From: josh@Happy-Man.com (Joshua_Putnam)
Subject: Re: Horsepower is a meaningless advertising term.
Reply-To: Joshua_Putnam@happy-man.com
Organization: Happy Man Corp., Vashon Island, WA 98070-7399
Distribution: usa
Date: Tue, 21 Jul 1992 20:54:50 GMT

In <1992Jul21.112458.4292@bb1t.monsanto.com> rghoff@bb1t.monsanto.com writes:

>In article <1992Jul21.100528.7286@ucsu.Colorado.EDU>, weverka@spot.Colorado.EDU (Robert T. Weverka) writes:
>> In <1992Jul20.182720.26606@bnr.ca> Peon w/o Email priv (Eric Youngblood) writes:
>>>In <1992Jul20.153457.20046@cbfsb.cb.att.com>, rjc@cbnewsf.cb.att.com writes:
>>>|> 
>>>|> I'm looking at 5hp, 80 Gal tank, cast iron, compressor.
>>>
>>>I will relate my experience purchasing a 3.5 HP 25
>>>gal unit. Paid $299 at Sears. After close inspection this *is* a high quality
>>>piece of equipment. It has a 120/240 15/17.5 amp GE electric direct drive motor.
>> Horsepower is a meaningless advertising term.
>> 
>> A normal house circuit produces no more than 15 Amps of current
>> at 120 volts.  This is 2.4 horsepower at 100% efficiency.
>> To get more horsepower you need to run special wiring.

>Ok, so what's the best number to use for comparisons?

I would look at two types of ratings:

(1) Power consumption of the motor.  If a 120v motor is listed as
drawing 15 amps, it is probably ROUGHLY twice as powerful as a similar
motor drawing 7.5 amps at 120v.  (I say roughly because motor
efficiency does vary.)  Lots of consumer compressors these days claim
to be 3.3 hp while using a standard 15A, 120v outlet.  Other
compressors claim to be 2 hp while drawing the same current.  The one
that says 2 hp may be using a less efficient motor, but more likely it
just comes from a more honest manufacturer.

(2) If you're looking at a complete compressor instead of just the
motor for a DIY setup, the manufacturer should have rated the
compressor's output of compressed air at various pressures, e.g.
10 scfm @ 90 psi; 15 scfm @ 40 psi.  If the compressor you are looking
at does not have such ratings, I'd suspect the manufacturer of hiding
something.  These ratings are usually less overstated than horsepower
ratings, but they do assume ideal conditions in most cases.

Output ratings are also useful for deciding what size of compressor
you need, since many (most?) air tools have air consumption ratings
using the same terminology.  If your compressor delivers 5 scfm @ 90
psi but your grinder needs 7 scfm, you'll need to stop once in a while
and wait for the tank pressure to build up.  A larger tank lets you go
longer before waiting, but also takes longer to refill.

If you have a tool that gets intermittent use (a wrench, nailer, etc.)
you won't need to supply its maximum air consumption continuously, so
a tank is an acceptable way to increase capacity.  If you have a tool
that will be used for long stretches (sander, grinder, sandblaster,
paint) you'll need an awfully big tank to compensate for an undersized
compressor.

-- 
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