# Re: [std-proposals] On the standardization of mp-units P3045R1

From: Tiago Freire <tmiguelf_at_[hidden]>
Date: Tue, 18 Jun 2024 13:44:09 +0000
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]> On Behalf Of Sebastian Wittmeier via Std-Proposals
Sent: Tuesday, June 18, 2024 15:03
To: std-proposals_at_[hidden]rg
Cc: Sebastian Wittmeier <wittmeier_at_[hidden]>
Subject: Re: [std-proposals] On the standardization of mp-units P3045R1

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_[hidden]<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_[hidden]<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]<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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