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timgunn

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Everything posted by timgunn

  1. As I understand it, motors tend to be designed for mass manufacture. Most of the bits of an 1800 RPM 145tc motor end up the same as those on the 3600 RPM 145tc motor built on the same production line. Only the stator windings will be different, and these don't rotate. All the moving parts of the 1800 RPM motor will therefore have been designed for 3600 RPM. Electrically, the motor will have been designed for 60 Hz (possibly both 60 Hz and 50 Hz, if aimed at a world market) and optimized for that frequency. Moving away from the design frequency will gradually increase internal losses until the motor becomes too inefficient to be useful. Most motor and VFD manufacturers seem to recommend a speed range of 10 Hz to 100 Hz for a 4-pole motor. My limited personal experience suggests they've got it about right. For a motor rated for 1800 RPM at 60 Hz, 10-100 Hz gives a speed range of 300-3000 RPM. A 3600 RPM motor could probably be run over the same 10-100 Hz frequency range, giving 600-6000 RPM. Motor life would probably be shortened dramatically, as everything above 3600 RPM would exceed the design speed. If using a TEFC motor, you need to be aware that you'll need to derate below about 25 Hz, as the cooling airflow is reduced at low speed and the motor is more prone to overheating. In most cases, you just need to be aware of it; I don't imagine you'll be leaning hard on the platen for long periods at such low speeds, so the derate will probably be taken care of naturally. (If I'm wrong on this, please somebody let me know soon, before I build my grinder) I have run 4-pole motors at 120 Hz, 3600 RPM, but they feel as if the torque is down. I've tried running below 10 Hz on a basic V/F drive (Voltage and Frequency are varied together) and the motor felt "coggy". Using a slightly higher-specced "Sensorless Vector" drive kept the same motor feeling smooth at 3 Hz. SV is worth knowing about if you need very low speed, but I don't think it's needed on a grinder.
  2. I'd be inclined to build from scratch, rather than substantially modify an existing unit, unless it is very clear from the outset that modifying will be cheaper and/or easier. I've built 5 electric HT ovens from scratch, 4 of them for work up to 18" long and one for work up to 42" long. All will work vertically or horizontally. I think the long one would probably work best at about 45 degrees; vertically, it's a long way to have to lift out a full-length workpiece. For your questions: First: Maybe, but it's fun. Second: You can buy elements from many places. As a general rule, the fewer constraints you give the supplier, the cheaper they are. Simple coiled 16 ga Kanthal A1 works well. I get mine from an ebay supplier in Portland, Oregon (I'm in England). No complaints. Third: What type of firebricks? You want the soft Insulating Fire Bricks. 2300 degF grade are ideal, as they are cheapest, softest (for shaping) and least dense. They have the lowest heat capacity and are the best insulators. If you have hard bricks, look for the soft ones. On my first couple of ovens, I filed the element grooves with a length of allthread. From number 3, I wised up and used a router to cut the grooves. Fourth: If you use the controller output to drive a contactor (basically, a big relay) and switch the 220V through the contactor, it should work OK. You'll need advice from someone familiar with US wiring codes though. Fifth: It's better to ask someone who knows you well I've built HT ovens that work and haven't killed or maimed anyone, which suggests it's not difficult. Sixth: If it's at all useful or sellable, I'd try to keep the kiln as it is for as long as possible. Some of my efforts: 18" oven with facility for vertical use with salt pot. 42" oven. usable horizontally or vertically.
  3. I took a punt on a far eastern diamond indenter off ebay a year or so back. Mine shipped from Hong Kong. I tried it in a friends tester and was getting readings 3 RC points below the ones he got with the indenter that came with his tester, on the same test block. As far as the weights go, the weight is the important thing. If you can weigh the weights from another machine of the same model, and make another weight that weighs the same, it'll work. A bag of lead shot would get the job done so long as the weight is right. If you are only going to be testing blades and are not going to be using the HRB scale, only one weight is needed.
  4. I've only seen 2 of the 55-gallon drum HT forges. Both worked very well. The first one was all-manual and had a surprisingly small burner; about 1/2" from memory. The other used two cheap Chinese propane torches and a PID controller; the (very) small torch was the continuous pilot, with the larger torch switched on and off by the controller. I'm pretty sure the gas jet in the big torch was 0.35mm/.014", which should give a fair idea of the heat input needed; I normally fit a 0.6mm (0.024") MIG tip, which actually measures about 0.7mm (.028"), in a 1" burner. Half the gas jet diameter should mean half the burner diameter, so a 1/2" burner would seem about right. I'd expect a 3/4" burner to work OK. It'll probably be running at quite a low pressure.
  5. Yes, look for Sodium Borate Pentahydrate. The active part is the Sodium Tetraborate. Ideally, you'd use anhydrous Sodium Tetraborate, but this is not usually as easy to obtain as the hydrated forms. The hydrated forms are the Pentahydrate, which has 5 water molecules associated with each molecule of Sodium Tetraborate, and the Decahydrate, which has 10 water molecules associated with each molecule of Sodium Tetraborate. Given that what we want is the Borax part, and that the water is undesirable, the Pentahydrate is better for our purposes than the Decahydrate. Both hydrated forms are available at similar prices-per-kilo, so the one with least water in is therefore usually better value. If you need to factor in shipping costs as well, the Pentahydrate will definitely be better value. When it comes into contact with hot steel, the first thing the associated water molecules do is boil off, making your Borax all frothy in the process, and causing a significant proportion of it to dribble off the work. Anhydrous Borax (I can't be bothered typing "Sodium Tetraborate" each time) froths very little, if at all. The Pentahydrate definitely froths and the Decahydrate froths more. The spillage that inevitably results from the hydrated forms frothing may be significant to you, especially if you have a gas forge with a refractory lining made from metal oxides (I'd expect less of a problem with solid fuel than with gas, but this expectation is based entirely on ignorance and supposition; I have not used solid fuel and have no real idea). "Metal Oxides" covers ceramic fibre blanket, most Insulating Fire Bricks, many hard fire bricks and a good few of the refractory cements and castables. Alumina (Aluminium Oxide) and/or Zirconia (Zirconium Oxide) are used in most of the refractory compositions we are likely to encounter. It's worth bearing in mind that the main reason we use Borax, or any other flux, is to dissolve the metal oxides that would otherwise contaminate our weld, and allow them to flow away from the weld. It would seem unrealistic to expect anything to work well as a flux, yet not damage a metal-oxide-refractory-lined forge. I have reduced the water content of the Decahydrate by heating it for a few hours in a domestic oven. Weighing it before and after and doing the sums, it seemed to get it down to the Pentahydrate, but no further. YMMV. Getting it up to melting (about 750 degC), letting it cool and pounding the resultant glassy solid to powder, gives Anhydrous Borax. Once made, it needs keeping in an airtight container to stop it absorbing moisture from the air. It's a lot of hassle, but some people, who's opinions I respect, say it's worth it.
  6. You can buy ready-wound elements. Many pottery equipment suppliers will wind them to your spec, albeit at a price. I got the coils for my ovens from ebay seller jrider12 in Portland, Oregon. He seems to have dropped off the radar lately, but ebay seller pmtoolco, also in Portland, seems to offer similar coils. Prices, shipped USPS flat-rate to the UK, were less than I could buy the Kanthal wire for from anywhere else. The coils I have are wound from 16ga (AWG?) wire. There are a number of far-East ebay sellers offering kiln elements very cheaply now, but these appear to be of very thin wire. The detail is sparse on the ebay listings, but they look like they are cut from a long length of continuous coil. If so, the working life will be shorter due to the smaller diameter and they'll probably need the ends uncoiling to make "tails" for connection. I asked to have my elements wound for 13A (to suit 13A UK fused mains plugs) at 110V and used 2, wired in series, for each of my 18" ovens; one on each side.
  7. There are a few pretty close UK equivalents to the AISI 10xx steels. CS70 is equivalent to 1070, CS80 to 1080 and CS95 to 1095. EN9 is a pretty close match for 1055. As Jake and others have said, a good starter steel if you want to make things that will cut, is BS1407 silver steel. Downsides are that it's prone to grain growth and benefits from careful attention to normalizing. It only tends to come as precision ground rod, so is expensive by weight. On the other hand, it's widely available in a good range of sizes. You can buy the diameter you need and immediately make stuff, rather than spending much of your time reducing your stock (you've intimated this may be a consideration for you with your light hammer). It can be bought in 330mmm, 1M and 3M lengths; If you buy the 1M lengths, you'll get at least one, probably two, goes at making something from each length before you need to use the tongs; I find it hard enough to learn one thing at a time and having enough length to use it as a handle really helps as a beginner. Cromwell are a decent source for BS1407 silver steel. For patternwelding steels, Mick Maxen at your third link really knows his stuff. If you've not looked at the BritishBlades forum, it's worth checking it out. Quite a few of the members are on here too, but BB is obviously UK-biased and many of the local solutions you'll be looking for can be found there. Dave Budd's work might also be worth a search. I gather his background is archaeology and he uses iron-age equipment. He's not large, but manages to do an impressive amount of work with a small hammer.
  8. I just cut the grooves straight in and pinned the elements fairly closely using little U-shaped staples pushed into the bricks. The staples were bent from 1mm Kanthal wire. The outside diameter of the coiled elements was only just under 10mm, so they were a pretty good fit in the grooves. They did feel a bit springy as I was fitting them in the first one, and I wasn't sure the staples would be enough to hold them in use. Everything seemed to stress-relieve on the first firing and I've not worried about it since. The element seems to sit quite happily in the groove. Not the best photo and the oven's upside-down in it; that's the thermocouple on the "floor". There seems to be an Oxide layer on both the element and thermocouple, so it's been up to temperature. This one's an 18" oven. The odd angle is because the oven was standing on end at the time; the plan was to allow it to be used with a salt-pot, should I one day feel the urge. The 42" oven hadn't been fired when this photo was taken
  9. Salem I'm no expert, but I've built 5 electric HT ovens so far and I've found out a few things on the way. The soft bricks tend to suck all of the moisture out of whatever jointing compound you use, so it's quite easy to stick the bricks together with a thick layer of the compound, but it gets more difficult as the joint gets thinner. Because I want a close joint, I've tended to fit them dry. Where I've been concerned about gaps, I've just slathered the outside of the joint with watered-down fire cement (Satanite and AP Green seem to be unavailable here, so I can't comment on them). If you do the inside as well, you need to avoid getting the mix on the elements. Cutting depends a lot on the actual bricks you use. JM23 branded bricks are the easiest and offer the best insulation. They cut with a cheap hardpoint tenon saw, no problem. They are quite chalky in texture. I've also used low-iron GD23LW bricks; bubbly in texture and horrible to cut; I killed a tenon saw in 3 cuts and used a new blade from a mechanical hacksaw for the rest of the oven. By the end, there was no set left on the saw blade. They are quite a bit harder than the JM23s and a little denser, so not such good insulators. I've also found unbranded 23-grade bricks. Denser still and even poorer insulators, the texture is more like a sand/cement mix and ease-of-cutting is somewhere between the other two types. The JM23s are more expensive here in the UK (they are only made in the US and in Italy) but I feel they are worth the extra cost. For my first 2 ovens, I hacksawed either side of the element grooves, broke out the centre bit by tilting the saw blade, then filed the base of the grooves with a suitably-sized piece of studding (allthread). For the last 3, I used a router, together with a shopvac and respirator. Infinitely easier and works for all 3 types of (soft) brick I've come across. I'd expected it to kill router cutters fast and bought several cheap ones, but I've probably done over 20 ft of 10mm wide groove and I'm still on the first one. It is possible to use the same oven for Austenitizing stainless and tempering Carbon steels, but it's not as straightforward as it might initially appear. I have found that there is a tendency for the oven temperature itself to overshoot the setpoint at tempering temperatures, as a result of the very high heat input needed to get the stainless Austenitizing temperatures. Added to this is the tendency to heat the workpiece by radiation fron the elements themselves, rather than from the oven as a whole. I've got it all down to what I think is an acceptable standard of control by paying attention to detail: a fast-responding control thermocouple, a short cycle time on the controller output and a ramp/soak controller. I'll admit to being slightly obsessive about temperature control, and to having access to instrumentation that few others on here will have, as part of my day job. As a good alternative to going high-end on the control, one very good knifemaker I know buries his hardened blades in dry sand in a fish-kettle for tempering and sticks the whole kaboodle in his HT oven . It gives a huge thermal mass and effectively eliminates the overshoot issue, but obviously adds many hours to the tempering time. As Daniel says, the biggest problem with using the same oven for both really seems to be the cooldown time. It takes several hours to cool the oven from around 800 degC (1472 degF) to around 200 degC (392 degF) and it's a long time to leave fully-hardened blades. For swords, a spell at intermediate temperature in the kitchen oven isn't even an option.
  10. Sounds like you've got it covered, so I'll just sit back and wait. I'm told patience is a virtue. Any information you can provide on the control system you mentioned would be gratefully received. My last 4 builds have used the AutomationDirect Solo controllers (They seem to be the same as the Omega CN 7200, 7500, 7600, 7800 series controllers). It's a good controller, but doesn't seem very user-friendly when it comes to the profile programming, especially for non-geeks. I need to get something pretty soon for my next build and if there's something that may be better available, I'd like to give it a try. My last build was a 42" sword-length oven with around 6 kW of input power. With hindsight, I could have gone a lot less. I basically just doubled up on the elements I had used on my earlier, 18", builds. This was partly because I only wanted to make one change at a time, and partly because I already had the elements, I'm basically lazy and, faced with a choice of 3 kW, 6 kW or winding fresh elements, it seemed like a no-brainer at the time. I really should try it on just one set of elements and see how hot it'll get. I've successfully taken the 18", 3 kW, version to 1176 degC/2150 degF, which was the highest HT temperature recommendation I could find at the time (S30V treated for maximum wear resistance) and it was still rising quite fast as it approached that temperature. I find lots of power tends to make accurate control at low temperatures a lot harder. Still on my to-do list is a tempering oven to go with the sword oven: a 4' long piece of 4" pipe, a fairly low-powered catering rod element (I'm guessing 1000W would do it) clamped to the outside of the pipe, along with a thermocouple and the whole thing insulated with Rockwool slab. The plan is to run it from the control box that runs the sword oven, once the blades are quenched and the sword oven is finished with.
  11. Looks the business, Dan. Nice use of the TIG shrouds. I just might have to steal that idea for a future build. Any chance, as you write it all up, you can major on the control/electrical side of things? Most of the oven/furnace write-ups I've seen have tended to gloss over all the boring-looking technical bits. Whereas most of the guys I've spoken to, who are considering building one, are happy enough about the physical build, but rather nervous about the electrics and control. Hopefully, all the questions will be answered anyway during the WIP, but there are one or two that spring to mind already: That looks like a lot of element length you have there. What sort of wattage are you going to be running, and what sort of temperature range are you intending to run it in? Carbon steels only, or stainless as well? Are you looking to use it for tempering too, or does the cool-down time rule that out? Overnight annealing suggests a ramp-and-soak controller. Have you found one that's affordable and easy to use? If so, please share.
  12. If you really need to contiuously measure welding temperature, you'll almost certainly need to go for a Platinum-based thermocouple with a ceramic sheath. The Omega thermocouple John uses is really about the upper limit for base-metal thermocouples. Where I feel it is most useful for the novice forge-welder is in establishing whether the (gas) forge is hot enough to weld high(-ish?) carbon steels. My feeling is that it's unlikely to help the experienced guys much, if at all. I originally recommended it because I feel that it offers the most bang for the buck. Over here, the additional cost for the Omegaclad XL sheath versus a 310 Stainless sheath from another supplier, works out at around 10 bucks or so. Once you factor in shipping and the readout, it adds less than 20% to the total system cost to get the extra 200 degC of usable range and this is enough to just get into the welding range. I come at this as a bit of a geek with an interest in tools and a background in gas burners and process control, rather than as a maker of knives. I had no idea at all what sort of temperature to aim for when building a propane forge a couple of years ago. I struggled to find objective information online, so I went to a hammerin, watched lots of folk making Damascus for two days, then measured the temperature of the forge using a type S (Platinum) thermocouple. It was doing 1280-1310 degC (2336-2390 degF). As is the way of such things, there followed a brief discussion on achievable temperatures, some burner adjustment and a melted forge lining. Peak temperature measured was 1470 degC (2678 degF). The thing I had wanted to achieve by the exercise, was a realistic value for a welding forge temperature for knifemaking. Several of the guys using the forge were very good smiths. A couple of them had guessed the temperature within 30 degC but one was 30 degC high and the other 30 degC low. Most were happy to admit they had no idea what the temperature was. Nobody had said it was too hot. Nobody had said it wasn't hot enough. If it had needed to be hotter, it could readily have been adjusted to make it hotter. Based on that experience, I'd say that for welding what the type K can do is tell you whether the forge temperature is in the right ball-park and give some indication of the temperature distribution. It'll be close to its limit though. Based on my limited understanding of metallurgy and welding, a base-metal thermocouple isn't going to be much use for measuring the higher welding temperatures needed for mild steel or wrought iron. My feeling is that continuous measurement in a welding forge is only likely to be helpful if you are sure the temperature distribution is even and that the thermocouple itself is measuring at a point that is at the same temperature as the working area, since the thermocouple is fragile and can't encroach on the working area itself. Effectively, it's only useful in a really well-designed and well-executed forge. I've not seen a forge that I feel is sufficiently well-designed and -executed to convince me that the continuous measurement would be useful, but I've seen plenty where a check measurement in the working zone would certainly be useful to some people. For HT, leaving the thermocouple in will not be a problem. For forging, it may not be a problem either, but a lot depends on your setup and how you work. For welding, I'd say you are better off taking it out. Personally, I'd take it out for forging too.
  13. There are a lot of things involved in putting together a PID controlled gas forge that works reasonably well. Few of them are immediately obvious to most people. Not all are immediately obvious to me, and I'm a seriously geeky type who deals with PID-controlled gas equipment for a living. The Don Fogg oil drum HT furnace lends itself well to simple PID control, but is about the only design I've seen that does (without going to analog control, which is getting rather specialized ). For the Don Fogg setup I've played with, a separate pilot burner was used and ran continuously. The main burner was switched on and off by the PID controller. The pilot burner was a small gas torch and the main burner was a larger gas torch. Both were teed off the same regulator, with the solenoid valve in the line to the big burner. A baffle plate was found to be needed in front of the burners to achieve reliable lighting of the main burner. Both burners were cheapies of far eastern origin. The PID controller was a cheap one off ebay, but had a relay output to switch the solenoid valve, rather than a DC output to drive an SSR. It wouldn't do anything that the manual version couldn't do, but was quicker, and needed much less fiddling, to get to temperature. The temperature of most gas forges is varied primarily by adjusting the gas:air mixture ratio to vary the flame temperature. The gasfeed can be fixed and the airfeed adjusted, or the airfeed can be fixed and the gasfeed adjusted. Either way, automating this system needs a means of varying flow smoothly across a range. This is generally more complex and expensive than simple time-proportioning (on/off) control.
  14. http://i1138.photobucket.com/albums/n537/HeathBesch/IMG_0090.jpg shows the photo for me Looks pretty good in the photo. What are you using for the burner? It's not clear from the photo, but I can't see a blower so assume it's some sort of atmospheric mixer, either with or without a Venturi. Photos never seem to give a true picture of either the temperature or the flame from the mouth. I suspect most digital cameras do something strange with auto white balance, so the colour is almost always misleading. Subject to the uncertainty in the photo though, it looks like you are running a bit rich there. As long as you are getting up to the temperature you need, that's fine, but if not, it will get hotter if you lean it off. If you're restricting the airflow, try increasing it if you need it any hotter. If you have the air fully open already, you might be better with a smaller gas jet; Once you can get it hotter than you need, you can restrict the air to bring the temperature down to just where you want it.
  15. I can't help on brands, but a couple of suggestions on things to look for: I don't know how it is on your side of the pond, but over here, a lot of the newer drills have a male chuck taper machined directly onto the end of the spindle. It's obviously cheaper to make than having a female Morse Taper and an MT-to-chuck-taper arbor, but it drastically reduces versatility. The long MT is designed to allow rapid tooling changes, whilst the short chuck tapers are effectively a permanent thing. With an MT spindle, you can change chuck/arbor assemblies easily, so you can have a big chuck (say 1/8" to 5/8")and a little one (say 0-5/16"). If you can't change them easily, you need to have one chuck and it's usually a compromise; maybe 1/16" to 1/2". If you can find an MT drill with a through-drilled spindle for use with a drawbar, you may even be able to use it for light milling. It obviously won't perform like a real milling machine and will need at least one collet for toolholding, but MT collets are pretty cheap. A high top speed can be useful, but a low bottom speed is far more useful to most people. At some point you'll need to drill hardened steel and a speed around 250 RPM works well for me with cheap Carbide multipurpose drills or, at a pinch, masonry drills for 5-6mm (3/16"-1/4") holes.
  16. There are several ways of doing the heating part, but the easiest, most accurate and often cheapest is to use electric heating coils. The Solo controller I linked to has a low-voltage DC output to switch a Solid State Relay. The SSR switches the mains power to the heating coils. The SSR is switched on and off on a cycle. I use a 2-second output cycle on my electric HT oven/furnace. For half power, it is on for 1 second, off for 1 second. For one quarter power, on for half a second, off for one-and-a-half seconds. You get the idea. When powerful heating elements are used at low temperatures (think tempering temperatures in a furnace that can reach Austenitizing temperatures for stainless steels), the rate at which the temperature rises whilst they are powered is very high. To avoid overshooting the temperature, it's necessary to have a short cycle time, hence my 2 seconds. I've also tried one second, but can't see any improvement, whilst 5 seconds gives measurably poorer control. For gas heating, the process needs to have enough thermal inertia to smooth out the temperature, through the much longer on/off cycle time needed for switching the gas solenoid valve. It's great for a salt-pot setup with lots of thermal mass, but much less good for a well insulated small oven/furnace. It actually works very well on a Don Fogg style oil-drum HT forge. Controllers are also available with analog outputs. These can run either modulating gas valves or phase-angle switching SSRs, but this is probably more of a specialist thing and doesn't offer anything "we" can't do with a switched output for less effort and money.
  17. Looks pretty good to to me, with the caveat that flame color is always different in photos. I'd strongly recommend you get a couple of MIG tips in the next 2 sizes down and rig up some form of adjustable air restrictor on the air inlet, though. The central cone is where the primary air (the air that gets drawn in at the Venturi) is burning with the gas. The outer, bushy, flame is where the excess gas mixes with secondary (ambient) air and completes the burn. When you stick it in a forge, with nowhere for the secondary air to get in, the bushy part of the flame will become the Dragons Breath, only mixing with the air it needs once it leavs the mouth of the forge. You currently have no means of reducing the air, and seem to have a rich flame already. It may do what you need it to do right off the bat, but if it doesn't, a smaller gas jet will lean off the burn and almost certainly raise the temperature. I know the intuitive thing is "more gas = hotter", but it doesn't always work like that. Once you've got the biggest jet just consistent with getting the forge too hot, you can use the air restrictor to tune the temperature. Reducing the air will reduce the temperature and increase the dragon's breath.
  18. It all depends on what you are doing. If you are setting the fuel:air mix on a forge, it's usually best to take it out once things are stable, just to save it getting damaged when you stick stuff in and out. Heat will tend to soak back along the thrmocouple, so you need to be careful of overheating the "cold" end. Handheld probes usually have a plastic handle. Mostly it's just common sense. Type K thermocouples tend to suffer "drift" above about 1000 degC. Type N was developed to overcome this, but all the really cheap readouts, like the TM902C, only take type K inputs. From that point of view, it's best to take them out when they're not doing anything useful. To keep it in perspective, the drift only really tends to show over days or weeks under continuous use in industrial processes; 3 hours a week for a year works out at the equivalent of about a week of continuos use, so it's probably not too big an issue for most small knifemakers. The sheaths tend to Oxidize and the oxide layer prevents further oxidation. If they get hot, then cold, the oxide layer can flake off, losing a little of the sheath on each cycle. Different sheath materials suffer from this to different degrees; 316L Stainless suffers when it cycles above about 850 degC, 310 Stainless above about 1100 degC. Nicrobel about 1250 degC, etc. From this point of view, leaving them in is better. At the end of the day, it's best to view thermocouples as consumables and just do whatever fits best with your way of working. If you are going to build your own HT furnace, it's worth getting a good ramp/soak controller. I have built five so far. After testing the first one, I have been buying Solo controllers from AutomationDirect, despite having a vitually unlimited supply of free non-ramp/soak controllers. The improvement in performance, especially when tempering, is huge. I use the 1/4 Din Solo SL9696VRE or 1/16 Din SL4848VR. I prefer the bigger 1/4 Din version because I have fat fingers and middle-aged eyesight. http://www.automationdirect.com/adc/Shopping/Catalog/Process_Control_-a-_Measurement/Temperature_-z-_Process_Controllers/1-z-4_DIN_Size_%28SL9696_Series%29/SL9696-VRE It's the cheapest thing I've found that will do the job well. It's worth keeping an eye on what is available though, as, like most electronics, controllers are getting better and cheaper all the time. I'm not 100% sure, but I think Omega sell the same controller with their own branding. http://www.omega.com/ppt/pptsc.asp?ref=CN7200_Series&Nav=temp06
  19. Thermocouples basically measure at the junction where the 2 different materials join, so broadly speaking, at the tip. For Mineral-Insulated thermocouples, there are 3 options. Exposed junctions have the outer sheath and insulating powder removed to expose the junction. This gives the fastest possible response but gives no tip protection. Insulated junctions have the end of the sheath closed and the insulating powder all around the junction. Slow response, as the temperature has to equalize across the insulation, but good protection and fine for slow-changing processes. Because of the insulation around the junction, the measured value tends to be the average temperature over perhaps a couple of sheath diameters at the end of the thermocouple, so possibly not the best choice if you have steep temperature gradients. Grounded junctions are like insulated junctions but with the junction itself welded to the sheath. The response is nearly as fast as the exposed junction and the mechanical protection is nearly as good as an insulated junction. They measure the temperature at the tip. The only real downside is that the junction is grounded through the sheath and this can cause interference in industrial process conditions. It's about ideal for use in a forge, but may not be the best choice in an electric furnace. That said, I use one in my homebuilt electric HT furnace. Omega have a lot of very good technical information on their site and it's worth a browse. As an ancient, who predates the interweb, I tended to carry a paper copy of the Labfacility Handbook for sitework, mainly because it was much smaller than the Omega catalog. It also tells you pretty much all you need to know about thermocouples. http://www.labfacility.co.uk/temperature-handbook.html Sheath materials make a difference to how long the thermocouple will last and I'm not sure that all sheath materials are suitable for grounded junctions. Thermocouple life is not necessarily a pure time-at-temperature thing, cycling matters as well. So does the atmosphere it works in.
  20. As the OP appreciates, there are 2 things needed for temperature measurement. One is the thermocouple, the other is the display. Realistically, you only need a simple handheld readout for checking temperatures. I use a TM902C bought off ebay. It's dirt cheap and accuracy seems good; the readings compare well with much more expensive big-name instruments. Downside is a Celsius-only readout. I'm in the UK and don't think in Farenheit, so it's not a problem for me, but it may be for some. It reads to 1300 degC, which is 2372 degF. If you need Farenheit, it's a safe bet that even the cheapest readouts will be at least as accurate as the thermocouple itself. It's worth spending on a good thermocouple, though. I'd recommend an Omega KHXL-14G-RSC-18 thermocouple. It is a type K, mineral insulated thermocouple with an Super Omegaclad XL sheath, able to stand a working temperature of 1335 degC, 2440 degF. It has a handle and a connection cable. The diameter is 6mm, 1/4", making it rigid enough to put the tip where you want it without sagging. It has a grounded junction for fast response and is 18" long. The fast response and 18" length are a real hand-saver. In one of the old paper catalogs, there was an option of additional length. I'd suggest another 6" would be worthwhile, if it's available, to give 24". http://www.omega.com/ppt/pptsc.asp?ref=KHXL_NHXL&Nav=tema06
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