HP and DM key Pressure
09-21-2020, 07:45 PM
Post: #1
 RPZ Junior Member Posts: 26 Joined: Sep 2019
HP and DM key Pressure
Hello All,

I recently had my interest in HP calculators renewed and
found this group. Thank you for this wealth of knowledge.

Besides HP calculators I also have acquired a DM42 and
a DM41X beta. Marvelous machines.

As others have noticed, the keys seem a bit stiff.
By how much?

I decided to make some measurements using a test tube and
lead shot to measure the pressure required. Quite tedious
and hard to place the bottom of the tube in the middle of the key.
Also spilled the tube once. Real mess on my tile floor.

It occurred to me that I have an old spring and dial scale
that might work. I placed the calculator on the scale and
pressed the key just as I would in use and then subtracted
the calculator weight. I only did the number keys.
Not very sophisticated but this method seems quite repeatable
and appeared to be accurate to + or - 1/2 ounce.

Calculator Key Activation Pressure, RPZ, 09-21-2020

Model ------- Condition --- Range in Ounces

HP 41CX ----- well used ------- 4 - 4

HP 25 -------- well used ------- 5 - 6

HP 12C ------- well used ------- 5 - 7

HP 35 -------- well used -------- 6 - 7

DM42 -------- almost new ----- 7 - 11

DM41X ------- almost new ----- 9 - 12

Some folks have mentioned that with use the keys break in.
These measurements will allow me to retest later and see
if that is true or if it is just in my head.

Regards, Rick

---eof
09-21-2020, 09:53 PM
Post: #2
 teenix Senior Member Posts: 1,376 Joined: May 2016
RE: HP and DM key Pressure
From the HP-45 patent document

cheers

Tony

Attached File(s) Thumbnail(s)

09-22-2020, 12:21 AM
Post: #3
 RPZ Junior Member Posts: 26 Joined: Sep 2019
RE: HP and DM key Pressure
Cheers,

Well, that's cool.

150 +- 25 grams is a range of 4.4 to 6.2 ounces. My readings
were in the ballpark. (had to use a calculator for that)

I also just noticed the much greater variation on the DMs that
might be throwing me off until they are broken in.

Regards, Rick
09-22-2020, 12:15 PM
Post: #4
 Dave Britten Senior Member Posts: 1,927 Joined: Dec 2013
RE: HP and DM key Pressure
Thanks for sharing these, I've been wondering about this for some time. Somehow it never occurred to me that placing the calculator on a kitchen scale and slowly pressing the keys with a blunt object could be used to get force readings. Maybe I'll have to try that with my wife's electronic measuring cup and see if I can get some readings for the DM41L/DM1XL models.
09-22-2020, 02:41 PM
Post: #5
 RPZ Junior Member Posts: 26 Joined: Sep 2019
RE: HP and DM key Pressure
Dear Dave,

I tried several things before the scale approach occurred to me.

Sometimes simplest is best.

I end up just using my finger just as I would use normally use. That feel in use is really what I was after.

I tried my digital kitchen scale and it also works, just had to press really slow to read the bouncing numbers.

Curious what you come up with.

Regards, Rick
09-22-2020, 03:31 PM
Post: #6
 Dave Britten Senior Member Posts: 1,927 Joined: Dec 2013
RE: HP and DM key Pressure
Got some interesting results. I tested my DM41L and DM42. For each, I did 8 tests using different keys around the keyboard. I did not include the ENTER key in the results, but the Voyager-type models generally require less force than the other keys if pressed in the top half, and the DM42 requires more force due to having the same length lever arm but two spring domes under it.

I only tested force required for the sound and feel of the key click, as I would have needed a more elaborate setup to watch both the calculator display and scale display simultaneously.

DM41L
Mean: 240 g (8.50 oz)
Std Dev: 13.4 g (0.473 oz)

DM42:
Mean: 265 g (9.33 oz)
Std Dev: 27.8 g (0.982 oz)

So there is more variability in the DM42 keyboard, and more force required in general.

And I used my 15C for the statistical calculations and unit conversion.
09-22-2020, 04:53 PM
Post: #7
 RPZ Junior Member Posts: 26 Joined: Sep 2019
RE: HP and DM key Pressure
Dear Dave,

That's what I did. Watched the dial as I felt for click.

Was initially going to build a hinged arm with a key pusher
and pan on top for gram weights. Too lazy.

You know my next question.

Will you measure your 15c? <grin>

And which keyboard do you prefer?

Regards, Rick
09-22-2020, 06:12 PM
Post: #8
 Dave Britten Senior Member Posts: 1,927 Joined: Dec 2013
RE: HP and DM key Pressure
(09-22-2020 04:53 PM)RPZ Wrote:  Dear Dave,

That's what I did. Watched the dial as I felt for click.

Was initially going to build a hinged arm with a key pusher
and pan on top for gram weights. Too lazy.

You know my next question.

Will you measure your 15c? <grin>

And which keyboard do you prefer?

Regards, Rick

I'll take some readings from the 15C when I get a chance. Mine is fairly well-worn, so it doesn't have very distinct key click. Neither does my 16C, and both are 1983 USA models, so I wonder if that's just typical with models from that factory/time period.

I definitely prefer the real HP keyboards, but I find SM's take on the Voyager keys perfectly acceptable. I was expecting the DM42 to need significantly more force, but that's not the case. I think it just feels like more because of the smaller keys leading to increased pressure, and the Enter key being particularly stiff.
09-22-2020, 06:40 PM
Post: #9
 Thomas Okken Senior Member Posts: 1,339 Joined: Feb 2014
RE: HP and DM key Pressure
(09-22-2020 06:12 PM)Dave Britten Wrote:  I'll take some readings from the 15C when I get a chance. Mine is fairly well-worn, so it doesn't have very distinct key click. Neither does my 16C, and both are 1983 USA models, so I wonder if that's just typical with models from that factory/time period.

I believe it is typical. I bought a 41CX when it first came out, and a 10C not much later, and even with those two brand-new machines, the difference in keyboard feel was very noticeable, with the 10C keyboard requiring more force and having a less distinct click.
09-22-2020, 07:15 PM
Post: #10
 Dave Britten Senior Member Posts: 1,927 Joined: Dec 2013
RE: HP and DM key Pressure
(09-22-2020 06:40 PM)Thomas Okken Wrote:
(09-22-2020 06:12 PM)Dave Britten Wrote:  I'll take some readings from the 15C when I get a chance. Mine is fairly well-worn, so it doesn't have very distinct key click. Neither does my 16C, and both are 1983 USA models, so I wonder if that's just typical with models from that factory/time period.

I believe it is typical. I bought a 41CX when it first came out, and a 10C not much later, and even with those two brand-new machines, the difference in keyboard feel was very noticeable, with the 10C keyboard requiring more force and having a less distinct click.

I forgot to mention that I've also got a 1986 USA 12C and a 1987 USA 11C, and those are considerably more clicky (though not as much as a 41).

I should measure my 42S too...
09-23-2020, 03:51 PM
Post: #11
 Dave Britten Senior Member Posts: 1,927 Joined: Dec 2013
RE: HP and DM key Pressure
More results:

15C (2337A)
Mean: 142 g (5.01 oz)
Std Dev: 7.18 g (0.253 oz)

11C (2706A)
Mean: 142 g (5.01 oz)
Std Dev: 8.78 g (0.310 oz)

42S (3134S)
Mean: 133 g (4.69 oz)
Std Dev: 8.72 g (0.308 oz)
09-23-2020, 04:31 PM
Post: #12
 RPZ Junior Member Posts: 26 Joined: Sep 2019
RE: HP and DM key Pressure
Wow

Maybe your 11c and 15c were made on the same day. <grin>
09-23-2020, 06:33 PM (This post was last modified: 09-23-2020 06:35 PM by ijabbott.)
Post: #13
 ijabbott Senior Member Posts: 1,051 Joined: Jul 2015
RE: HP and DM key Pressure
(09-23-2020 04:31 PM)RPZ Wrote:  Wow