Two Why: Units in Mathcad and Excel

[ValeryOchkov]

PS

Is there a short name ofthe base US pressure units?

[FredLusk]

Valery…

R (Rankine) is in Mathcad 15.

The pressure unit I use most often is psi (pounds-force per square inch). I am a civil engineer and psi is the unit I use in water system modeling and design and for other hydraulic applications. When applicable, it is sometimes expressed as psia (psi absolute) and psig (psi gauge). Zero on the psig scale is one atmosphere.

Here are some common pressure units used in the US which are also built into Mathcad Prime 3.0. I didn't check Mathcad 15.

atm = atmosphere. 1 atm = 14.7 psi (±). Thus x psig + 1 atm = (x + 14.7) psia.

in_Hg = inches of mercury. This unit is commonly used in mechanical engineering applications. I have run across this unit when designing natural gas distribution systems, but I always convert to psi.

ksf = kips per square foot. 1 kip = 1000 lbf (kip stands for kilo-pounds-force). This unit is commonly used in geotechnical engineering and foundation engineering.

ksi = kips per square inch. This unit is commonly used in structural engeering for material stresses.

psf = pounds-force per square foot. This unit is commonly used in geotechnical engineering and foundation engineering.

psi = pounds-force per square inch. 1 psi = 6.9 kPa (±)

One common unit missing from Mathcad Prime 3.0 is in_water. It is similar to in_Hg, but is based on the density of water instead of mercury. This unit is also used by mechanical engineers. Two other less common units are ton per square inch and ton per square foot where 1 ton = 2000 lbf. I have never used these last two.

I hope this helps.

Fred

[ValeryOchkov]

Thanks, Fred!

But the questions were:

Why we have not R with temperature when we have US Units system...

[Fred_Kohlhepp]

Because PTC/Mathcad is still hoping that the USA will convert to metric?

[RichardJ]

Because there really is no "US units system". Unit usage in the US is a mess. I have never heard of anyone using Rankine for anything. Many scientists and engineers have never even heard of it.

[ValeryOchkov]

Richard Jackson wrote:

I have never heard of anyone using Rankine for anything.

This reference book >>> has two parts:

- SI with Kelvin

- US with Rankine

And second.

1Btu/lb/R=1 kcal/kg/K

[RichardJ]

What reference book?

[ValeryOchkov]

Richard Jackson wrote:

What reference book?

[RichardJ]

Maybe there are some engineers in the US that actually use Rankine, but they must be few and far between. Assorted "weird" US engineering units (for example kips) appear all the time on these forums, but I don't recall ever seeing a worksheet in which someone actually used Rankine.

[FredLusk]

Richard…

When I design natural gas distribution systems (typically at pressures below 30 psig), I use the Isothermal-Compressible Flow Equation (ICFEq), which includes a term for absolute temperature and which means Rankine for me.

.

I have played around with several of the published empirical and implicit equations (e.g. Mueller, Pacific Gas & Electric's equation, etc.) and was not satisfied with the their accuracy over the entire range of solutions I need. Thus, I decided to use the physically correct and more universally applicable ICFEq.

[RichardJ]

OK. So now I know one engineer that uses Rankine

[ValeryOchkov]

Richard Jackson wrote:

but I don't recall ever seeing a worksheet in which someone actually used Rankine.

If you see Btu/hr/ft/F as an unit if thrmal conductivity you see R not F.

[FredLusk]

Vallery…

R is in Mathcad, both 15 and Prime.

Fred

[RichardJ]

I tihnk Valery's complaint is that the default unit when an expression is evaluated is K, not R, even when US units are selected as the default.

[ValeryOchkov]

Richard Jackson wrote:

I tihnk Valery's complaint is that the default unit when an expression is evaluated is K, not R, even when US units are selected as the default.

Thanks, Richard!

It was my question

[ValeryOchkov]

Valery Ochkov wrote:

Richard Jackson wrote:

I tihnk Valery's complaint is that the default unit when an expression is evaluated is K, not R, even when US units are selected as the default.

Thanks, Richard!

It was my question

Sorry, R (gas constant) is not OK - must be R (Rankine) not K!

[FredLusk]

Now I understand his question.

[ValeryOchkov]

Fred Lusk wrote:

Now I understand his question.

Thanks!

Now I understand my question too!

What is the base unit of temperature in US unit system - Renkine or Kelvine?

In NIST - Rankine (see the reference book above), in PTC - Kelvine.

[FredLusk · Accepted solution]

Valery…

The base temperature unit in the US system *is* Rankine. Mathcad gets it wrong.

However, I and Mathcad are both unit-bilingual, so this Mathcad error is mearly an annoyance. BTW, I was just looking at the definition of the ICAO Standard Atmosphere (for something totally unrelated to this conversation and even my own work) and found that the standard lapse rate used to calculate temperatues in the lower atmosphere is officially defined as "-1.98°C per 1000 feet." Yep, mixed unit systems…at least according to the three semingly knowledgeable websites I checked.

In the US, the federal government requires the use of SI units for its projects…at least, they're supposed to. Most, if not all, state and local agencies still use the US unit system (the California Department of Transporation tried SI for a while, but gave up).

As a civil engineer, I have designed and/or managed the design of site and infrastructure improvements for three federal prisons and three other federal projects. In all six cases, the use of SI units was a problem for some members of the design team and for most members of the construction and construction management teams. I once even had a drafter ask me how to draw a 3:1 slope in metric (that's 3 horizontal to 1 vertical). I did it for him…fortunately he was fired soon after.

The worst situation was a couple of buildings at a local naval air station. I won't go into all the details, but at the project kick-off meeting, the architect complained about having to use SI units. The Navy's project manager said he and his staff didn't like SI units either so he made a command decision to design the project using US units. I piped up and said that the topographic survey the Navy had provided us was in SI units and I was not going to accept the liability of converting the data to SI (it would have been a fairly simple procedure, but it was not my responsibility). The Navy's PM said he didn't have the money for a new survey (maybe $7,000 out of a budget of $20,000,000 ) and the previous surveyor was no longer under contract. So, in the best tradition of King Solomon, the Navy's PM decreed that the site work would be designed using SI units and the buildings using US units. I told the Navy's PM and the rest of the design team that this was a VERY bad idea, but that my project engineer and I were fully able to make it work. And we did.

Fred

[jroth]

I used to use Rankine in the oil & power industry. The oil and power plant guys (US) like to express things in terms of Fahrenheit, which to do the thermo calculations, must be converted to Rankine. Rankine and Fahrenheit are of the same magnitude in difference; i.e.; del F = del R, but R is the full scale based on absolute zero is zero R. Most of the English or B.G. or Imperial (same systems) express entropy specific heats in Btu/lbm*R. Power industry also likes that here on earth you can express 1lbf = 1lbm, as the gravity is assumed to be g = 32.2 ft/s^2. Makes going from how much mass there is to force very easy. Have a 30lbm plate, well it weights 30lbf. It has stuck around since the 70's from oil industry engineering boom.

As a note, the Imperial system is a complete nightmare for any type of space calculations due to the way the lbf and lbm are defined. The lbm is actually defined as: 1lbm = 1lbf/32.2ft/s^2. I was taught in school a new rather memorable unit called 'slug' which was the equivalent to the kg and defined in the same way as the kg. 1lbf = 1 slug * 1ft/s^2, exactly like the newton.

Either way, Rankine isn't the defualt expression of measurement generally. For the Imperial system it is usually degrees F, but MathCad has problems calculating units from deg F I have found becuase the zero F is offset from the absolute zero by 459.67 degrees.

As a note: I was always told it is called the Imperial system as the units of measurement originated in Britain. But yes, I like the SI system better.

[ValeryOchkov]

Fred Lusk wrote:

Valery…

I was just looking at the definition of the ICAO Standard Atmosphere (for something totally unrelated to this conversation and even my own work) and found that the standard lapse rate used to calculate temperatues in the lower atmosphere is officially defined as "-1.98°C per 1000 feet." Yep, mixed unit systems…at least according to the three semingly knowledgeable websites I checked.

Fred

Not only mixed but... go to bugs - see http://communities.ptc.com/groups/mathcad-paradoxes