Subject: Re: pyrometer & Gingery Furnace

From: [email-masked] (Barry Workman)
Date: Mon, 6 Feb 1995 02:14:17 GMT

[email-masked] (David Adams) writes:

>A couple of posters have mentioned the use of a "pyrometer" 
>for measuring furnace temperature.  What is this?  Where
>can I get one?  Can they be made?  Are they just high
>temp thermocouple devices?

>---
>	-David  [email-masked]

>-- 
>	-David  [email-masked]

A pyrometer measures tempratures at peak -- rather an instant read
type of device.  A potentiometer measures an average temprature.  
Both devices use thermocouples for their source.  Pyrometers are of
greater use in the home shop/furnace/foundry anthough the 
potentiometer is a better device.  We used both for measuring the
EGT (Exhaust Gas Temprature) and CIT (Compressor Inlet Temprature)
on jet engines at test cell.  The Pyrometer was used to identify
any hot spots in the combustion stream while the potentiometer was
used to insure that we were not getting an overall overtemp situation.

I have seen very good (and expensive) pyrometers -- Allan Frisbie has
one such with a probe and everything.
-- 
    - Barry Workman

---

From: [email-masked] (Steve Hampson)
Date: 7 Feb 1995 22:29:53 GMT

> used to insure that we were not getting an overall overtemp situation.

A potentiometer is a variable resistor and has nothing (by itself) to
do with measuring temperature.  A potentiometer, or "pot", is the
device that is connected to the volume knob on your radio.  You are
refering to an optical pyrometer.

My Omega Infrared Temperature Handbook shows many different models of
infrared or optical pyrometers.  While low temperature models are as
little as $200, the models with useful range for us (2500 F) start at
$1800. 

I have a Fluke 51 digital pyrometer and a custom made 3' long type K
probe with about 10' of cable that I use in my gas forge.  The Fluke 51
was about $100. That was a close-out price, I think they sell for about
$150.  The probe was about another $50.  The Fluke will display up to
2500 F while most digital pyrometers will only display up to 1999 F. 
I've been told that the type K probe is only good up to 1800 F, but
this one is still working fine at temperatures up to 2400 F after more
than 5 years of occasional use.

For about $200, Omega sells a two display, programmable digital
controller that can accept type R probes.  The type R is good to more
than 3000 F but can be quite expensive.

--
Steve Hampson                           
Blue Sky Research
534 SW 3rd Ave
Portland, OR 97204
503-222-9571
503-222-1643 fax

---

From: [email-masked] (Barry Workman)
Date: Thu, 9 Feb 1995 20:45:52 GMT

After 4 years of running test cells with J-57, j-33, TF-30, and some
smaller military jet engines on them, I think I know what the names
of our test equipment was.  Both were mil spec.  Both had the instrument
names engraved on the front.  Both connected to the thermocouples that
were in the gas stream.  Do not get your simple-minded electronic 
experiance get in the way, and realize that there may be more than one
device called a potentiometer.
-- 
    - Barry Workman

---

From: [email-masked] (James Kirkpatrick)
Date: 10 Feb 95 08:42:18 MST

[near-flames from Barry omitted]

Barry's right, "potentiometer" does have more than one meaning.  If you
take the word apart, it essentially means "potential meter".  I vaguely
remember that one way to measure a signal is via a Wheatstone bridge,
and that to balance the bridge a variable resistor is used in one leg.
The whole apparatus may have originally been called a potentiometer.
Current "popular" usage however is to use the term to refer to just the
variable resistor, or "pot".

However, using such a term in a correct but unusual sense does cause
confusion (not Barry's fault), as we can see.  Perhaps it's the military's
"inertia" that causes it to continue using, and specifying, equipment
using seemingly archaic terminology.  Or maybe "the industry" really does
use these terms behind the backs of the populace.

My Webster's defines potentiometer as "an instrument for measuring
electromotive forces, or a voltage divider."  Note the common usage is
the *second* definition!  The definition for pyrometer is a bit longer"
"an instrument for measuring temperatures esp. when beyond the range
of mercurial thermometers usu. by the increase of electric resistance
in a metal, by the generation of electric current by a thermocouple,
or by the increase in intensity of light radiated by an incandescent body."
If we use the above, then the distinction between a pyrometer and a
thermocouple is simply that a thermocouple is *one type* of pyrometer,
and that a pyrometer usually measures temperatures higher than a
mercury thermometer.

Let's get back to metalworking!

Jim

---

From: [email-masked] (BrnzCaster)
Date: 10 Feb 1995 17:34:43 -0500

Barry ... us old guys have to fight back sometimes. :)
I've been measuring temperatures for 35 years with a wood-box L&N 
potentiometer and Type-K couple.
To me, an SCR is still a Saturable Core Reactor and I measure temperature
in degrees Centigrade.
(Anyway, the Gingery charcoal furnance is too basic for temperature
measurement. :)

Winston

-------------------------------------
Winston W. Harness
Metal Casting Services
[email-masked] / [email-masked]

---

From: [email-masked] (Steve Hampson)
Date: 11 Feb 1995 04:49:34 GMT

> device called a potentiometer.

Well, pardon me all to hell...