quantity Temperature = (28.0 * deg_C).in(K);

quantity n_ = 0.04401 * kg / mol;

quantity R_boltzman = 8.314 * N * m / (K * mol);

quantity mass = 40.0 * kg;

quantity Pressure = R_boltzman * Temperature * mass / n_ / Volume;

std::cout << Pressure << std::endl;

```



We get the following result:

`211581 N/m2`

(=211.581kPa = 2,09 atm)

But the correct answer is actually: 2275.629kPa = 22.5 atm

(11 time s higher than what mp-units calculated)



How is this considered a design feature and not a bug? (note that other similar libraries don’t have this problem)

And how do the authors think this design choice impacts on safety and user-friendliness?





Thanks.





 -- Std-Proposals mailing list Std-Proposals_at_[hidden]<mailto:Std-Proposals_at_[hidden]> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals


--
Std-Proposals mailing list
Std-Proposals_at_[hidden]<mailto:Std-Proposals_at_[hidden]>
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals