Arduino R290 Suction Line Superheat Calc.

[buffalobillpatrick]

I convert function input parameter "float PSI" into array index:

//convert PSI to array index
work = ((PSI - 30.0) / 10.0);

I = work; //drop decimals to get array index,
//I is an int (whole number) work is a float (decimals)



I use the decimal portion to interpolate between array elements:

decimals = work - I; //restore decimals for interpolation between array points

Val = R290_Saturation_Temp[I]; //get array value
Next_val = R290_Saturation_Temp[I+1]; //get Next array value

Span = Next_val - Val; //Calc Span

Saturation_Temp = Val + (Span * decimals);//interpolate between array points

float calc_R290_Saturated_Temp(float PSI, int Which_sensor)
{ 

//Array element [0]  contains Saturation_Temp *F for 30PSI,
//Array element [1]  contains Saturation_Temp *F for 40PSI,...................>
//Array element [27] contains Saturation_Temp *F for 300PSI,
//Array element [28] contains Saturation_Temp *F for 310PSI


float R290_Saturation_Temp[] = 
     {7.93, 18.80, 28.42, 36.91, 44.59, 51.63, 58.13, 64.21, 69.90, 75.27, 
      80.35, 85.18, 89.78, 94.19, 98.41, 102.50, 106.40, 110.20, 113.80, 
      117.30, 120.80, 124.10, 127.30, 130.40, 133.50, 136.50, 139.40, 142.20, 
      145.00};


float Val, Next_val, Span, Saturation_Temp, decimals, work, low, hi; 

int I;

  if (Which_sensor == Sensor_150){ low = 30.0; hi = 150.0; }
  else{ /*Sensor_300*/             low = 60.0; hi = 300.0; }
  
  if ((PSI >= low) && (PSI <= hi)) //Bounds check
  {
  // in bounds, continue
  
  //convert PSI to array index
  work = ((PSI - 30.0) / 10.0);
  
  I = work;          //drop decimals to get array index
                         //I is an int (whole number) work is a float (contains decimals)

  decimals = work - I; //restore decimals for interpolation between array points  
  
  Val = R290_Saturation_Temp[I];           //get array value
  Next_val = R290_Saturation_Temp[I+1];    //get Next array value
  
  Span = Next_val - Val;                   //Calc Span

  Saturation_Temp = Val + (Span * decimals);//interpolate between array points

  return (Saturation_Temp);        

  }  //end if in bounds
  
  else {return (-1);}       // error return, out of bounds 

}  // end calc_R290_Saturated_Temp

[AC_Hacker]

BBP,

OK, it didn't take as long as I thought, I had saved data from before.

Here is the heart of the matter, if you need me to make it into a line of code, let me know.

4th Degree Polynomial Fit: y=a+bx+cx^2+dx^3...
Coefficient Data:
a = -4.03085122205E+001
b = 1.81496452942E+000
c = -1.01201804003E-002
d = 3.05234258195E-005
e = -3.37963005786E-008

Right now, mandolin is calling me...

Best,

-AC

[Mikesolar]

Originally Posted by buffalobillpatrick

"The Emerson EX2 pulse modulated electric valve is an electrically driven expansion valve that provides precise temperature control. It is designed for use with R-12, R-22,
R-134a, R-404A, R-407C and R-502 refrigerants. - See more at: http://www.emersonclimate.com/en-US/products/valves/expansion_valves/Pages/electronic_expansion_valve.aspx#sthash.AodE1Cgy.dp uf"

Did anyone read the link in post #21 ?

Yup, and I know it is made for liquid injection purposes. I haven't seen any reference to it being used as a main EEV. It needs the solenoid to activate it and it is a normally closed valve like most solenoid. Without power to it, it will be closed. Not what you want from an EEV but fine for injection.

[Mikesolar]

Originally Posted by AC_Hacker

Yep, EEV for HVAC must be stepper (or servo, which would be too expensive, with no added benefit).

Tell me more about what you have in mind for the EEV?

-AC

There is a way to control a heat pump based solely on subcooled liquid temp. I would like to try to make the EEV measure the subcooled temp just before it goes to the evap. The control only has to look at one internal temp (liquid line) and try to compare it to the outdoor ambient (and I am not sure this is necessary) to adjust the valve.
It is called a flooded evaporator.

[AC_Hacker]

BBP,

I haven't read every post here, but I think that you need an incrementally-variable EEV.

The PWM thingy wants square-wave input with varying duty cycle, in order to 'mimic' analog control.

I think that an incrementally variable (analog) valve would be better.

The EEVs I've seen use a little teeny stepper motor that turns the adjustment screw.


-AC

[Mikesolar]

Originally Posted by AC_Hacker

BBP,

I haven't read every post here, but I think that you need an incrementally-variable EEV.

The PWM thingy wants square-wave input with varying duty cycle, in order to 'mimic' analog control.

I think that an incrementally variable (analog) valve would be better.

The EEVs I've seen use a little teeny stepper motor that turns the adjustment screw.


-AC

Yes, I have one in my hands. A Sporlan SER-B

[AC_Hacker]

Originally Posted by Mikesolar

Yes, I have one in my hands. A Sporlan SER-B

Then on with the show...

-AC

[Mikesolar]

Now to just get a program that will monitor subcooling and adjust the valve accordingly. It also has to open it a bit at start up till the temps settles.

[buffalobillpatrick]

A/C did I get this right?

How did you come up with such an accurate formula??

float calc_R290_Saturated_Temp(float x)
{ 

float a = -4.03085122205E+001;
float b = 1.81496452942E+000;
float c = -1.01201804003E-002;
float d = 3.05234258195E-005;
float e = -3.37963005786E-008;

 return (a + (b * x) + (c * x*x) + (d * x*x*x) + (e * x*x*x*x));
       
}  // end calc_R290_Saturated_Temp

If so, either the formula or the table are off by a little bit.

OR Arduino floating point math is a bit in-accurate.
I would bet on this option, as Arduino does floating point by actually doing integer add operations.

This one line of code: y = ( a + (b * x) + (c * x*x) + (d * x*x*x) + (e * x*x*x*x));
Produces 348 bytes of Arduino object code.

At 30PSI: formula = 5.83*F, table = 7.93*F, difference = -2.1*F
At 300PSI: formula = 143.75*F, table = 142.2*F, difference = +1.55*F

[jeff5may]

Here is your datasheet for the ex2. It seems to be a damped solenoid valve with a period of about 15 seconds from full shut to full on. A 6 second period on a pwm would not make it walk around much between pulses.

http://www.emersonclimate.com/europe...EX2__35016.pdf

The EX3 and up to EX8 are true stepper motor driven designs. All these EX series valves seem to be PRICEY, if they can be found.