digital thermometer for amateur HT

[petertha]

I have not seen any examples where people use these low cost digital thermometers for simple heat treating & I'm wondering why. I'm talking kind of amateur level, back yard, simplistic HT here like a small O1 part. The usual advise is heat until a certain shade of orangey red or until no longer magnetic. Personally I found the magnet thing to be a bit of a high temperature clown juggling act. If the magnet sticks, you are fiddling to remove it. If you bring the hot part up to a fixed magnet, its already cooling outside the firebricks. Wouldn't it be easier to just contact the metal with a probe & read the number? Or is there more to it than that?

http://www.ebay.ca/itm/DM6802B-Doub...86458846&tpos=top&ttype=price&talgo=undefined

 

[kquiggle]

I have one of these - they are a great bargain and come with two temperature probes. I have used mine with a hot plate to measure temperature for shrink fitting small parts, but I have not tried it for use with hardening metal. I think that could work, but the problem would be getting good contact between the probe and the metal for a long enough time to get a reading. One solution might be to form a small enclosure with ceramic wool or firebrick to form a small "oven" and measure the temperature within the enclosure. I also used the digital thermometer to measure the temperature of the toaster oven I use for annealing - I found the temperature dial on the oven to be fairly accurate. My long term plan is to build a coffee-can forge and use the digital thermometer to measure the forge temperature. I'll be interested to hear from anyone else who has one of these about how they have put it to use.

 

[gunna]

I would be very surprised if the supplied probes are capable of surviving 1300 degrees. That figure is just the maximum value that the meter can indicate, think full-scale on an analog meter.
Ian.

 

[kquiggle]

gunna -

You raise an important point (but prepare to be surprised). The online specs for the digital thermometer include a note that the probes should not be used above 250 °C. However, I decided to check this and ran a few quick tests. You can see details and pictures on the testing at the link below, but here's the TLDR:

With a protective metal sleeve I was able to heat the probe to up to 1,800 °F (982 °C) multiple times; it survived with only minor damage (a heat shrink sleeve burned away, but this does not impact function).

Details here:

https://sites.google.com/site/lagad...lathes-mills-etc/review---digital-thermometer

 

[gunna]

Well done, I hadn't thought of the protective sleeve idea, but I guess it may be how the very high temperature probes are made anyway.
Ian.

 

[prof65]

Some ebay vendors are just cheating. They sell you a 1300 C digital thermometer, but the cheap thermocouple that come with it has a very limited temperature range (usually 150 or 200 C). 1300 C is the maximum range of the thermometer, NOT the probe (sometime they forget to mention this...).
For higher temperature the thermometer is fine but you have to buy another probe. An high-temperature thermocouple can cost more than the thermometer itself.

 

[kquiggle]

I found a number of high temp K-type thermocouples on-line for around $6 (but others with prices ranging up to $45). All of the high temp probes I found had stainless steel sleeves. If I ever get around to making a small furnace for myself, I will probably re-do my sleeve idea using stainless steel. Getting back to the original post, it does not seem practical to use this type of digital thermometer to assist with heat treating when doing the basic "hold it in a torch flame" method, because it would be difficult or impossible to hold the workpiece and the probe at the same temperature. However, with a furnace type arrangement in which you could bake the probe and the workpiece together, this might work. I think even the so called "one brick furnaces" might do the trick. Here's one example of a one brick furnace: [ame]https://www.youtube.com/watch?v=qOqgVfgpvH0[/ame]

 

[petertha]

it does not seem practical to use this type of digital thermometer to assist with heat treating when doing the basic "hold it in a torch flame" method, because it would be difficult or impossible to hold the workpiece and the probe at the same temperature.

Thanks for the vid. The way I envisioned it for my own application is: the part is pre-laid out on the firebrick, more or less open mode either in a trough and/or additional vertical bricks & roof for heat containment backstop. My right hand holds the torch & heats the part. My left hand has the temp probe ready mode. So as it gets to red, kind of in one quick fluid motion pull the flame away & touch the probe to metal, then return to heating as required.

The downside is with heat removed the part starts cooling. But my hope would be I could get a reading in a lot less time lag than fumbling around with a magnet & hopefully bit more accurate. I guess it would be a visual feedback thing, another 50-deg to go, once there holding that color of red etc. My understanding of the magnet method is you get it to that temp & then 'a bit more' & 'try & maintain that shade' for a while until quenching. Just seems more variable to me.

A real HT oven is the way to go. And I almost had my wife convinced she could pay half & use for making clay metal jewelry... as long as she scored 99% on a 400 question safety test that I would prepare But a/p another post, HT ovens (aside from cost) resolved the temperature issue but other considerations can come into play: foil bagging to prevent oxygen, method to have parts contacting quench oil etc.

I thought I'd ease into this with simple methods.

 

[gus]

30 years ago I bought a USA Made Digital Thermometer recommended by a foundry supplier for $80. Came with a Digital Meter,cord and a long broomstick probe with the thermocouple house in a steel tube to probe aluminium melt. Was advised to coat probe end with liquid clay before dipping in. Aluminium will leached away steel probe. The additives will also help corrode and eat away bare probe. Casted hundreds of aluminium parts using this meter.
Somewhere in a magazine was an article using ammeter with 0------100 micro meter. Constant Wire connected to meter would give a readout but you would have to experiment with a range of wires. Its quite a wide range from type k----------?? Some calibration required. Device is crude but seems to work very well for my backyard aluminium foundry friend. His dad used eyeball to judge aluminium temperature for pouring.
May make it a project to build a DIY Temperature gage using old fashion ammeter.

 

[petertha]

That's interesting Gus. I looked up aluminum, melting point ~ 1220F. The (austenite?) target temp range for this tool steel is 1450-1500F. Melting point is a long ways off at 2600-2800F so it will be solid where I want to measure. Hoping its just a matter of quick contact to get a reading. I can see liquid metal creating havoc with a probes.

Question for the meter/probe guys: how fast for the reading to occur between ambient & elevated temps in the 1450 F range? Does the display bounce around a lot or does it settle in pretty quick?

 

[gus]

That's interesting Gus. I looked up aluminum, melting point ~ 1220F. The (austenite?) target temp range for this tool steel is 1450-1500F. Melting point is a long ways off at 2600-2800F so it will be solid where I want to measure. Hoping its just a matter of quick contact to get a reading. I can see liquid metal creating havoc with a probes.

Question for the meter/probe guys: how fast for the reading to occur between ambient & elevated temps in the 1450 F range? Does the display bounce around a lot or does it settle in pretty quick?

Hi Petertha,

Only good for checking aluminium melt and pour temperature. Device good for 800 C. Was doing gravity casting of aluminium into CI Molds. 690----700C was best for the Condensate Trap. Below 690C I get cold run.Above 700C I get too much shrinkage. The digital readout was very steady. A 15----20 secs prod dip will give good reading.

 

[kquiggle]

The specs for the digital thermometer I have say it has a reading rate of 2.5 times per second (once every 400 msec), but it does not say what the probe response time is. However, the tip of the probe is quite small (very low mass) so I would guess that it would equilibrate fairly quickly.

Here is a method which might work to do what the original poster suggested:

1. Using the arrangement I used in my original testing (three firebricks set up to form a "corner" in which the part is heated in a flame), flame heat the part while holding it with pliers or other suitable tool for holder incandescent-hot metal.
2. Lay the probe on top of the firebrick (but away from the flame).
3. Flame heat the part.
4. When the part has gotten hot enough (as judged by color),hold the part down on top of the probe and take a reading.

The above method should solve the "I need three hands to do this" problem, and should enable good contact between the part and probe. In addition, the firebrick should provide some insulation to help keep the part hot. Pre-heating the firebrick under the probe might also be helpful.


This is all speculation, but maybe I'll give it a try to see if it actually works. If I do try it, I will add it to my web page on this and post an update here as well. In the meanwhile, you can see my existing web page at the link below - it includes photos of the 3-brick method.



https://sites.google.com/site/lagad...lathes-mills-etc/review---digital-thermometer

 

[Wizard69]

Having a fair bit of experience with thermal systems if like to point out that contact probes need to be in very tight contact tith the item being probed. In many cases the thermocouple is bolted down tight to the item being measured. That and the fact that a hand held probe only reads surface temperature has me questioning if such probes are worth the effort. A magnetic test frankly is just as reliable in my mind.

The other issue with heat treat, is soak time which is especially important with thicker pieces. Simply measuring the surface temperature will not assure you of achieving the desired core temperature.

Now that being said it is a good idea to know what your oven temperature is so I'm not dismissing the value in these meters. What you do need though is a thermocouple designed to operate in the ovens environment and locate it such that it actually reflects oven temperature.

 

[petertha]

Those are all good points Wizard69. The magnet is giving feedback about the entire mass. Maybe what I need to do is just organize my space & procedure better. Ideally the magnet needs to be fixed & in in vey close proximity so if I am going to grip the part & quickly move it for magnet contact test, it doesn't lose much heat. I rigged up a magnet on the end of a rod that I could touch the part. Trouble was, when it attracted the (smallish) part stuck & I was then fumbling around trying to de-stick it, wasted time, cooling part...