DM-15L Review

[dm319]

This is a brief review of the DM-15L which I got last week.



Cost/shipping

I bought this for one of my children who expressed an
interest in having a 'weird' calculator. They are 10 years
old and will start secondary school in the UK next Autumn
2025. This model was selected based on calculator exam
requirements and the possibility of a two line display.

I bought this from the SM Amazon store for £150. I did this
deliberately to avoid having to figure out customs charges
for the package. It took exactly 14 days which felt like a
long time, but only because of anticipation, and doesn't
compare to the length of time I have deliberated about it.



Appearance

This calculator feels very weighty, maybe the weightiest of
all pocket calculators I have come across, apart from the
DM42. The sand-blasted titanium rear plate has a rough feel
and very industrial look. It is mated pretty perfectly onto
the plastic top plate.

There is a brushed steel face plate which looks good to me,
but there is a very slight asymmetry in its positioning
which will annoy a perfectionist. The key legends are very
clear, especially the yellow, the blue depends on the
lighting.

Overall, the colours are quite earthy, and the weight gives
it a slight bomb-proof feel. In fact it wouldn't look too
out of place next to world war 2 field equipment in terms of
colours. The hand-stiched leather pouch fits it perfectly
(maybe too tightly but that will improve with time) and
looks terrific.



Keys

The keys are well-formed. Others have mentioned the key
presses are short travel, but it doesn't 'feel' short. If
you are used to HP-65, it will feel short, if you are used
to a Casio or Sharp scientific this has more travel.

It is very tactile and the 'click' is muted. I switched it
to 48Mhz mode fairly quickly which is meant to be better for
key presses and calculates quicker. The most important
thing for me is reliability of key presses, and it does very
well here.

Unfortunately I don't have a good collection of high-quality
HP calculators to compare it against, but my second hand
HP-12c platinum is especially bad in this regard. My rogue
HP-12c is much better than the 12c platinum, but not quite
as good as this. My DM42 is the king of reliable key presses
and is a bit better.

Compared to modern and old Sharps, Casios and Texas
Instruments, this is better than any of them. By which I
mean that often these other brands have a very flat feel to
them, with very little tactility or travel.

There is a very slight discrepancy between the tactile bump
and the registration of the key press. This is familiar and
expected to me given to my experience in mechanical clicky
keyboards. It's normally not a problem as long as you
develop a feel for how hard you are pressing the keys.

Overall, I find the feel of the keys to be excellent.

Display

Is clear to me and similar to the best LCD screens out
there. The DM42 with its high contrast memory display is
better of course. The fonts frankly look terrible on the
stock photos, but it looks good in real life. I don't know
why this is. It is imminently usable and clear.

Functionality

As this was bought to be used in secondary school in the UK,
this is the perspective I'm looking at this from.

Firstly, it is RPN. That's a given in this forum of course,
but the competition for this calculator mainly comes in the
form of a Casio, Sharp or TI scientific non-graphing. I
really wanted an RPN calculator for my kid to learn on as I
genuinely think it is better for 'calculating' than
algebraic. At the moment, for less than 1/10th the cost, I
can get a Casio with a natural layout and which can
calculate to greater accuracy.

On the other hand with the algebraics you can end up at the
mercy of wherever arguments are ongoing regarding order of
operations, which is something you decide for yourself with
RPN. Casio have opted for 'multiplication by juxtaposition'
(a silly term for this IMO) as needing clarification with
brackets. At the same time they are now treating scientific
notation as not receiving any special precedence which means
that 1÷5e2 is evaluated as 20 rather than 1/500th.

Without this turning into an anti-algebraic rant, there are
also issues that changing the format of the answer results
in a re-calculation, which can be very confusing if you have
chained a calculation using 'ANS' from previously and
require it in a different format.



RPN avoids all these pitfalls, but requires that you
understand what you are doing fully, but really, if you are
getting this calculator for school, it is because you have
prioritised RPN over almost everything else.

Despite being based on 43 year old technology, the DM-15L
beats every other modern scientific calculator in complex
number handling. The modern TI-30X pro mathprint and TI-36X
pro which I believe are top of the range scientifics can not
raise complex numbers to anything other than 1, 2 and 3.

It seems to also have the largest matrix handling of any
modern scientifics with up to 8x8 matrices. I'm impressed
that the factorial works continuously and on negative numbers
presumably using the gamma function.

There is no fraction support. Statistics are limited to
sum, mean, standard deviation and linear regression. This
might be an issue in later A-level maths when calculators
may be expected to calculate probability distributions and
medians.

It does have a numerical solver and integrator, which
matches it with the top-of-the-range scientifics from the
major brands. It is programmable, though this will be of
little use for school as programs will need to be cleared
pre exams.

It will not attempt to show results as radicals or fractions
of Pi as many Casios will do these days. On the other hand,
even though this isn't a full CAS system, I think this is
half-way cheating, and it also results in errors when the
calculator gets it wrong.

This isn't the only modern RPN calculator you can buy.
There is also the HP-15CE which is the official modern
re-incarnation of this device. This is very slightly less
expensive here in the UK, but stock is running out rapidly.
It is effectively the same calculator on different hardware,
using a classic 7-segment display and angled HP buttons.

I didn't consider the DM-42 or DM-32 as these are
significantly more expensive and may run foul of rules
around exam use.


Overall Thoughts

This is a very tidy and capable package and, in some ways,
the ideal calculating machine. I am unsure whether this
will work out for my sons education, and I'm documenting the
experience here. The device is incredibly powerful in
some specific areas, more so than any modern scientific
calculator, which is a testament to the effort which went
into its original design. In other ways it doesn't have
functionality which some might expect of even cheap modern
scientifics - fractions, radical and pi representations,
advanced spreadsheets, tables, formulas and statistical
calculations.

However, I don't think these things necessarily help
students to learn, if anything, it might be detrimental, and
learning the principals of maths is more important. To this
end, hopefully the DM-15L will be good, but we will have to
see.