Imposing dimensional consistency


I recently purchased a Privileg 892NC calculator at a garage sale. It is a financial calculator which seems to be very similar to the Commodore F4146R. The keyboards are identical but the 892C is about an inch longer.

I am working my way through the capabilities of the 892C. Page 35 of the manual presents the solution to an elementary interest problem; i.e., for n = 3, I = 7% and a present value of 1000 what will be the future value? The solution sequence is given as

C 1000 PV 7 I 3 N CMP FV

where C is a clear key and CMP is a compute key which acts similarly to the CPT key on the TI-MBA and TI-BA-55. The answer is given as 1255.043 . The demonstration continues with the sequence

- DIS PV =

where the DIS PV sequence recalls the inputted present value of 1000 and the subtraction then yields the accumulated interest of 255.043 . All well and good up to that point, but for some reason I decided to see what would happen if instead of the - DIS PV = sequence I used - DIS I = . To my surprise the machine refuses to do that calculation. It will also refuse to perform in response to the - DIS N = sequence. It seems that the programmer recognized that the dimensional inconsistency was such that the calculation shoudn't be allowed. That's an idea that warms the heart of this old aero engineer.

The machine will accept a numneric keyboard entry after the minus sign. Apparently, the programmer made the reasonable assumption that if the user made an entry from the numeric keyboard then the user prbably know what he was doing.

There are several other Commodore and Privileg financial calculators. I don't have any of them in my collection so I can't say whether or not they impose dimensional consistency.

Calculators in my collection which permit the user to recall the contents of the financial registers but which do NOT impose a check on dimensional consistency include the TI-MBA, the TI-BA-55, the HP-10B, the HP-12C, the HP-17B-II, the HP-38C and the Sharp EL-733A.

Does anyone know if there are other financial calculators which impose dimensional consistency?



Hi, Palmer --

The "dimensional consistency" of TVM calculations you describe is that PMT, PV, and FV are monetary values in some arbitrary currency, while N and I%YR (or I) are not. Therefore, the operations of addition and subtraction between one of the former group and one of the latter group can be considered invalid.

However, it should be OK (for example) to multiply N by PMT or by I. Can you multiply FV by unity, then add it to N?

The HP-14B and HP-20b HP allow TVM variables of different groups to be added or subtracted -- to include register arithmetic -- and I expect that the same is true of all HP business calc's.

-- KS

Edited: 27 Dec 2008, 4:49 p.m. after one or more responses were posted



You wrote:

However, it should be OK (for example) to multiply N by PMT or by I. Can you multiply FV by unity, then add it to N?

It won't multiply by either I or by N. Some curious things happen when I add PMT to the mix but I may not be doing everything as required by the machine. It's slow going since the only manual I have is in German and I'm not conversant in German. I have translated a little bit with the help of a German to English dictionary but that is really S L O W going.

It has occurred to me that what is happening may be accidental. I'll do some more testing this weekend and post the results.



Joerg Woerner translated some parts of the manual. Thank you, Joerg.

The problem from page 33 described at the top of this thread:

Example: What is the total amount of earned interests, if you invest DM 1000 for 3 years with an annual interest rate of 7% and yearly payment?


C 1000 PV 7 I 3 N CMP FV


1255,043 total accrued value


- DIS PV =


255.043, total of the earned interest

Please notice how the DIS key is used to use a previous entry.

To calculate the compounded interest, you have to subtract the daily value from the final result.

If you calculated FV, you simply have to press - DIS PV = and the calculation will be done in a logical order.

Description of the INT key from page 22:

INT Interest Amount key

This key is used in conjunction with the CMP key to calculate bank discounting and accrued interest

Description of the DIS key from page 23:

DIS Display kay

Recalls an entry to verify. Maybe used before the CMP key without interfering previous entries.

Comment 1: If you press DIS immediately after the CMP key, e.g. with CMP DIS PV, the previous entry will be displayed. To continue the calculation you HAVE to press CMP again.

Comment 2: In some cases the recall of previous entries is allowed after a calculation. Please refer to the example "aufgelaufener Zinseszinsbetrag" on page 34.


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