Alternative thermistor

Discussion in 'E3D-v6 and Lite6' started by paultnl, Sep 20, 2015.

Tags:
  1. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    I find the current thermistor setup very fiddly, probably due to my fat fingers. This looks like it could be a good alternative if it fits into the existing screw hole http://www.ebay.co.uk/itm/321790767371
    Has anyone tried these? I think I will risk £2.69 and give it a go.
     
  2. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    The epoxy used in those is only rated to 260C, so you can't perform the final step of the E3D assembly process correctly which requires heating beyond 260C.

    It will be a less accurate measurement as some of the sensor is exposed to ambient temperatures by hanging out like that.

    It is the right thread to fit the existing screw hole, though.

    A better alternative is the PT100 sensor and block:

    (If in the US) http://www.filastruder.com/collections/e3d-spare-parts-and-accessories/products/e3d-pt100-sensor
    (If elsewhere) http://e3d-online.com/V6-PT100-Sensor-Kit?search=PT100

    Its rated to 400C and is less fiddly than a thermistor. Better strain relieved too, which your eBay link is not.
     
  3. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    Fair points but for two things, 1 it is less than a tenth of the cost of the PT100 and 2 you are out of stock :-(

    Seriously I take the point but as I mainly print with PLA the max temp wouldn't be a problem and it would be easy to swap thermistors after assembly.

    I assume that the thermistor is down in the threaded part of the assembly so I doubt ambient temperature will be an issue.

    I will experiment with an open mind and report my results. I will probably upgrade to the PT100 in the fullness of time.
     
  4. Mike Kelly

    Mike Kelly Volunteer

    Joined:
    Mar 7, 2014
    Messages:
    444
    Likes Received:
    89
    You don't need to actually heat it above 260 for final tightening. You can heat it to 260 and just tighten it there no problem.

    They're fine and pretty popular.
     
  5. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    Not sure what you mean, I've been in stock for a couple weeks now and the website reflects that.
     
  6. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    You may have noticed my use of £ pricing, I am in the UK so looked at the E3D site, no stock :-(
     
    #6 paultnl, Sep 21, 2015
    Last edited: Sep 21, 2015
  7. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    According to the supplier they are rated to 300 degrees, time will tell. I have fitted a couple to my Chimera and so far they are working well and are much easier to fit.
     
  8. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    The thermistor is rated to 300C, but the whole assembly is not. It is made of PTFE (ok to 245C) and epoxy (ok to 260C).
     
  9. Mike Kelly

    Mike Kelly Volunteer

    Joined:
    Mar 7, 2014
    Messages:
    444
    Likes Received:
    89
    The new Epoxy is rated to 280C and the PTFE is safe to 300C
     
  10. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    Do you have a data sheet supporting that? Dupont themselves say not to take PTFE over 260C, which is why E3D limits the Lite6 to 245C (to allow for overshoot).
     
  11. Mike Kelly

    Mike Kelly Volunteer

    Joined:
    Mar 7, 2014
    Messages:
    444
    Likes Received:
    89
  12. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    I'm afraid you're misunderstanding. The Prometheus uses silicone sleeving, not PTFE.

    Here's a real data sheet:

    http://catalog.wshampshire.com/Asset/psg_teflon_ptfe.pdf

    The thermal limit of PTFE is 260C... and it starts getting squishy before that. Youtube and Wikipedia aren't good sources when it comes to health and safety. While 300C may not be hot enough to create a significant health risk to humans, some pet birds are particularly sensitive to the products produces from the offgassing process. If PTFE was truly good to 300C, we wouldn't need the E3Dv6.
     
    #12 elmoret, Oct 2, 2015
    Last edited: Oct 2, 2015
  13. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    The PTFE is on the leads outside the assembly and subject to ambient temperature so I doubt that the specs will be exceeded in actual use. The main body is made of a brass like material which I am sure can withstand much higher temperatures.

    Rather than see this thread descend into a war of specs I will do some potentially destructive testing tomorrow. Would running the head at a displayed temperature of 280 for an hour satisfy everyone?

    Results to follow.
     
    #13 paultnl, Oct 2, 2015
    Last edited: Oct 2, 2015
  14. paultnl

    paultnl Member

    Joined:
    Aug 17, 2015
    Messages:
    21
    Likes Received:
    0
    As I have a 12v system, I couldn't get it to go beyond 276. Kept it there for an hour with no visible ill effects. The PTFE covered wires do not get any noticeable heat and have no strain on them as I zip tie the connector they go in to.

    Next upgrade solid state relays so I can run 24v heaters.
     
  15. Mike Kelly

    Mike Kelly Volunteer

    Joined:
    Mar 7, 2014
    Messages:
    444
    Likes Received:
    89
    Yeah like Paul mentions, the PTFE isn't experiencing the same temperature as the sensing point. It will be significantly cooler in the same way measuring the external temperature of the heater block will be less than the internal. Plus add in convective cooling and I doubt it will even be 200C while running at 280. I think you're stretching for reasons to be critical Tim.
     
  16. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    The PTFE runs right up to the thermistor bead, which is experiencing the temperature that is being sensed. The plain facts are that PTFE is not rated for anything over 260C - how about if the leads come in contact with the block for instance? It's just not a good design choice.

    I have no motive to stretch for reasons to be critical, I've talked to two different manufacturers of these and could buy them in bulk and resell for a tidy profit if I thought they were suitable.
     
  17. Mike Kelly

    Mike Kelly Volunteer

    Joined:
    Mar 7, 2014
    Messages:
    444
    Likes Received:
    89
    For a PHD mech E I figured you'd understand more about heat gradients. The internal temperature (280C) will not be the same as even the external of the heater block. Even less after the thermistor stud, and even less on the leads where the PTFE is.

    Tim I think you need to reevaluate where your criticisms lay. Just because you don't sell them doesn't make them dangerous.

    Further, PFOA, the harmful offgas from PTFE has long been removed from PTFE.
     
  18. elmoret

    elmoret Administrator

    Joined:
    Mar 1, 2014
    Messages:
    586
    Likes Received:
    91
    I'm not sure why its ok to do personal attacks here, but I received an A in Heat Transfer in college.

    Yes, the internal portion of the block is hotter than the external surface, but not by much owing to the fact that aluminum is such a good conductor of heat. In my (thermocouple based) testing, the temperature is different by about 5C, so 275C in the case you proposed. Again, in these sensors the PTFE runs right up to the thermistor bead. If the thermistor bead is measuring 280C, the PTFE in contact with the bead is also at 280C. That is beyond the maximum operating temperature of PTFE, as shown in the data sheet I linked above.

    While it is true that some PTFE is now made without PFOA, that is not the case for all PTFE. For instance:
    There is no statement about the source/composition of the PTFE used in the referenced sensor, so it is reckless to make assumptions.

    Dupont's testing has shown PTFE to start off gassing toxic particulates at as little as 230C. Plain and simple, there is no support for it being taken to 300C as stated earlier in this thread.

    All of this arguing when there's a suitable solution that doesn't push materials past their data sheet maximum operating temperature, and is included with every hotend!

    Hopefully this puts the matter to rest, no sense in arguing it further.
     
    #18 elmoret, Oct 7, 2015
    Last edited: Oct 7, 2015
  19. Sanjay

    Sanjay Administrator
    Staff Member

    Joined:
    Feb 27, 2014
    Messages:
    125
    Likes Received:
    118
    I don't have anything against these little stud thermistors, they work decently enough and are really easy to use within their temp range. They do have a bit of lowered performance in terms of response and accuracy, but if you're just after something easy then they're probably just fine.

    On tightening, as long as you tighten the hotend at, or above the temperature at which you're printing then you're fine and things will seal. Tighten at 250C, and print at 220C, no problems. Tighten at 250C and print at 260C = leaks. If you're going to print at 380C with an upgraded sensor, then you need to re-tighten at that temp or higher. Thinking about it I should add that to the documentation....

    Using PTFE at 280C unsettles me somewhat. Although it hasn't melted or suchlike at those temperatures you've certainly lost a great deal of mechanical properties, probably not enough to pose a risk of the insulation being breached, but enough to be wary at least.

    You will certainly see off-gassing and PTFE shedding toxic particulate matter at those temperatures though. Although DuPont (and many others) made efforts to eliminate the amount of PFOA in final tefon coated cookware and reduce other heat produced emissions from their cookware grades some time ago this applies only to their cookware range. Most industrial PTFE is still produced using PFOA.

    Furthermore PFOA is just one of the concerns, I'll quote from another article:
    (on gasses produced from over-heating PTFE)
    It's also significant to bear in mind that PTFE used in heatshrink and sleevings is probably a low-molecular weight grade, for improved flexibility, but these grades are some of the worst in terms of heat resilience and off-gassing.

    I have yet to find a PTFE sleeving that has a temp rating of over 260C.

    A good solution here might be to move to a polyimide (Kapton), which is a bit nicer at higher temperatures.

    We're trying to move to a system where we use a single sensor cartridge geometry for all sensor types. Currently we've got the PT100s in a nice 3mm diameter cartridge package, we're in the sample phase of getting thermistors in those same packages, as well as thermocouples (some people still want them for niche applications). Thereafter we should be able to run a single block type, with easily changed sensors. There is nothing I would like more than to get rid of that whole fiddly thermistor assembly process. We'll probably use fibreglass sleeving, but Polyimide is in the mix too.
     
    Mike Kelly likes this.
  20. Sanjay

    Sanjay Administrator
    Staff Member

    Joined:
    Feb 27, 2014
    Messages:
    125
    Likes Received:
    118
    Mike got his comment in before I hit the button on mine!

    I don't know anything about the internal construction of the stud type things, never cut one up. I get that the sleeving is likely at a lower temp and all from the bead itself if the PTFE is not full depth inside the stud. Not sure how the bare thermistor leads are kept from touching the stud or eachother after that though - just careful positioning of everything when the potting compound is added?

    It's worth bearing in mind with all of this that we're using a thermistor with a 3% variance in accuracy, along with the balancing resistors on the controller boards having their own resistance tolerance too. In a bad case you might see as much as 10-15C error.

    For me, personally, PTFE on an item running at or above 260C is sailing a bit close to the wind, and not something I'd do on my products. Especially when you throw in measurement error, and environmental variables like the user adding insulation (not uncommon). I don't think the world is going to end if you run PTFE sleeving at 280C, but it is starting to exceed the specs of the material supplied by the manufacturer and I wouldn't consider it particularly good practice overall.
     

Share This Page