×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Reading a physically moving temperature
2

Reading a physically moving temperature

Reading a physically moving temperature

(OP)
I have a bean roasting application where there's a perforated rotating horizontal stainless steel drum in an oven.

Max temp is 375°F (200C)

We're trying to get the temperature of the roasting contents as it tumbles in the drum.

Easiest would be a temp transmitter inside the drum but there aren't any batteries that will put up with 200C.

Any ideas?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

Temperature sensitive RFID type device?
Is there any way you can get a shot at the contents with an IR gun?
A thermo-couple inside the drum brought out to contacts outside the drum. Wipers make contact one or more times a revolution?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

How is the drum or oven heated?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

What about infrared pyrometry? They can even be outside of the oven looking in.

Alternately, an imaging pyrometer might more easily show you the non-uniformity in temperature distribution. Relatively inexpensive compared to the old days.
https://www.amazon.com/dp/B072J49BX7/ref=emc_b_5_i

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Reading a physically moving temperature

(OP)
Bill;

Quote:

Temperature sensitive RFID type device?
RFID temp sensor! Hadn't thought of that at all. Hunted them down.. 150C max. Dang!

Quote:

Is there any way you can get a shot at the contents with an IR gun?
Racking my brain trying to figure out a way to do that.

Quote:

A thermo-couple inside the drum brought out to contacts outside the drum. Wipers make contact one or more times a revolution?
Can T/Cs work thru slip rings??!

Quote:

How is the drum or oven heated?
3phase CALRODs 230V 33A

IR;

Quote:

What about infrared pyrometry? They can even be outside of the oven looking in.
That's what I've been focusing on but the problem is that even if I managed to read inside from outside it's still once removed from the INSIDE of the rotating drum.

Quote:

Alternately, an imaging pyrometer
All I can think of is using a pixelated thermal imager that looks at the drum taking snapshots. The beans would be colder than the oven until they're not. Looking pixel to pixel I could find the lowest temp pixel which would be the still cool beans. Once the oven temp shifts to cooler then the beans would become the hotter pixels as seen thru the drum holes.

That would be a lot've work though.










Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

For slip rings look here:

Link

I'd be inclined to the RTD version, because the cold junction issue disappears.

RE: Reading a physically moving temperature

What rpm is the drum, and how long is a typical roasting cycle? Is this a permanent instrumentation of the drum or a one-off temporary setup for testing/R+D?

If not permanent: I have taken dynamic (0-5000 Hz) strain gage data from a part on a lathe rotating at up to 2000 rpm, with a strain gaged part in the chuck and the gage wires running out through the headstock. We just gave ourselves about 15 feet of slack wire and us two engineers holding the wire suspended (and letting it twist up) while the machinist made the interrupted cuts on the part, approx. 2.5 to 3 minutes of run time. Between tests, we would have him reverse the lathe to un-twist the wire.

If permanent: A high-ohm RTD probe might give a stronger signal (less noise from the slip rings) than a thermocouple, and be more stable over the long term. You should be able to get platinum RTD's in the 1k-ohm range fairly cheaply, and in nice stainless steel sheaths that can be clamped or tack welded onto the drum walls. Better, but possibly bulkier, run the wires for the RTD's up and into a hollow shaft for the drum, then down the shaft and outside the oven to a place that is cool or can be cooled, then connect where you have RTD transmitters (attached to and rotating with the shaft) that shift the signal to 4-20ma prior to passing through slip rings to your recording instrument.

Lastly, a bit more experimental...but...we used to take temperature readings of an operating rocket thruster nozzle in a vacuum chamber using an optical pyrometer. Because of the layout of equipment, and the fact that you really don't want direct or even <90 degreee impingement of the hot gasses on a (even quartz) window, we had to use a mirror to reflect the image of the nozzle back to the pyrometer, roughly 150 degrees of optical path bending. We did some calibration work to see what difference the mirror made - it was minimal for our purposes and easily calibrated out. So...put a mirror canted at 45 degrees along the axis of the drum, and "look" at the drum walls with the ir gun or pyrometer looking down the axis. Calibrate the gun by sticking a test block (suitably heated and instrumented) in the drum whilst static. In your case, you will want to be able to access the mirror to clean it regularly (don't know, but suspect roasting coffee beans give off a lot of smut and oils that might occlude the mirror over time), and possibly to recalibrate after a roasting cycle and before cleaning so you could get an idea of how much the signal drifted during the roasting process.

R+D baby! Have fun.

RE: Reading a physically moving temperature

How long does the roasting cycle last? I have sent TC loggers through furnaces at 1500F.
Sealed boxes with lots of good insulation.
Maybe clip one to a fin inside during a run?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Reading a physically moving temperature

Time for either the smallest wax motor or finding a material with a curie temp in this range. Perhaps a tiny bi-metal device the size of a bean. The wax motor and bi-metal device could be linked to a variable air-transformer that would change the response frequency to an mm-wave RF pulse. Not sure there is a coffee bean size off the shelf transponder.

It seems like the difficult problem is to measure the temp of the beans and not the air or the rotating drum.

Perhaps sampling? Drop a bean like a bingo/lottery ball and take it's temp shortly after it leaves. The drum could have a pocket added that only opened at a certain rotation position.

RE: Reading a physically moving temperature

If this is a more permanent installation, I would think a probe placed somewhere along the shaft would be useful... doesn't need to rotate with the drum, but it would definitely be on the inside of the oven.

If you want the temp of the actual contents (and not just overall oven itself), a well-aimed pyrometer is probably the only way to go.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Reading a physically moving temperature

If you just want the max. temperature, take a bunch of beans and coat them with temperature-sensitive paints of various ranges - see:

https://markal.com/products/tempilaq-advanced-2

It looks like they have temperature indicators at 25 deg. F increments around 375 F, implying a precision of about 10~15 F.

Of course, the paint may wear/rub off of the surface of the bean in tumbling. May have to make a set of R+D beans, with recessed pockets to hold the paint...this could require tooling jigs for machining paint pockets in coffee beans, and some 3-axis milling machine CAM coding... Or, maybe make bean simulators from wood or ceramic, so they are more durable and could be re-used.

RE: Reading a physically moving temperature

One last thing, I promise...

Does the drum shaft have to go completely through the drum? Could the end bearings not be cantilevered? If they could, there is an easy path through the non-driven end by making the shaft hollow, and passing a fixed (non rotating) probe through that end. It may get beaten up by rotating into/out of the beans as they tumble...but maybe if the probe is built stoutly enough it could hold up.

RE: Reading a physically moving temperature

Does the center shaft spin?

RE: Reading a physically moving temperature

My thought was to investigate RFID technologies and then roll your own with high temperature components.
You may mount your circuit on the outside of the drum with only the probe on the inside.
I am guessing that the beans are a tumbling layer in the bottom portion of the drum.
You could power the device with an RF field and take trend readings as the device was in the lower part of the cycle and immersed in the beans.

For the slip ring idea, I wasn't thinking slip rings.
I visualized metal contacts about an inch or two long, mounted on the exterior of the drum.
Stationary would be small wheels that would make about 240 degrees of rotation as they were contacted by and turned by the contacts on the drum.
Once clear of the drum contacts a spring would return them to a stop. They could be jumper connected with a flexible jumper.
The axis wheels could be inclined slightly to the axis of the drum to give a slight scrubbing action to keep the surfaces clean.

Is this a batch operation or a continuous operation?
Is the drum supported by a bearing at each end?
Is it possible to modify the drum in any way?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

(OP)
danw2; Nice. We'll have to consider this. May be too thick but it might work.
Totally with you on the RTD angle.

btrue; RPM is what you'd see in a meat rotisserie. Maybe 2RPM?
Roasting cycle is about an hour and change.

We want to rest control onto the beans instead of dead reckoned outside the drum air temp and fixed time.

Quote:

into a hollow shaft for the drum, then down the shaft and outside the oven to a place that is cool or can be cooled
I think this is where we'll have to go. Currently one end of the shaft is gear driven and very technical to get through, like center drilling the 90 degree gearbox shaft. But the non-driven side, while solid, isn't doing much of anything. I think that can be re-tooled to make the center hollow allowing a completely non-rotating solution to present itself.

Yeah an IR mirror! Perhaps a truncated-cone so only the lower edge would be viewed by the thermal imager which would only take the bottom-of-the-drum temperature where the beans will always be..


EdStainless; Yes! We used to do the same thing with rocket fuel mixers and sterilization chambers. Insulate and thermal mass the logger and toss it in. Unfortunately this is a production system that runs about 3 batches over 4 hours and will eventually be longer runs in 7 ovens so I see keeping track of probably 30 thermal loggers with batteries charged and in order as unworkable. They'd also need to get the data out as we need the data realtime to control the roasts.

3DDave; LOL. Crazy waxmotorbean! In about an hour it would probably look like a bean and one would eventually go down process.
I was trying to come up with a bean sampler too! Nasty mechanics.

MacGyverS2000; Permanent installation, yes. It would need to NOT rotate so the bean pile at the bottom of the drum is what's being monitored. They sort of slither up the side 90° then fold-over and slide to the bottom again.

btrue; Not going to work because it turns out the beans come in all sizes and fore instance smaller beans take longer to heat than larger ones because of the resulting interstitial air spaces (the heating medium -air) can't get between them to heat them up. Also a different weight batch shifts everything. Too many variables to do any characterization.

Yay the hollow shaft angle. That's where this looks like it's going. You saying "cantilevered" made me think of actually sitting one end of the drum on two bearings so there is nothing going on with the axle at all. It could be a 4 inch hole! The beans can't get up to the axis ever... hmmm.

Tug; Gadzooks!

Bill; Batch. Any electronics including the ones to make an RF powered simple device would probably have thermal diffusion lifetime problems. Also to drive the RF would require dealing with acres of SS sheetmetal wrappings. Ugh.

Bearings on the ends which had been the chicken rotisserie support system providing three chicken shafts. The drums are custom designed and made so they could be changed. I'm hoping only one exists at this time.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

Too bad the shaft is rotating. If they have the room maybe they can convert is to a stationary shaft. It might be cheaper than a thermocouple slip ring or pyrometer.

3DDave's idea might still work with a rotating shaft. Put a hole in it and a stationary auger. Let out a dozen or so beans into an insulated container and quickly measure the temperature. I know, a little too Rube Goldbergsmile.

RE: Reading a physically moving temperature

The mechanics aren't too tough. Put a small hole in the side of the container with a little door/catch on it - look at marble traps on marble machine/sculptures. Release the bean with a small lever that is moved to press it open when it's even with the axle so the other beans don't spill out. As the drum turns an unselected bean will fall out back into the moving pile anyway and a different bean will drop in on the next cycle - that way the bean is not permanently trapped.

Plenty of marble pick-up mechanisms to choose from.

RE: Reading a physically moving temperature

Forget my other suggestions. I am tending towards the hollow shaft solution.
How about replacing the NDE shaft with a 4" stainless pipe centered and supported by three cam followers?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

If the roaster is well designed, with good air flow though the beans, there is little benefit to reading the actual bean temperature. Reading the air temperature after it has passed through the tumbling beans will be just as useful. When the heaters are at full power there will be a large delta T between the inlet and outlet temperature of the air. As the beans heat up to oven temperature, the heater power will reduce to a low level and the beans will approach air temperature.

It's like boiling eggs, is there any benefit to knowing the egg temperature? There is a large difference between the center and surface. The same with beans. What is needed is a consistent and repeatable process.

RE: Reading a physically moving temperature

As Compositpro said

I come from the plastics industry where drying plastics is critical to end part performance.

All the (modern vacuum) dryers I have ever used always monitored the outlet temperature of the bulk material. Most had the drying time started when the outlet temperature reached the inputted value.


Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Reading a physically moving temperature

The question for sensing the air temp is whether the roasting process is converting a large amount of the energy into some transformation process. Certainly forcing water to evaporate is one of those conversions. It helps if that transformation is below the temp of any other transformation - the melting point of plastic, for example.

Are the beans being dehydrated or is there some process akin to caramelizing that doesn't see such a large shift or may even be an accelerant? For example, if the beans were to darken then they may absorb IR more readily, causing a runaway where they absorb more until they are charcoal.

RE: Reading a physically moving temperature

(OP)
Compositepro; This is a converted chicken roaster (many of them). So NO not "well designed". Also NOT a continuous process. Batches of all sizes, moistures, initial temperatures, and mass. The air doesn't pass anywhere although a little is sucked in around the non-drive bearing shaft and blown out the other side around the drive side. Unlike boiling eggs the heating medium is no where near as thermally dense.

Pud; This thing is more like cooking the beans not drying them.

3DDave; Nothing so odd as an endothermic type result.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

(OP)
Bill; I like the cam follower. Unfortunately all I find would cause bad geometry issues because they tend to be thick and with long studs.



Cam followers would force the support shaft to be way too long blocking the view to the bottom of the roasting drum.

Looking for a Flange Mount Bearing is causing the same dang problem because any that have a 2" ID, helping the non-contact T/C get the angle to see the bottom, expect to support thousands of pounds and so have extremely wide bearings.

Anyone have a thin idea?




Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

Would the cam followers not be used on the rim of the drum and be outside the perimeter?

RE: Reading a physically moving temperature

It makes one wonder why no one else is roasting coffee beans in a chicken roaster. All you need is the right thermometer.upsidedown

Adding a blower in the oven would be a far better improvement than any other.

RE: Reading a physically moving temperature

It's for chicken finger coffee. New market. Unlimited growth potential.

RE: Reading a physically moving temperature

(OP)

Quote:

Would the cam followers not be used on the rim of the drum and be outside the perimeter?

Hadn't considered that. See the picture of the rim above. Think it would work?


Comp; Actually many use these ovens converted just like this for roasting. The guy I'm working with has used this thing for more than a year. Regular roasters are 10x more expensive and operate like a flying a plane, requiring a skilled operator. This thing works okay being run by any employee after seeing it run and 5 minutes of instruction.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

My thought was that the center shaft be removed. The drive end bearing and the shaft support may need to be modified.
The Non-drive end shaft would be replaced with a large diameter tube supported by cam followers.
Or
The entire drum could be supported at the non-drive end by cam followers similar to a clothes drier drum or a front loading washer drum.
Size and weight is no object. Think large transit mix concrete truck.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

(OP)
Bill; Yeah, I got all that. The problem is that high temp cam followers are a hundreds apiece and we'd need three per roaster. Next, they all come on absurdly long studs without any threads for half their length. This either has them crowding the drum end or if we go with normal temp and mount them on the outside wall pointing away from the heated space they'd require a reeeeeally long axle tube. Night as well go with a flange bearing at that point.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

(OP)
How about this..

Given the drum is like this.



Cut a single hole in the side of the oven.
Mount a non-contact T/C in the hole looking at the side of the drum just above the end spider outer ring.
Mount it so it sees the three spider bars as they pass in front of it but otherwise see's the beans thru the perforations just above the ring.



This sensor has a 350ms response time. I should be able to see the oven temperature looking at the spider arms VERSES the bean temperature when the spider arms are elsewhere.

The difference between the two will show the difference between the beans and the oven itself.


Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

I like that idea of comparing the spider arm temp to the perforated steel/product temp.

I don't follow the IR market so I don't know if that Raytek temp sensor is a wideband, linear 4-20mA model or whether Raytek has adopted a line of the cheaper units that are a thermocouple thermopile with a bowed response over some relatively specific, narrow temperature range and which expect a T/C analog input with CJ compensation (like an Exergen non-contact sensor).

When screen is passing by, the IR will 'see' a signal that is the combination of the relatively low emissivity heat emitted from the perforated steel and the relatively high amount of emitted from the product and when the spider arm passes the signal will be the low level emitted heat of the steel, biasing the reading toward the organic, but on the low side because of the steel contribution.

The linear IR's always have an emissivity adjustment and my memory from years ago recalls that 'back when' the Exergen thermopiles expect the AI to do an offset for emissivity. Assume you use adjust to a higher emissivity to read the organics.

If the exterior steel is all clean and shiny, it wouldn't surprise me if the spider arm temperature was perceived/indicated as a lower temperature than the organic product because organics typically have high emissivity, so they emit more heat than the steel and the sensor emissivity is set high. If any of the steel is dirty, then high emissivity dirt affects the reading. Spray painting the spider arm flat black (automotive exhaust spray paint) will make the spider arm steel high emissivity and get you close to the actual steel drum temperature. I think you'd want shiny perforated steel to reduce the contribution of low emissivity perforated steel heat and maximize the contribution of organic product heat to the sensor.

Field calibrate the temperature reading by stopping the drum, inserting an RTD hand probe and taking an reading to determine your 'real' temperature and either tweaking the emissivity adjustment (don't let the operators see you do it and glyptol the adjustment screw when you're done) or add the difference between reported and real as an offset in the temp indicator.

RE: Reading a physically moving temperature

And I just remembered another condition.
Working on freezer conveyors we had to use edible lubricants. I am not sure how that would work at high temperatures.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

Well, if the IR sensor can read "through" the mesh, then why not put it on the side or perhaps at 45 degrees below the axis, and aiming at the cylindrical wall? For that matter, you could have an array of them aiming at different locations along the length of the drum, and at various elevations above bottom-dead-center. Wondering if there is a good IR-transparent material you could embed at various points along the mesh...how toxic is gallium arsenide...hmm...

RE: Reading a physically moving temperature

Are there any RFID devices that can condition or interface with a temperature measuring device? Use the RFID as a slip ring assembly. So temperature probe attached to rotating shaft, wires out center bore in shaft to rotating RFID on outside of heated basket.

RE: Reading a physically moving temperature

Old school low tech- dial/probe thermometer?
https://www.grainger.com/product/460V91?ef_id=EAIa...

EDITED- Did not realize it was 200C so beyond the range of meat thermometers, added a link above

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RE: Reading a physically moving temperature

(OP)
danw2; I just grabbed an example non-contact T/C for illustration. Raytek has a dozen different models and there are about 6 different companies that make them. You can get every kind of output you can think of so I suspect I can find one that will be sufficient. I like this one as I can use a meter and a simple hole in sheetmetal and try it to see what happens. A completely different job than having to come up with a different bearing system for the drum. If the $200 probe doesn't work a screw and a little square of sheetmetal repairs the test modification.

Paint in service like this is verboten. It'll have to work without or a piece of black anodized aluminum will have to be used instead.

I'll float this by the guy and see if he want to "do the study".

Thanks for the emissivity thoughts. I hadn't considered that looking at the spider could actually give a lower reading but it could!

The new drum he's working on has MUCH larger hole sizes which should probably help this whole scheme.

Bill; Anything in the food path needs "food consideration" for shore. Paint is generally BAD an screws and nuts that can land in the food are also very bad.

btrueblood; I'd like to use several but they ain't cheap and in this case it's all X8 so I have to be somewhat conservative. There are also simple physical constraints. For instance it would a problem to try to have the sensor point up from the bottom. The sensor would require raising the entire oven the sensor length and most of these ovens are already double stacked. Ugh. Also NCT/Cs have like a 4:1 viewing ratio. For every inch away the width of the view greatly increases so it's seeing a larger and larger region.

Brian; As was mentioned above RFID craps out at a max of 150°C, too low.


analogkid2digitalman; Damn! That's outta the box. If the entire thing can sustain 200C then with it screwed into the drum one could simply photo the dial and do video comparisons to read the dial. Interesting. LOL .

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

I like AK2DM's suggestion.
Read the probe optically just before if exits the beans as the drum rotates.
Use a "sample and hold" circuit to update the bean temperature signal once per drum revolution.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

You need to watch for the case temperature limit for the bimetallic thermometer (old school low tech). Wika limit is 200 DegF:



and it's likely that other manufacturers' specs are around that same number.

But could you put the probe through a flat sheet of aluminum or SS shield to prevent radiant heat from getting to the case/dial assembly in order to keep the case temperature down? Conductive heat through the probe would be minimal.

The bigger issue is the fact that the probe doesn't stay in the organic product, no matter where you install it, the probe rotates in and out of the product and spends a good portion of the rotation in the air, not the product. If the IR concept is feasible, the IR could be aimed at the lower section where gravity is going to keep a pile of the organic product.

RE: Reading a physically moving temperature

I remember trouble shooting an instrument that measured the height of molten glass in a glass furnace.
A probe was lowered mechanically until the tip of the probe touched the surface of the molten glass.
That completed a circuit that stopped the mechanism, read the position of the mechanism, froze the reading and sent it to a PID controller on the feed system and then returned to the top of the stroke.
The cycle repeated continuously. If the glass level was rising or falling, the signal would be updated and the PID would react accordingly.
Will the temperature of the beans change enough from one revolution to the next to introduce significant errors?
If so, use three or four dial thermometers.
(Molten glass is a conductor of electricity and a circuit was completed through the mass of the molten glass.
Another industry, another glass furnace. This one was heated by passing an electric current through the molten glass.
At startup, the glass was first melted with gas heaters and then the electric resistance heat took over.)

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Reading a physically moving temperature

(OP)

Quote:

The bigger issue is the fact that the probe doesn't stay in the organic product

I failed to consider that. It would (if it didn't have a 100°C case limit) be taking only a mechanical average of the beans/air temperature and I dare say it would be about 85% air temp verses 15% bean time.

Oh well.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Reading a physically moving temperature

Quote (itsmoked)

Brian; As was mentioned above RFID craps out at a max of 150°C, too low.

The shaft would be extended out of the heated area so the RFID was in a cooler area. Maybe that's not possible for the hardware they currently have.

RE: Reading a physically moving temperature

Quote:

85% air temp verses 15% bean time.
If the thermal time constant of the dial thermometer is less that 1/5 of the transit time in the beans the reading will reach the bean temperature while the probe is in the beans. Take a reading just before the probe comes out of the beans.
Does your wife have a meat thermometer with a narrow probe? Try a simple test;
Ambient at 21 degree C
100C - 21C = 79C
63% x 79C = 50C
100C - 50C = 50C
Take the thermometer at room temperature and immerse it in boiling water. The time to reach an indication of 50C will be one time constant. You need five times that to reach 99+% of the measured value.
This should let you know if you are in the ballpark for time constants with a dial type thermometer.

This is based on:

Or more simply:
1/e
Please don't think of me as irrational. grin

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close