Units for Time

Use this forum to post Vensim related questions.
Post Reply
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Units for Time

Post by Bena.m »

Hi there!
I'm using PLE+ and the time series that I'm working with is related to every 8 hours, it means that in the model settings I need to customize the units for time instead of hour, day, week... as every 8 hours, I tried to simply write it in the bar of "units for time" and it worked, but when I run SDM-Doc I faced with strange views, so I wanted to ask you if we're allowed to customize the units for time and if yes, how we can solve the problem of SDM-Doc?
thank you
Administrator
Super Administrator
Posts: 4573
Joined: Wed Mar 05, 2003 3:10 am

Re: Units for Time

Post by Administrator »

Bena.m wrote: Fri Jan 07, 2022 12:28 pmI need to customize the units for time instead of hour, day, week... as every 8 hours
I'm not sure you are doing this correctly, but you haven't really provided enough information to help.

The units for time would remain "hours", it's just your data that is available every 8 hours. So you either need to interpolate between the data points, or use the raw values. Changing the time step to 8 hours just to match the data is most likely the wrong thing to do.
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Re: Units for Time

Post by Bena.m »

thank you so much for your response.
in fact, I wanted to change the units for time in order to have a more clear interpretation of graphs,
because I'm not sure that if we keep the units for time "hours", it means that we must provide data for 24 consecutive hours every day, or we can explain that x-axis which is "Time(hour)" represent the data every 8 hours in a day?
I hope I could explain what is my question...
Administrator
Super Administrator
Posts: 4573
Joined: Wed Mar 05, 2003 3:10 am

Re: Units for Time

Post by Administrator »

Have you done any of the tutorials in Vensim yet? It might help you understand how time should be used.

http://www.public.asu.edu/~kirkwood/sysdyn/SDRes.htm

On this page you will find a really good Vensim PLE tutorial which takes you through how to build a simple model.
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Re: Units for Time

Post by Bena.m »

Yes of course I've done, also I checked the Vensim help, but couldn't find the answer to this question,
however, thank you for the link I will check it out.
tomfid
Administrator
Posts: 3806
Joined: Wed May 24, 2006 4:54 am

Re: Units for Time

Post by tomfid »

You could make the time unit "hour" and set TIME STEP = 8.

However, I'd be careful about matching the time step to the data interval. Usually a real system will have some processes that are faster than the measurement interval.
https://metasd.com/2017/01/forrester-on ... ous-flows/
https://metasd.com/2017/11/discrete-time-stinks/
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Re: Units for Time

Post by Bena.m »

thank you so much for your reply.

I tried that for 3 years data and it doesn't work properly, can it happen because TIME STEP=8 is so big and it leads to an error?...

however, when I changed the "Units for Time = every 8 hours" it worked as it was expected to and the problem occurred just when I run SDM-Doc...
so also I wanted to ask you since according to Vensim Help we're allowed to type in any units we like, why you don't suggest changing the "Units for Time"?
tomfid
Administrator
Posts: 3806
Joined: Wed May 24, 2006 4:54 am

Re: Units for Time

Post by tomfid »

Certainly changing the time unit to "EightHrs" or something like that works too.

I'm not sure what you mean by "doesn't work properly". This is hard to evaluate without seeing the model. I would guess that you have parameters with implicit time constants linked to one 8h interval, so using TIME STEP = 8 ~ hours makes all the gains 8x too big.

Does your model have units for all variables, and does it pass a unit check?
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Re: Units for Time

Post by Bena.m »

for example by using TIME STEP =8, if then else functions don't start working when the condition is met

you're right with Unit for Times = 8, the model doesn't pass the unit check, so maybe it's better I work on TIME STEP as you said...
tomfid
Administrator
Posts: 3806
Joined: Wed May 24, 2006 4:54 am

Re: Units for Time

Post by tomfid »

The TIME STEP shouldn't have any influence on IF THEN ELSE, unless the interpretation of the inputs changes. I'd have to see a specific example to comment further.

For the sake of concreteness, here's an example:

BankBalance = INTEG( InterestIncome, 100 ) ~ $
InterestIncome = BankBalance*InterestRate ~ $/year
InterestRate = 0.12 ~ fraction/year
TIME STEP = 1 ~ year

I could rewrite this as

BankBalance = INTEG( InterestIncome, 100 ) ~ $
InterestIncome = BankBalance*InterestRate ~ $/month
InterestRate = 0.01 ~ fraction/month
TIME STEP = 12 ~ month

The results of these two models will be identical (except for the time axis).

However, if you convert the first model to the second by changing units and TIME STEP, but you don't adjust the InterestRate parameter, the second version will result in explosive growth, not matching the first.
Bena.m
Member
Posts: 23
Joined: Thu Dec 23, 2021 10:32 am
Vensim version: PLE+

Re: Units for Time

Post by Bena.m »

Thank you so much for your complete explanation
I will work on that considering your useful points
Post Reply