Equipment for measuring hot end temperature?
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@zapta if you are manufacturing printers and supplying filament to go with them, then you may require consistency between all the machines so that you can recommend what temperatures to use. In that case, it may be worth using a PT100 or PT1000 sensor instead of a thermistor, to get better consistency.
If the machine is a one-off then you don't need to know the heater block temperature accurately, you just need to know what indicated temperature gives good prints, which you will need to determine anyway. The slight downside is that if you need to change the thermistor, you might possibly need to determine the best indicated temperature again.
Duet WiFi hardware designer and firmware engineer
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Thanks @dc42. It seems that my best chance of having a reasonable absolute measurement at a reasonable price is to using a PT100x and just trust its datasheet, possibly calibrating it at 100C for extra accuracy.
They seem to come at different tolerances, but I couldn't find the specification of E3D PT1000. Anybody happens to have it?
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I found this RTD which seems to be a good reference for absolute temperature. It's Class A (accurate), reasonably priced, and liquid proof.
One thing I don't understand is the formula in its datasheet, I expected to have a linear relation between temperature and resistance but it has a SQRT. Am I missing something?
https://atlas-scientific.com/probes/pt-1000-temperature-probe/
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@zapta said in Equipment for measuring hot end temperature?:
One thing I don't understand is the formula in its datasheet, I expected to have a linear relation between temperature and resistance but it has a SQRT. Am I missing something?
The relationship is not completely linear over the full temperature range of the sensor (though it approximates it over small ranges). The above formula is a simplified version of the full conversion normally used. See https://techoverflow.net/2016/01/02/accurate-calculation-of-pt100pt1000-temperature-from-resistance/ for lots more details. RRF uses a lookup table to generate the temperate readings.
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Thanks @gloomyandy, this is very useful. Until today I thought that the common model for PTx is linear, and you are right, Duet indeed uses a table with 10C intervals and linear interpolation in between (same table for pt100 and pt1000).
https://github.com/Duet3D/RepRapFirmware/blob/3.4-dev/src/Heating/Sensors/TemperatureSensor.cpp#L278
And that python package is exactly what I need. I will look at it.
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If anybody is planning to measure their sensor with boiling water, apparently there is a category of products to do just that. They are called thermowell, and are metal tubes that are closed on one end, with optional means to attached it on the other hand.
I just ordered one from amazon. Ali express also has them.
https://www.amazon.com/gp/product/B0BRW5FSGJ?th=1
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Apparently, calibrating a temperature sensor is easy, if you have the right equipment.
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@zapta hah, let me know if that equipment works out if you choose to buy it.
I think alot of the discussion on this thread is because we (the FDM printing community) have (traditionally) not bothered too much about absolutes in temperature (as every batch of filament is different so a bit of recalibration is needed). Even when we do want to be more accurate, consistency between different readings (the same filament on multiple printers with the same hotend uses the same temperature) was more important than absolutes. Given filament manufacturers give wide ranges of correct printing temperatures and hotend geometries mean that nozzle temperature vs temperature measurement are widely variable between hotend designs, and all this changes based on flow rate and sometimes part cooling, the general consensus is "vaguely good enough is good enough".
I am curious why thats not the case for your application?
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Hi @T3P3Tony, I don't have a specific application that require high accuracy but more a research/challenge project trying to get accurate temperature measurement. I am used to reasonably priced and accurate meters for voltage, current, frequency, weight, time, capacitance, distance, and more, and am surprised that it's not the same with temperature (not counting medical range thermometers which are inexpensive and sensitive).
I manage to get stable reading at milidegrees C, such that the reading changes when I enter the room, I presume due to air movement, but would like to be able to anchor it at absolute values, an my best bet at this point is to rely on the spec of a class A RTD.
My post is not specific to Duet, it's just the best place I know with people that may have useful information.
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@zapta The problem with your analogy with other measuring devices is that they mostly have the sensor encapsulated within the device whereas for fdm printing, we use a remote sensor. Whatever is being measured has to be converted to an electrical signal of some sort. With electronic scales for example, the device which responds to changes in force or pressure is usually a load cell (or several connected using a Wheatstone bridge). So it's reasonably easy to calibrate the device as a whole by checking it against known standard weights. On our printers, the sensor is nothing more than a device which changes resistance with temperature. So we actually measure resistance then do a mathematical conversion to get temperature. We can calibrate the whole against a known standard but if we change the sensor then we have to recalibrate. The same would apply to our electronic scales if we changed the load cells. In fact, load cells are notoriously prone to drifting and scales are usually "tared" before use.
Ultimately most electronic measuring devices can drift over time. So the only way to be sure of accurate readings is to calibrate the device (including its sensor) against a known, traceable standard, then take a measurement, then re-check the calibration. This is in fact what we used to do when I was involved in measuring vehicle exhaust emissions.
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@zapta A class A RTD is probably the most reliable, affordable temperature measurement device you can use to cover a moderately wide range with no special calibration. A thermocouple of appropriate type can be used without any calibration, and fairly good accuracy, over a very wide range, but the resolution isn't so good. If you want to go first-class, an SPRT (standard platinum resistance thermometer) is ideal, but costs. These are RTDs manufactured to extremely precise specification (loosely wound, special platinum wire, so thermal stresses don't change it, etc.).
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@deckingman, my goal is to first calibrate the sensor itself, when calibrate the sensor itself, removed from the printer. It's funny that you mention load cells because I also play with load cell and having no problem calibrating them with known weights. A known temperature is much difficult to achieve. I am still waiting for the thermowell to calibrate at 100c with water (I tried nylon bags and it leaked and deionized water but they still conduct electricity). I also saw that people calibrate with molten reference tin at ~230 but haven't tried it yet.
@mendenmh, you are right and this is the best class A RTD I could find in my price range (it's a hobby project). https://atlas-scientific.com/probes/pt-1000-temperature-probe/ . I calibrate the resistance measurement with 0.01% resistors I got from ebay and will use short two wires RTD wiring.
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@zapta Make sure when you do the boiling water 100C point, that you look up the local barometric pressure, and then use a conversion table to get the correct boiling point. It varies quite bit, especially if you are at altitude. This is why an old explorers trick in the mountains was to keep a thermometer, and put it in boiling water to measure the altitude.
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Thanks @mendenmh. I am close to sea level but will make sure to check the barometric pressure. I also saw all kind of other rules of thumb (e.g. immersion depth) and realize that I will just scratch the surface with my limited knowledge and means.
E.g.
http://www.isotechna.com/v/vspfiles/pdf_articles/Temperature Calibration-Depths of Immersion.pdf
