HP 67 Collection: October 2026

HP 67 Collection: October 2026

All examples are done in FIX 4. (DSP 4)

Earth's Gravity at a Specific Latitude

The true earth gravity force depends on several factors, such as latitude and altitude. This estimation takes the latitude (north/south), in degrees-minutes-seconds, and latitude, in kilometers, into account.

The constant 9.80665 m/s^2 is an accepted average, the true force varies.

g_s = 9.806 – 0.026 * cos(2 * φ)

g_h = g_s * (6378.1 / (6378.1 + h))

HP 67 Code:

LBL A: 001 31, 25, 11

DEG: 002 35, 41

H←: 003 31, 74

2 : 004 02

× : 005 71

COS: 006 31, 63

. : 007 83

0 : 008 00

2 : 009 02

6 : 010 06

× : 011 71

9 : 012 09

. : 013 83

8 : 014 08

0 : 015 00

6 : 016 06

x<>y: 017 35, 52

- : 018 51

x<>y: 019 35, 52

6 : 020 06

3 : 021 03

7 : 022 07

8 : 023 08

. : 024 83

1 : 025 01

+ : 026 61

LST x: 027 35, 82

x<>y: 028 35, 52

÷ : 029 81

× : 030 71

RTN : 031 35, 22

Inputs:

Y: h

X: φ

Examples:

Inputs: h = 0, φ = 2°33’: g ≈ 9.7801

Inputs: h = 0.762, φ = 52°17’: g ≈ 9.8114

Source 

Grainger Engineering Office of Marketing and Communications. (answer written by Rebecca H.) (2016, November 21). “How gravitational force varies at different locations on Earth.” Illinois. https://van.physics.illinois.edu/ask/listing/64061. Retrieved March 10, 2026.

Triangulation

With angles α and ß:

base = dist * (1/tan α + 1/tan ß)

dist = base ÷ (1/tan α + 1/tan ß)

Labels:

LBL A: Enter angles α and ß in the current angle mode. Returns 1/tan α + 1/tan ß.

LBL B: Base. When a number is entered prior to pressing [ B ], the number is stored as a base. Otherwise, the program solves for a base.

LBL C: Distance (of a line that bisects the base). When a number is entered prior to pressing [ C ], the number is stored as a base. Otherwise, the program solves for a base.

HP 67 Code:

LBL A: 001 31, 25, 11 (enter the angles)

TAN: 002 31, 64

1/x: 003 35, 62

x<>y: 004 35, 52

TAN: 005 31, 64

1/x: 006 35, 62

+: 007 61

STO 5: 008 33, 05

RTN: 009 35, 22

LBL B: 010 31, 25, 12 (base)

STO 3: 011 33, 03

F? 3: 012 35, 71, 03

RTN: 013 35, 22

RCL 4: 014 34, 04

RCL 5: 015 34, 05

× : 016 71

STO 3: 017 33, 03

RTN: 018 35, 22

LBL C: 019 31, 25, 13 (distance)

STO 4: 020 33, 04

F? 3: 021 35, 71, 03

RTN: 022 35, 22

RCL 3: 023 34, 03

RCL 5: 024 34, 05

÷ : 025 81

STO 4: 026 33, 04

RTN: 027 35, 22

Examples:

α = 30°, ß = 65°

Set degree mode. 30 [ ENTER ] 65 [ A ]. Result: 2.1984

Distance = 24, solve for base. 24 [ C ] [ B ]. Result: 52.7606

Base = 24, solve for distance. 24 [ B ] [ C ]. Result: 10.9172

Source:

“Triangulation (surveying)” Wikipedia. https://en.wikipedia.org/wiki/Triangulation_(surveying) (last edited October 24, 2025). Retried March 17, 2026

Time of Runoff on Turf

t = 2.5 * (L / (S * I^2))^(1/3)

t: time of runoff (rain off) in minutes

2.5: turf constant

L: distance from remote area in feet

S: slope of the terrain

I: rain intensity in inches per hour

Input Stack:

Z: L

Y: S

X: I

Output Stack:

X: t

HP 67 Code:

LBL A: 001 31, 25, 11

x² : 002 32, 54

× : 003 71

÷ : 004 81

3 : 005 03

1/x: 006 35, 62

y^x: 007 35, 63

2 : 008 02

. : 009 83

5 : 010 05

× : 011 71

RTN: 012 35, 22

Examples:

Inputs: Z: 900 ft, Y: 0.11, X: 4 in/hr. Result: 19.9917 min

Inputs: Z: 650 ft, Y: 0.33, X: 0.06 in/hr. Result: 204.4744 min

Source

Texas Instruments. Slide Rule Calculator SR-50A Manual. 1975. Manual from datamath.org: http://www.datamath.net/Manuals/SR-50A_US.pdf pg. 59

Beamwidth of a Radio Telescope

This program calculates the beam width of a radio telescope in degree-minutes-seconds.

Θ = 67625 ÷ (D * f)

Θ = beam width

D = the diameter of a circular telescope in feet

f = observing frequency in MHz (megahertz)

Inputs:

Y: D

X: f

Outputs:

X: Θ

HP 67 Code:

LBL B: 001 31, 25, 12

× : 002 71

1/x : 003 35, 62

6 : 004 06

7 : 005 07

6 : 006 06

2 : 007 02

5 : 008 05

× : 009 71

→H.MS: 10 32, 74

RTN: 11 35, 22

Examples:

Inputs: X: 1980 MHz, Y: 106 ft. Output: Θ: 0.1920 (19’20”)

Inputs: X: 1665 MHz, Y: 84 ft. Output: Θ: 0.2901 (29’01”)

Source

Ball, John A. Algorithms for RPN Calculators John Wiley & Sons: New York, NY. 1978. ISBN 0-471-03078-8. pg. 259

Fan Laws

p2 = (rpm2 ÷ rpm1)^n * p1

(p1 and p2 stand for various parameters, see the input syntax below)

Inputs:

1^st law: (cfm: cubic feet per minute, rpm: revolutions per minute)

Calculate cfm2:

T: cfm1

Z: rpm2

Y: rpm1

X: 1

2^nd Law: (sp: static pressure, rpm: revolutions per minute)

Calculate sp2:

T: sp1

Z: rpm2

Y: rpm1

X: 2

3^rd Law: (hp: horsepower, rpm: revolutions per minute)

Calculate hp2:

T: hp1

Z: rpm2

Y: rpm1

X: 3

HP 67 Code:

LBL B: 001 31, 25, 12

R↓ : 002 35, 53

÷ : 003 81

R↑ : 004 35, 54

y^x: 005 35, 63

× : 006 71

RTN: 007 35, 22

Examples

1^st Law: T: cfm1 = 2000, Z: rpm2 = 1100, Y: rpm1 = 950, X: n = 1

Result: cfm2 ≈ 2315.7895

2^nd Law: T: sp1 = 0.5, Z: rpm2 = 1100, Y: rpm1 = 950, X: n = 2

Result: sp2 ≈ 0.6704

3^rd Law: T: hp1 = 1.5, Z: rpm2 = 1100, Y: rpm1 = 950, X: n = 3

Result: hp2 ≈ 2.3286

Source

Gladstone, John. Air Conditioning Testing and Balancing: A Field Practice Manual A Van Nostrand Reinhhold Book. August 1976. pp. 24-32

More HP 67 goodness to come in November,

Eddie

All original content copyright, © 2011-2026. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.

Review: Calculated Industries 4090 Sheet Metal/HVAC Pro Review

Review:  Calculated Industries 4090 Sheet Metal/HVAC Pro Review 

Quick Facts

Model: 4090 Sheet Metal/HVAC Pro 

Company: Calculated Industries

Years: approx. 2004 - Present*

Memory Register:  1 independent memory, 3 general memory registers

Battery:  2 x LR44 batteries

Screen:  8 digits with alpha numeric prompts, 12 digit accuracy

Logic:  AOS (Algebraic), Chain 

Price:   retails about $89.95; iOS and Android app available for $29.99; used calculators can be found for less 

Web Page from Calculated Industries:  https://www.calculated.com/mobile/prd722/Sheet-Metal-HVAC-Pro.html

If you purchase it out of the box, either by repackage or retail, both an Armadillo Case and a hard slide cover come with the calculator.  In the box set has a full user's manual, which is really nice and gives an extensive amount of examples and details.  

History

The Sheet Metal/HVAC Pro was exclusive to the International Training Institute (ITI)(https://www.sheetmetal-iti.org/) until 2021, when Calculated Industries made the calculator available to the Public  (7 seconds into this video:  https://www.youtube.com/watch?v=wzmB8UAEjls).

I purchased a used Sheet Metal/HVAC Pro which is why you see the ITI logo in the pictures.  I am very confident both versions have the substantially the same amount of functions.  

Features

* Trigonometry and Right Triangles

* Square, Square Root, Cube, Cube Root

* Fan Laws

* Circle and Arc Calculations

* Column and Cone Calculations

* VP (velocity pressure)/FPM (feet per minute) conversions

* Offset calculations

* Stairs, Hip/Valleys, Jack

* And one of the best known features to come with Calculated Industries industry calculators:  feet-inch-fraction and meter-millimeter calculations. Fractions of inches can be set from 1/2" to 1/64".  I usually work with 1/16".  

I think this is the only calculator from Calculator Industries to have a exponent (10^xx) key (x 10^y), allow for numbers to be entered using scientific notation.  The maximum number is 9.999999 * 10^76.  

Trigonometry

The only angle unit the trigonometric functions work with is degrees.  Two unique features that come with the Sheet Metal/HVAC Pro are the ArcK constant and the Law of Cosines function.  

An ArcK constant, which is equal to π/180 ≈ 0.017453.  This constant is helpful for circle arc calculations.

The Law of Cosines function where given the lengths of three sides A, B, and C, calculate the angle measurements of each of the angle and the area of the triangle using Heron's formula.   This is one of my favorite features of the Sheet Metal/HVAC Pro, as going through the User Guide a lot of pipe work involves determining lengths and angles of triangles.  

There are four keys that deal with right triangles, the multi-purpose [ x (Run) ], [ y (Rise) ],  [ r (Diag) ], and [ θ (Pitch) ] keys.  Enter the given knowns and then just press the remaining keys to solve for the missing parameters.  

Fan Laws

The A and A new (which represent the CFM - cubic feet per minute), and the B and B new registers deal with the three fan laws.  The B and B new parameters change depending on which fan law is used.

Fan Law 1:  B and B new represent RPM (revolutions per minute)

Fan Law 2:  B and B new represent SP (static pressure)

Fan Law 3:  B and B new represent BHP (brake horsepower)

Stairs and Roofs

The Sheet Metal/HVAC Pro has the stair, hip valley, and jack rafter calculations, the same as the construction calculators Construction Pro 5 and the Construction Master Pro.   You can set the default riser height of the stairs, as well as calculate the number of required risers, stairs, the tread width, the stringer length.  The HVAC Pro also works with roofs:  rafter length, pitch, jack rafters including irregular jacks.  

Offset Calculations

The offset calculations on the Sheet Metal/HVAC Pro slightly differ from the offset calculations offered on the Pipe Trades Pro.  The offset deal with sheet metals, and the calculations included are:

*  centerline radius

*  wrapper length

*  throat and heel radius

Verdict

As always, Calculated Industries offers quality calculators that are specific to industrial needs and the Sheet Metal/HVAC Pro is no exception.  The key quality is great, the display has helpful prompts, and the Armadillo cases given solid protection.   I'm happy that this calculator is now available to the public.

Eddie 

All original content copyright, © 2011-2022.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author. 

Swiss Micros DM41X: Applications

Swiss Micros DM41X: Applications

Swiss Micros DM41X Program:  Euclid Algorithm

Registers:

R01:  A

R02: B

R03:  C  (used)

Finds the GCD of A and B, where A and B are positive integers and A > B.

01 LBL^T EUCLID

02 ^T GCD A>B

03 AVIEW

04 PSE

05 ^T A?

06 PROMPT

07 STO 01

08 ^T B?

09 PROMPT

10 STO 02

11 LBL 00

12 RCL 01

13 ENTER

14 ENTER

15 RCL 02

16 /

17 LASTX

18 X<>Y

19 INT

20 *

21 - 

22 STO 03

23 X=0?

24 GTO 01

25 RCL 02

26 STO 01

27 X<>Y

28 STO 02

29 GTO 00

30 LBL 01

31 ^T GCD=

32 ARCL 02

33 AVIEW

34 END

Examples:

A = 100, B = 20, GCD = 20

A = 78, B =24, GCD = 6

Swiss Micros DM41X Program: Fan Laws 

The program will calculate RPM_new (Revolutions per Minute), SP_new (Static Pressure), and BHP_new (Brake Horsepower).  

Inputs:

CFM_old:  Cubic Feet of Minute - old

CFM_new:  Cubic Feet of Minute - new

RPM_old:  Revolutions per Minute - old

SP_old:  Static Pressure - old

BHP_old:  Brake Horsepower - old

Outputs:

RPM_new

SP_new

BHP_new

01 LBL^T FANLAWS

02 CLA

03 ^T CFM.OLD?

04 PROMPT

05 STO 01

06 ^T CFM.NEW?

07 PROMPT

08 STO 02

09 X<>Y

10 /

11 STO 04

12 STO 06

13 ST* 06

14 STO 08

15 ST* 08

16 ST* 08

17 ^T RPM.OLD?

18 PROMPT

19 STO 03

20 ST* 04

21 ^T SP.OLD

22 PROMPT

23 STO 05

24 ST* 06

25 ^T BHP.OLD?

26 PROMPT

27 STO 07

28 ST* 08

29 ^T RPM.NEW=

30 ARCL 04

31 AVIEW

32 STOP

33 ^T SP.NEW=

34 ARCL 06

35 AVIEW

36 STOP

37 ^T BHP.NEW=

38 ARCL 08

39 AVIEW

40 END

Variables:

R01 = CFM.OLD

R02 = CFM.NEW

R03 = RPM.OLD

R04 = RPM.NEW

R05 = SP.OLD

R06 = SP.NEW

R07 = BHP.OLD

R08 = BHP.NEW

The program uses a lot of storage arithmetic.  

Example

Inputs:

CFM.OLD:  1250 CFM

CFM.NEW: 1600 CFM

RPM.OLD:  840 RPM

SP.OLD: 4 in

BHP.OLD: 7 BHP

Results:

RPM.NEW: 1075.2 RPM

SP.NEW: 6.5536 in

BHP.OLD:  14.680064 BHP

Source:

Calculated Industries "Sheet Metal/HVAC Pro Calc User's Guide" 2021

Swiss Micros DM41X Program: Johnson-Nyquist Noise Analysis

Equations Used:

Power (in Watts):

P = kb * T * Δf

RMS Voltage (in Volts):

v_n = √(4 * R * kb * T * Δf) = √(4 * R * P)

Current (in Amps):

i_n = √((4 * T * kb * Δf / R) = v_n / R

Inputs:

T = temperature in Kelvin  (°C + 273.15)

Δf = bandwidth, difference of frequencies in Hz

R = resistance in ohms (Ω)

Constants:  Boltzmann's Constant

kb ≈ 1.380649 * 10^-23 J/K

Program:

01 LBL^T NOISE

02 ^T TEMP? <K>

03 PROMPT

04 ^T BANDWIDTH?

05 PROMPT

06 *

07 1.308649E-23

08 *

09 ^T POW=

10 ARCL X

11 AVIEW

12 STOP

13 ^T R?

14 PROMPT

15 *

16 LASTX

17 X<>Y

18 4

19 *

20 SQRT

21 ^T V=

22 ARCL X

23 AVIEW

24 STOP

25 X<>Y

26 /

27 ^T I=

28 ARCL X

29 AVIEW

30 END

Example:

Temperature:  299.68 K

Bandwidth:  10,500 Hz

Resistance:  1375 Ω

Results:

Power:  4.3444E-17 W

Volts:  4.8882E-7 V

Current:  3.5550E-10 A

"Johnson-Nyquist Noise" Wikipedia.  Retrieved February 15, 2015 https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

Eddie

All original content copyright, © 2011-2021.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.