HVAC Psychrometric LT Online Help

The HVAC Psychrometric LT application allows you to easily perform single-state HVAC psychrometric calculations. This app takes advantage of all the incredible iPhone and iPad user interface functionality such as allowing you to easily change input values by sliding a control with your finger and immediately viewing the calculated results. Also, you can quickly toggle amongst 20 different combinations of psychrometric input parameters all without leaving the main input screen.

This is a universal app that works on both the iPhone and iPad and adjusts the screen size accordingly. This help screen will focus on the iPhone version, but the input explanations also apply to the iPad version.

Overview

When you first open the Psychrometric LT application, you will see 3 tabs at the bottom of your screen:

1. The "Psychrometric" tab displays when you start up the application and allows you to use the psychrometric functionality. See the "Psychrometric Tab" section below.
2. The "Global Settings" tab allows you to specify global settings such as whether to use English (Imperial) or Metric units. Read the "Global Settings" section below for more details.
3. The "Help" tab displays this help screen.

Psychrometric Tab

This tab automatically appears when you first start the Psychrometric LT application. Two selections appear at the top of the device screen:

1. Single-State Psychrometrics: Select this row to open the input form that allows you to input values for up to 20 different combinations of 2 psychrometric parameters and instantly view the calculated results for a single stream of air. See below for more details.
2. Email Results: Select this row to display an HTML report that displays all of the inputs and results. You can also email this HTML report along with the corresponding spreadsheet attachment.

Single-State Psychrometrics

The "Single-State Psychrometrics" form allows you to specify up to 20 combinations of two psychrometric parameters for one stream of air and instantly view the results all in the same screen. The following is an explanation of the input controls:

The user input area contains 2 buttons, 2 input boxes, and 2 slider controls that allow you to specify the types and values of up to 20 combinations of psychrometric parameters.

1. Parameter 1 Button: This button allows you to toggle through all 8 psychrometric parameters. They toggle in the order that the output values appear in the calculated results area of the screen. As you press the button, the parameter name text will change on the button, itself, and also the value in the text box (discussed below) will update with the latest calculated value for that parameter type. In addition, the parameter 2 button name may change in response to changes you make on parameter 1. For example, if the parameter 2 button displays "Wet-Bulb Temp:" and you toggle the parameter 1 button to "Wet-Bulb Temp:", the parameter 2 button will switch to the next parameter in the list: "Partial Pressure:". The reason for this is that you cannot specify values for 2 of the same parameter types. In addition, there are some combinations of psychrometric input parameters that are not supported due to the complexity and number of iterations required to calculate the remaining properties. The following is a list of the input parameter combinations that are indeed supported:
   
   1. Dry-Bulb Temp / Humidity Ratio
   2. Dry-Bulb Temp / Relative Humidity
   3. Dry-Bulb Temp / Wet-Bulb Temp
   4. Dry-Bulb Temp / Partial Pressure
   5. Dry-Bulb Temp / Enthalpy
   6. Dry-Bulb Temp / Specific Volume
   7. Dry-Bulb Temp / Dew-Point Temp
   8. Humidity Ratio / Relative Humidity
   9. Humidity Ratio / Wet-Bulb Temp
   10. Humidity Ratio / Enthalpy
   11. Humidity Ratio / Specific Volume
   12. Relative Humidity / Wet-Bulb Temp
   13. Relative Humidity / Partial Pressure
   14. Relative Humidity / Enthalpy
   15. Relative Humidity / Specific Volume
   16. Relative Humidity / Dew-Point Temp
   17. Wet-Bulb Temp / Partial Pressure
   18. Enthalpy / Specific Volume
   19. Enthalpy / Dew-Point Temp
   20. Specific Volume / Dew-Point Temp
2. Parameter 1 Textbox: Input a number into this textbox that represents the value of the parameter that you selected by pressing the parameter 1 button (described above). After the value has been inputted, the calculated results will automatically update and the slider control indicator will relocate to the appropriate position. You cannot input a value greater than the maximum value on the slider control below the textbox. Also, if you input a value that in combination with the parameter 2 value (discussed below) produces invalid psychrometric properties (ie - the relative humidity is less than or greater than 0 or 100%), then the value of "N/A" will be displayed for all of the calculated psychrometric results.
3. Parameter 1 Slider: Depending upon the selection you made in the button described above, you can alter the parameter 1 value using this slider control. As you slide the indicator along the control, the results will immediately update. Depending upon the "step" value that you inputted in the "Psych. Properties" form, the values will increase/decrease along the slider by the "step" amount. See the explanation of the "Psych. Properties" form below.
4. Parameter 2 Button: The allowable parameter types for this button will depend upon the selection you made from the parameter 1 button discussed above. For example, if the parameter 1 button selection is "Humidity Ratio:", then the following psychrometric properties are available for the parameter 2 button:
   1. Dry-Bulb Temp
   2. Relative Humidity
   3. Wet-Bulb Temp
   4. Partial Pressure
   5. Enthalpy
   6. Specific Volume
   Notice that "Dew-Point Temperature" is missing from this list due to the complexity and number of iterations required for calculating the other 6 psychrometric properties.
5. Parameter 2 Textbox: Input a number into this textbox that represents the value of the parameter that you selected by pressing the parameter 2 button (described above). After the value has been inputted, the calculated results will automatically update and the slider control indicator will relocate to the appropriate position. You cannot input a value greater than the maximum value on the slider control below the textbox. Also, if you input a value that in combination with the parameter 1 value (discussed above) produces invalid psychrometric properties (ie - the relative humidity is less than or greater than 0 or 100%), then the value of "N/A" will be displayed for all of the calculated psychrometric results.
6. Parameter 2 Slider: Depending upon the selection you made in the button described above, you can alter the parameter 2 value using this slider control. As you slide the indicator along the control, the results will immediately update. Depending upon the "step" value that you inputted in the "Psych. Properties" form, the values will increase/decrease along the slider by the "step" amount. See the explanation of the "Psych. Properties" form below.

The following is an explanation of the calculated values. The values that are displayed include the 2 inputted parameter values and the remaining 6 calculated psychrometric values.:

1. Dry-Bulb Temperature (F or C): This is the calculated or inputted dry-bulb temperature that exists along the x-axis of the psychrometric chart.
2. Humidity Ratio (lbs/lb or grains or kg/kg or grams): This is the ratio of the mass of moisture content to mass of dry air. It is the basis for the vertical y-axis of the psychrometric chart.
3. Relative Humidity (%): This is the ratio of actual water vapor pressure in air to the pressure of saturated water vapor in the air at the same temperature.
4. Wet-Bulb Temperature (F or C): This temperature is registered by a thermometer whose bulb is covered by a wetted wick and exposed to a current of rapidly moving air. It is located along an angled axis on the psychrometric chart.
5. Partial Pressure (psia, psig, atm, etc. or kPa, Pa, etc.): This is the partial vapor pressure exerted by the water vapor contained in the air.
6. Enthalpy (BTU or kJ): This is the quantity of heat in the air above an arbitrary datum. The datum for dry air is 0 F (-17.7 C), for the moisture content, 32 F (0 C) water.
7. Specific Volume (ft³/lb or m³/kg): This is the volume of the air/water mixture with regard to mass.
8. Dew Point Temperature (F or C): This is the temperature at which moisture condensation begins when the air is cooled.

Email Results

This screen is accessed from the "Email Results" row located on the startup screen. It displays an HTML report that displays all of the inputs and results. You can also email this HTML report along with the corresponding spreadsheet attachment (.csv file).

Psychrometric Properties

This screen is accessed by clicking the round button with the "i" inside it located at the top right-hand portion of the Psychrometric LT input screen. The form will flip around and a number of additional psychrometric property inputs will appear. This same form is accessible from the "Single-State" and "Mixed-Air" psychrometric input modes. The following is an explanation of each input:

1. Humidity Ratio Units: This input allows you to select the humidity ratio units that are displayed in the main psychrometric input form. If you press the button located on the far right of this cell, a picker control will display that lists all of the available units. The following English units are displayed:
   1. lbs/lb
   2. grains/lb
   The following Metric units are displayed:
   1. kgs/kg
   2. grams/kg
   Select the units that you want displayed and press the "Back" button to return to the previous form. All values and units will be updated accordingly.
2. Pressure Units: This input allows you to select the pressure units that are displayed in the main psychrometric input form. If you press the button located on the far right of this cell, a picker control will display that lists all of the available units. The following English units are displayed:
   1. psia (lbs/in^2 absolute)
   2. psig (lbs/in^2 gauge)
   3. in wg a (inches of water gauge absolute)
   4. in wg g (inches of water gauge gauge)
   5. atm a (atmospheres absolute)
   6. atm g (atmospheres gauge)
   The following Metric units are displayed
   1. kPa abs (kiloPascals absolute)
   2. kPa gauge (kiloPascals gauge)
   3. m wg a (meters of water gauge absolute)
   4. m wg g (meters of water gauge gauge)
   Select the units that you want displayed and press the "Back" button to return to the previous form. All values and units will be updated accordingly.
3. Temperature Units: This input allows you to select the temperature units that are displayed in the main psychrometric input form. If you press the button located on the far right of this cell, a picker control will display that lists all of the available units. The following English units are displayed
   1. F (Fahrenheit)
   2. R (Rankine)
   The following Metric units are displayed
   1. C (Celcius)
   2. K (Kelvin)
   Select the units that you want displayed and press the "Back" button to return to the previous form. All values and units will be updated accordingly.
4. Airflow Units: This input allows you to select the airflow units that are displayed in the main psychrometric input form (this is for the mixed-air psychrometric input mode, only). If you press the button located on the far right of this cell, a picker control will display that lists all of the available units. The following English units are displayed
   1. CFM
   2. lb/min
   The following Metric units are displayed
   1. L/s
   2. kg/min
   Select the units that you want displayed and press the "Back" button to return to the previous form. All units will be updated accordingly. The inputted airflow values for streams 1, 2, and mixed will update according to the new units based upon the specific volumes of each of the streams. The maximum airflow value will update according to an average of the specific volumes of the two air streams or if the air stream values are invalid, it will use the specific volume for air at 72 deg. F and 50% relative humidity.
5. Altitude (ft or m): This input allows you to specify the elevation above sea level of the air stream(s) that you are analyzing. This value is used in an internal lookup table to determine the atmospheric air pressure in psia or kPa. After inputting this value, the atmospheric air pressure value (discussed below) will update accordingly.
6. Atmospheric Air Pressure (psia, kPa, etc.): This value is automatically updated according to the altitude that you specified above, but you can override this value to any appropriate atmospheric pressure. The atmospheric pressure does factor significantly in the calculated psychrometric results.
7. Max Temperature (F/R or C/K): This input allows you to specify the maximum allowable temperature input for the dry-bulb, wet-bulb, and dewpoint temperature inputs. After returning to the main psychrometric input screen, the maximum value on the slider control will update according to the new value. You will not be allowed to specify a value greater than this maximum temperature.
8. Temperature Step Value: This input allows you to specify the "step" value by which the temperature value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "5" degrees, the temperature will increase/decrease by 5 degrees as you slide the slider control indicator. If you specify 0, the value update will be continuous.
9. Max Humidity Ratio (lbs/lb, grains/lb or kgs/kg or gram/kg): This input allows you to specify the maximum allowable humidity ratio input value. After returning to the main psychrometric input screen, the maximum value on the slider control will update according to the new value.
10. Humidity Ratio Step Value: This input allows you to specify the "step" value by which the humidity ratio value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "0.0020", the humidity ratio will increase/decrease by 0.0020 lbs/lb (or other units) as you slide the slider control indicator. If you specify 0, the value update will be continuous.
11. Max Relative Humidity (%): This input allows you to specify the maximum allowable relative humidity input value. After returning to the main psychrometric input screen, the maximum value on the slider control will update according to the new value.
12. Relative Humidity Step Value: This input allows you to specify the "step" value by which the relative humidity value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "1%", the relative humidity will increase/decrease by 1% as you slide the slider control indicator. If you specify 0, the value update will be continuous.
13. Max Partial Pressure (psia, etc. or kPa a, etc.): This input allows you to specify the maximum allowable partial pressure input. After returning to the main psychrometric input screen, the slider control will update according to the new value.
14. Partial Pressure Step Value: This input allows you to specify the "step" value by which the pressure value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "0.1", the pressure will increase/decrease by 0.1 psia (or other units) as you slide the slider control indicator. If you specify 0, the value update will be continuous.
15. Maximum Enthalpy (BTU/lb or kJ/kg): This input allows you to specify the maximum allowable enthalpy input. After returning to the main psychrometric input screen, the slider control will update according to the new value.
16. Enthalpy Step Value: This input allows you to specify the "step" value by which the enthalpy value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "5", the enthalpy will increase/decrease by "5" BTU/lb (kJ/kgs) as you slide the slider control indicator. If you specify 0, the value update will be continuous.
17. Maximum Specific Volume (ft^3/lb or m^3/kg): This input allows you to specify the maximum allowable specific volume input. After returning to the main psychrometric input screen, the slider control will update according to the new value.
18. Specific Volume Step Value: This input allows you to specify the "step" value by which the volume value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "5", the volume will increase/decrease by "5" ft^3/lb (m^3/kgs) as you slide the slider control indicator. If you specify 0, the value update will be continuous.
19. Maximum Airflow (CFM, lb/min or L/s, kg/min): This input allows you to specify the maximum allowable airflow input. After returning to the main psychrometric input screen, the slider control will update according to the new value. (Please note: This maximum airflow value is applicable to the mixed-air psychrometric input mode discussed below.)
20. Airflow Step Value: This input allows you to specify the "step" value by which the airflow value will increase/decrease as you slide the indicator along the slider control on the main psychrometric input form. For example, if you specify "100", the airflow will increase/decrease by 100 CFM (or other units) as you slide the slider control indicator. If you specify 0, the value update will be continuous. (Please note: This airflow step value is applicable to the mixed-air psychrometric input mode discussed below.)
    
    Please note that all of the "step" units discussed above remain agnostic to any unit conversions that occur. For example, if you select a 5 degree step value for temperature in English units and you switch to Metric, the value of "5" will remain.

As an added feature, you can shake the device, and the values in this form will reset to the initial settings that existed when you first installed the Psychrometric LT application. You will be asked for confirmation prior to reset. This feature can be disabled in the "Global Settings" form discussed below.

To return to the main input screen, click the button at the top right of the screen, and the form will "flip" over revealing the main inputs along with updated values.

Global Settings Tab

This screen can be accessed by selecting the "Global Settings" tab on the home-page screen that first appears when you start the Psychrometric application. The following is an explanation of each of the inputs:

1. Display Units: This selector allows you to specify whether to display all values in English (Imperial - IP) or Metric (SI) units. When you return to the main input screen, all values will reflect the new units.
2. Where to Display Results on Screen: This selector allows you to specify where to display the calculated results on the device screen. You can display them at the top or the bottom. When you run the program for the first time, the inputs will appear at the botom of the screen and the results will appear at the top.
3. Shake Device to Reset Values: This selector allows you to specify whether to activate the device "shake" (or accelerometer) functionality in the "Psychrometric Properties" screen. You can reset the values to the initial settings that existing when you first installed the Psychrometric LT application.

Calculations Used

All of the calculations used in the HVAC Psychrometric LT application are derived from Chapter 1 of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 2017 Handbook of Fundamentals.