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# pressure

Arduino library for pressure conversion.


## Description

Arduino library to convert between several pressure units (formats).
The class consists of a number of setters and getters and internally it uses millibar. 
In fact the class just hides all conversion constants to and frm millibar.

Pressure is implemented as a float so this limits the precision of the value.

If other constants / conversions are needed please open an issue on GitHub.

Note: as a conversion uses two steps the conversion error might be (bit) larger 
than in a single conversion step as there are two multiplications involved.

Note: constants need to be verified.


#### Related

- https://github.com/RobTillaart/temperature (a bit)


## Interface

```cpp
#include "pressure.h"
```

#### Constructor

- **pressure(float value = 0.0)** Constructor, with optional initial value.


#### Setters

- **void setMilliBar(float value)** sets pressure in milliBar.
- **void setBar(float value)** sets pressure in bar.
- **void setPSI(float value)** sets pressure in PSI = Pounds per Square Inch.
- **void setATM(float value)** sets pressure in Atmosphere.
- **void setDynes(float value)** sets pressure in Dynes.
- **void setInchHg(float value)** sets pressure in inches mercury.
- **void setInchH2O(float value)** sets pressure in inches water.
- **void setPascal(float value)** sets pressure in Pascal. Note this is the **SI** unit.
- **void setTORR(float value)** sets pressure in TORR.
- **void setCmHg(float value)** sets pressure in centimetre mercury.
- **void setCmH2O(float value)** sets pressure in centimetre water.
- **void setMSW(float value)** sets pressure in Meters of Sea Water. (under water pressure unit).


#### Getters

- **float getMilliBar()** returns pressure in milliBar.
- **float getBar()** returns pressure in bar.
- **float getPSI()** returns pressure in PSI = Pounds per Square Inch.
- **float getATM()** returns pressure in Atmosphere.
- **float getDynes()** returns pressure in Dynes.
- **float getInchHg()** returns pressure in inches mercury.
- **float getInchH2O()** returns pressure in inches water.
- **float getPascal()** returns pressure in Pascal. Note this is the **SI** unit.
- **float getTORR()** returns pressure in TORR.
- **float getCmHg()** returns pressure in centimetre mercury.
- **float getCmH2O()** returns pressure in centimetre water.
- **float getMSW()** returns pressure in Meters of Sea Water. (under water pressure unit).


#### Gas law (experimental see below)

The **change()** function is applied to the current internal pressure.
All these functions return pressure in milliBar.

- **float change(float T1, float T2, float V1, float V2, float N1, float N2)**
  - apply changing temperature (**Kelvin**), 
  - volume (m3) and moles.
  - If an parameter does not change fill in 1 for both before (T1,V1, N1) and after (T2,V2,n2).
- **float changeTemperature(float T1, float T2)** only change temperature. T in **Kelvin**.
- **float changeVolume(float V1, float V2)** only change volume.
- **float changeMole(float N1, float N2)** only change moles.


#### Constants

The library has a number of constants to convert units. See the pressure.h file.
These constants can be used to write specific convertors or define specific constants.

A dedicated conversion is faster as it has only one float multiplication runtime.
The constants will multiply compile time!


```cpp
inline float PSI2MSW(float value)
{
  return value * (PSI2MILLIBAR * MILLIBAR2MSW);
}
```

or
```cpp 
#define PSI2MSW     (PSI2MILLIBAR * MILLIBAR2MSW)
...
float out = in * PSI2MSW;
```


## Operation

```cpp
pressure P;

...

P.setDynes(1000);
Serial.print("mBar: ");
Serial.println(P.getMilliBar()); // 1000 Dynes in mBar
Serial.print("TORR: ");
Serial.println(P.getTORR());     // 1000 Dynes in Torr
```

#### Experimental

Note: names changed in 0.3.0


Apply the ideal gas law : **(P x V) / (n x T) = Constant**

- **float change(float T1, float T2, float V1, float V2, float N1, float N2)**
  - T (temperature) in Kelvin,
  - V (volume) in identical units, 
  - N (# atoms) in mole
  - wrapper around next three.
- **float changeTemperature(float T1, float T2)** only change temperature. T in Kelvin.
- **float changeVolume(float V1, float V2)** only change volume.
- **float changeMole(float N1, float N2)** only change moles.

in code
```cpp
pressure P;
P.setPressure(...);
P.change(T1, T2, V1, V2, N1, N2);  // apply all changes.
x = P.getPressure()
```


Some temperature converters

```cpp
Kelvin = Celsius + 273.15;
Kelvin = (Fahrenheit - 32) \* 5 / 9 + 273.15;
Kelvin = Fahrenheit \* 5 / 9 + 290.93;  // one operator less.
```


## Future

#### Must

- update documentation
- find a good reference for conversion constants.
  - https://www.semicore.com/reference/pressure-conversion-reference


#### Should

- test with gas law.


#### Could

- **float AddMillibar(float value)** simple math with all units (12x)
  - ```return _pressure += value * factor;```
  - does **subtractMillibar()** make more sense?
- **float setMillibar(float value)** return pressure in millibar all setters
  - ```return _pressure = value * factor;```


#### Wont (unless requested)

- **float changeTC(float T1, float T2)** only change temperature, T in Celsius.
- **float changeTF(float T1, float T2)** only change temperature, T in Fahrenheit.
- move code to .cpp file
- rename parameters so they make more sense?  (simple enough)
```
  void  setMilliBar(float milliBar )  { _pressure = milliBar; };
  void  setBar(float Bar)             { _pressure = Bar * BAR2MILLIBAR; };
  void  setPSI(float PSI)             { _pressure = PSI * PSI2MILLIBAR; };
```
- defaults for functions?
  - none  <<<<<<<<<<<<<<<<<<<<<<<<<<<
  - 0 like constructor?
  - 1 to get conversion constant?