MoistAirTab Frequently Asked Questions: Contents
No fear! Just upgrade to MoistAirTab V2.0 which is fully compatible with Office 2010 on Windows 7 (either the 32- or the 64- bit version).
If you have a single-user license, please call or email us directly to access the discounted upgrade. If you are in a multiple-user environment, please ask your IT administrator to contact us for the upgrade.
The relative humidity is defined as a fraction between 0 and 1. To enter the relative humidity, either use a number between 0 and 1, or enter it as a percentage, e.g. 25.4% (Please note that you must include the percent symbol % after the number - In Excel, a percent is always represented as a fraction).
You may have accidentally "unloaded" MoistAirTab from your Excel. To load MoistAirTab back into Excel, you simply need to put a check mark before MoistAirTab in the "Add-Ins..." dialog-box accessible from the Excel -> Tools -> Add-Ins... menu.
If MositAirTab doesn't show up on the add-in list there, you probably have accidentally "uninstalled" MoistAirTab. If that is the case, you'll need to reinstall the software from the original program diskette.
There are two possibilities for this:
- You may have changed the Units specification from Metric/SI to English from the Options dialog-box. Please make sure that the Default Pressure is set at the correct value in the selected unit. MoistAirTab V1.0 does not automatically convert the value of the default pressure when the unit is switched. For example, the default pressure is 1 bar in Metric/SI units. When you change the units to English, the default pressure value remains 1, while the "Unit" has changed to psia. (The program will get smarter in a future release and you will get a free upgrade!). But in the mean time, you need to specify the default pressure value in psia unit, e.g. 14.69.
- Even when you make the above change, you may still notice differences between printed psychrometric charts in English units versus the numbers generated by MositAirTab in Enthalpy and Entropy values. The reason is that some versions of printed charts use a different reference state for defining the thermodynamic path functions. For example, the popular Psychrometric Chart published by Carrier Corporation in 1947 and 1959 in English units use 0oF as the reference temperature whereas MoistAirTab uses 0oC (32oF) as the reference temperature (as recommended by ASHRAE). Please note that the choice of a reference state is arbitrary and should not affect the end result when using either the printed chart or MoistAirTab as one normally only deals with the changes in enthalpy and entropy rather than the absolute quantities.
For those who prefer to use the English units (I-P units) and insist that 0oF be used as the reference temperature for enthalpy and entropy, please note that the numeric difference between 32oF and 0oF is 7.686951 Btu/lb-dry-air in enthalpy and 0.016166 Btu/lb-dry-air/oF in entropy. To shift the enthalpy or entropy value from 32oF to 0oF you only need to add the value noted above from the enthalpy or entropy value obtained from the MoistAirTab. No shift is needed for latent heats or other properties.
To "unload" MoistAirTab, uncheck MoistAirTab in the "Add-Ins..." dialog-box accessible from the Excel -> Tools -> Add-Ins... menu..
To "reload" MoistAirTab, just put back the check mark before MoistAirTab. This will load MoistAirTab every time you start Excel.
The "$" used in the formula is a convention shared by all spreadsheets for anchoring the cell reference to either the row, the column or both (a fixed cell location). As a spreadsheet add-in, the MoistAirTab complies the rules and conventions of the host spreadsheet.
MoistAirTab anchors all cell references to fixed cell locations by fixing both column and row locations ($Column$Row). When you wish to copy formulae with cell references, depending on your formulation, you will need to modify the anchors by removing either one or both of the "$" signs before copying the formula. If you single click the left mouse bottom when the mouse pointer is placed on the cell reference (for example, $B$12) in the formula bar on top of your Excel spreadsheet, and toggle the 4-way F4 key on your keyboard, you will see that the fixed cell reference is first changed to fixed column reference. Toggle again, changed to fixed row reference. Toggle once more, changed to no anchoring. Toggle once again, changed back to fixed cell reference. Alternatively, you could just go into the formula and delete the unwanted $ signs before copying the formula.
This is intentional. Our experience in spreadsheet engineering has taught us that when we embed values directly into formulae, it becomes more difficult later to recreate the logic or debug the algorithm, since we only see the result of the formula, instead of a full description of input parameters. Referencing a cell reference for input to a formula not only documents our data source, but also allows Excel's auditing tools to track all precedents and dependents thereby aiding debugging.
If you insist, you can directly input the value, but you will need to put up with some annoying Excel behavior. When you enter a value in any of the edit boxes, Excel will warn you that the cell reference in incorrect. Since you know better, simply ignore it and Excel will still proceed to compute the results.