When using “quantity” 1°C = 1°K, this is by design in mp-units (you cannot change this, it is intrinsic to the design). Only quantity_point converts 28°C to 301K.

From: Std-Proposals <std-proposals-bounces_at_[hidden].org> On Behalf Of Sebastian Wittmeier via Std-Proposals
Sent: Tuesday, June 18, 2024 14:43
To: std-proposals_at_lists.isocpp.org
Cc: Sebastian Wittmeier <wittmeier_at_projectalpha.org>
Subject: Re: [std-proposals] On the standardization of mp-units P3045R1


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_lists.isocpp.org<mailto:std-proposals_at_[hidden]>;
CC: Tiago Freire <tmiguelf_at_[hidden]<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.





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