Scheduled formulas not working

[WillCurry]

I am trying to create a schedule for Fan Powered Boxes, but the results of the calculations are wrong. I have added a parameter for each box to indicate leaving and entering water temperatures, but when i try to find the difference in the schedule, it gives the wrong answer. I changed the formula to be a straight calculation of 180 - 160, and it STILL gives the wrong answer (it seems to think 180-160= -440 )??

anyone else run across this? any ideas on what i am doing wrong?

[sec-k]

Ok, I'm pulling this topic out of the attic and dusting it off. I'm having a problem with a formula I have built into one of my families and need some help with it.

Here is the formula: (((1.085 * (_Design Supply Heating Airflow Calc / 1 CFM) * (Delta T / 1 °F) / 3.413))) * 1 W

Where, in this particular example, _Design Supply Heating Airflow Calc = 490 CFM and Delta T = 30.43 °F

The Delta T value comes from another formula, (Leaving Air Temp - Mixed Air Temp) + 0 °F

If I plug in those numbers in place of the parameters, I get the correct answer of 4740.143 Watts. When I use the parameter values, the result is 165.724 Watts. The issue seems to be with the "Delta T / 1 °F" part of the formula. I assume this stems from the Kelvin/Fahrenheit conversion, but don't know what to do to fix it. Any ideas?

[davidvs]

Hi Jason. I MAY have an answer for you, but it's rather messy. It relies on using a temporary variable of type Piping Temperature, and setting Piping Temperature units to Kelvin. There's also a bit of roundoff error induced, but I doubt it's a big deal.



1) Set Piping Temperature units to Kelvin

2) Create the parameter Delta T of type Piping Temperature

3) Set it to the difference between Leaving Air Temp and Mixed Air Temp

4) Create a Number parameter called Delta T Unitless Kelvin, set it to this formula:



(Delta T / -459.67 °F) * -510.7445



this should give you a Delta T value in Kelvin (= Celsius) degrees



5) Create a Number parameter called Delta T Unitless Fahrenheit and set it to this formula:



Delta T Unitless Kelvin * 1.8



this should convert that delta to the equivalent Fahrenheit change in temperature



Use Delta T Unitless Fahrenheit in your formula in place of (Delta T / 1 °F)



Edited by: DavidVS on May 19, 2009 1:45 PM. Tweaked the Delta T Unitless Kelvin formula to reduce roundoff error and make more sense.

[sec-k]

I got it to work without changing the temperature setting to Kelvin.

My “Delta T” formula is Leaving Air Temp – Mixed Air Temp, which equals -429.24°F, (or really 30.43°F)

Then, I made a “Delta T Unitless” parameter and its formula is ((Delta T / -459.67 °F) * -510.7445) * 1.8, which results in a unitless number 30.43. I then plugged the “Delta T Unitless” in place of (Delta T / 1 °F) and it worked.

Then I thought, hey, I can just do this with one formula. So I made a new Delta T (number) parameter and set the formula to this:

(((Leaving Air Temp - Mixed Air Temp) / -459.67 °F) * -510.7445) * 1.8, which does the exact same thing, just in one step.

Very messy, but I guess I have no other option.

Thanks for the help!

[davidvs]

Great! Glad to hear your improved approach is working. I don't think it will work if you change your Temperature units away from Fahrenheit, though...at the very least you'd have to stop multiplying by 1.8

I agree with others, though: The real solution is that Revit needs separate data type definitions for Temperature and for Temperature Difference.

[test_52]

I thought I had this whole temperature difference figured out but my formula is still giving me an inccorect answer. Hopefully it is something easy that I have been overlooking since I have been staring at it so long.

I am trying to calculate the gpm of a coil using a formula that contains CFM, entering and leaving air temperatures and entering and leaving water temperatures. Here is the formula that it should be calculating:

GPM = (CFM * 1.085 * Delta Tair) / (500 * Deta Twater)

Here is how I have it written in Revit using my shared parameters:

GPM = (CFM * 1.085 * (LV. AIR TEMP. - ENT. AIR TEMP. + 0 °F)) / (500 * (ENT. H2O TEMP. - LV. H2O TEMP. + 0 °F))

I used the +0°F to offset the K problem, but the formula is still giving me the wrong answer.

For example, with CFM = 300, LV. AIR TEMP.=85, ENT. AIR TEMP.=55, ENT. H2O TMEP.=140 AND LV. H2O TEMP.=120 the GPM should be 0.98 gpm but revit is giving me 4.97.

Is it something I am missing or can revit not calculate the gpm correctly?

Any help would be great!

[CoreyDaun]

Can you report the details on the Shared Parameters or post an image of the dialog box with the formula? Attached is the image of my test. I removed the "+ 0 °F" for 'GPM 1' because that made the results incorrect. I made both of the test parameters Numbers, so they are unitless.

[test_52]

I have attached a screen shot that shows my shared parameters and my formula. One question about yours, why do you divide by 1 cfm at the end? When I tried this, it automatically changes the 1.085 to 1cfm. Also, I wonder if the problem is that I don't have GPM as unitless, I have it set to gpm as the units.

[test_52]

Ok, when I set up a unitless paramater and entered the formula like you did it works. Why won't this work when the units are set to gpm? It seems weird that it needs to be unitless.

Oh well. Thanks a ton for the help! At least know I have numbers that I can work with. I can try to find a work around to get the units added back so I can map it to the coil connections.

Thanks again!

[CoreyDaun]

It should work without having to be unitless - I was just doing it fast and didn't go to Project Units to set the tolerance to two decimal places. My question is how did the "+ 0° F" change the equation? Well, if it works, it works. Revit exists through work-arounds...

[test_52]

Sorry to keep harping on this and I really appreciate your help, but I can't figure out why this won't work. The formula works great if the GPM parameter is set to unitless, but if I have the GPM parameter set up as a Pipe Flow type, I get very strange answers out of the formula.

I though I could come up with a work around by having a parameter of GPM unitless and then have a second parameter of GPM with the correct units. The correct unit GPM would equal (GPM unitless * 1gpm). This gives me the correct answer in the family, but once the family is in my project, the correct unit GPM won't update as the CFM parameter is changed. The correct unit GPM always shows 0.

Has anyone found a way around this?

Thanks again in advance!