Heat bed temperature anomally

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brid971
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Joined: Tue Dec 12, 2017 7:02 pm

Heat bed temperature anomally

Postby brid971 » Tue Dec 12, 2017 7:27 pm

I recently received a Raise3D N2 printer but have a problem with the heat-bed not reaching the temperature indicated on the Raise3D panel. I was wondering if anyone else has had a similar problem and how it can be resolved. I have sent the following details to the manufacturers and are awaiting their response.

Raise3D N2 Heat Bed temperature anomaly.


When I received the Raise3D N2 printer I set it up in a room with an ambient air temperature of about 7 Deg C. Granted, I bit on the low side but I thought with an enclosed unit, I would be able to maintain the required temperatures for successful operation. My initial prints with small objects using PLA were successful but larger prints were a bit problematical. I will not detail all the problems I had with the various prints, suffice to say that those requiring higher bed and extrusion temperatures failed, mainly due to poor adhesion to the bed plate.

I carried out a number of tests measuring the heat bed temperature with and without the 'BuildTak' plate and with the printer located in two different rooms (different ambient air temperatures). The first room had an ambient air temperature of about 7 Deg C. In the second room the ambient air temperature was 23 Deg C. In each case I controlled the temperature of the heat bed with the 'Tune' button on the Raise3D panel. I used two DS1820 digital temperature sensors inside the Raise3d enclosure. One measured the air temperature, the other measured the heat bed temperature. The sensor on the heat bed was held down in contact with the bed by a piece of polystyrene and a weight. Both were connected to a bluetooth module allowing me to monitor the temperatures without opening the printer door.

I don't know how to attached all the details of my findings which included photographs and tables of measurements, however in summary I found that the bed temperature over its full range was about 18 % lower than the indicated temp on the Raise3D panel. This was the case for the cold room and the warm room.

I wonder if anyone can suggest a solution.

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walshlg
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Re: Heat bed temperature anomally

Postby walshlg » Wed Dec 13, 2017 5:42 pm

try pid tuning

brid971
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Re: Heat bed temperature anomally

Postby brid971 » Thu Dec 14, 2017 1:28 pm

Excuse my ignorance but I have no idea what you mean by PID tuning.

I have just completed further tests that I believe shows very clearly that the Raise3D system is not measuring the bed temperature correctly. I would be interesting to know if this problem is unique to my unit or if all raise3D printer show this discrepancy.

Test of Heat Bed Temperature on Raise 3D, N2

The following test describes very clearly that the Raise3D control system is not reading the heat bed thermistor temperature correctly. The error in in the order of 17% at the upper end of the temperature range.

The thermistor fitted to the Raise3D heat bed is described by the manufacturer as an
NTC 100K 0805 with a tolerance of 1% to 5%. No further details were provide but it is assumed that the thermistor has a +- 5% tolerance with a beta vale of 4500. My initial hand calculations at room temperature suggested a tolerance of +- 6% but following further examination and the use of an online calculator the thermistor proved to be within tolerance at room temperature.

Test Procedure

Using the tune button on the raise3D panel the temperature of the heat bed was set at 110 degC. A DS1820 digital sensor was positioned directly at the centre of the top of the heat bed. The BuildTak plate was not fitted.
When the heat bed reached its maximum temperature the Raise3D panel showed a tempearture of of 110 degC.
I allowed the temperature to stabilise before noting that the DS1820 sensor was reading 91 degC.
This discrepancy matches the results of previous test showing a discrepancy of 17%.

With the raise3D panel indicating a steady 110 degC for the heat bed, I set the heat bed temperature to zero. I immediately disconnected the thermistor from the Raise 3d panel and connected it to the arduino. The temperature had dropped to 86 deg C, as read by the DS1820 sensor, before I could start reading the data from the arduino. Following this initial test I then reconnected the thermistor to the raise3d panel and completed the test again recording the DS1820 sensor temperature and Raise3D indicated temperature on the panel as the temperatures dropped.

The results are shown below

DS 1830 sensor located at centre of heat bed on top with BuilTak plate
Indicated temperature shown on Raise3D panel
Arduino reading directly from Thermistor
Percentage discrepancy between DS 1820 readings and Raise3D panel readings


Note: Don't know how to attach the table showing the data



Conclusion

1. There is no significant difference in the temperatures read be the DS 1820 sensor and the thermistor temperature read by the arduino. This indicates that the thermisor attched to the heat bed is working with tolerance.
2. There is a significant difference between the Raise3d panel readings with those of the Arduino and DS1820. As much as 17 % difference at the highest temperature.
3. This clearly indicates that the Raise3d panel is not recording the correct temperature of the heat bed by a significant amount.

ABH
Posts: 182
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Re: Heat bed temperature anomally

Postby ABH » Sun Dec 17, 2017 9:48 pm

"PID tuning" was a bad advice anyway, for two reasons:
1. The priner is not using a PID regulator for the heatbed.
2. If it had been using ta PID regulator, then a PID regulator will never be responsible for a static temperature offset (there's an integral term in PID, that will always remove a static offset, independent of tuning).

So, for your actual "problem":
There will always be a temperature gradient across your glass plate and across the Buildtak. This is unavoidable because there's a heat flow through the system and glass/Buildtak are not good thermal conductors.
The thermistor measures a temperature very close to the heating element. This is not the same temperature as you see on top of the Buildtak. It is just like that, and you need to take this offset into account when setting the bed temperature.

That being said, then there could also be an error in your DS1820 reading. The wires going to the DS1820 should be in good mechanical contact with the heatbed for at least 10 cm or more. Otherwise the wires can introduce an error in the temperature measurement, because copper in the wire is a phenomenal heat conductor.

brid971
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Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Mon Dec 18, 2017 10:38 am

I disconnected the thermistor from the heat bed and connected it directly to an Arduino. The readings I obtained confirmed that the raise3d system is reading the temperature from the thermistor incorrectly. Around 17% difference at the highest temperature. For those not familiar with the Arduino, its a small microprocessor that allows you to receive and send all sorts of analogue and digital signals for monitoring and controlling systems. Measuring the temperature of the thermistor is a relatively easy task with the Arduino even for those that have very little knowledge of programming language or electronics. I would encourage others to try this test.

ABH
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Re: Heat bed temperature anomally

Postby ABH » Mon Dec 18, 2017 2:37 pm

The easiest way is to use an ordinary ohmmeter, but whatever method you use, you need to know what type of thermistor is used in the heatbed. Raise3D printers use a thermistor like this one: NTCS0603E3104FXT, with a very specific NTC resistance curve.
There are hundreds of other thermistor types.
If you use an ohmmeter to measure the resistance of the NTC, then you can use this formula for converting to temperature:
NTC.png

firesped
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Re: Heat bed temperature anomally

Postby firesped » Mon Dec 18, 2017 4:54 pm

First off. The motion controller board is an Arduino board. an ATmega 2560 RUMBA (R.eprap U.niversal M.ega B.oard with A.llegro driver). http://reprap.org/wiki/RUMBA

So you just connected the bed temp sensor from one Arduino to another Arduino. The Marlin firmware running the RUMBA Board is compiled with Arduino IDE. The RUMBA board is connect to the touchscreen with a USB cable providing a serial connection. The touchscreen is a PCduino Arcadia. It steams data to the motion controller and recieves feed back from the motion controller over that serial connection.

Where did you get the information that the Raise3D printers use a "NTC 100K 0805" for the heat bed. search the internet for raise3d and "NTC 100K 0805" produces no real links with any type of information as such as that. the marlin firmware shows it as a type 8 temp sensor. "100k 0603 SMD Vishay NTCS0603E3104FXT (4.7k pullup)"

NTC 100k 0805 doesn't even tell us anything we really need to know. We need to know the pullup. What are you setting your arduino to read the thermistor as? as ABH said, it affects the reading you get off it. If it is set wrong, it will generate the wrong results.

Is your DS1820 sensor configured correctly for your device it is connected too? What does it read inside you freezer if you go stick it in there?
RL name: Michael Nolen
printers:
raise3D N2 kickstarter Early Bird
Trinus Deluxe (running smoothieware on Azteeg X5 GT board)
Monoprice Maker Select v2

EldRick
Posts: 345
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Re: Heat bed temperature anomally

Postby EldRick » Mon Dec 18, 2017 5:56 pm

Ditch the Buildtak. Gluestick on glass works better.

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Wed Dec 20, 2017 1:22 pm

I agree that the Marlin software lists the thermistor sensor as an NTCS0603E3104FXT. However the manufactures advised me that it was an NTC 100k 0805. They were unable to provide any other information. I measured the pull up resistor. It is 10K. I assumed a beta value of 4500. I tried all the different beta values I could find for this sensor. Using 4500 produced the smallest anomally. I also used an infrared thermometer to check the values obtained from the DS1820. I also appreciate that I have just connected from one Arduino to another. So I also appreciate that if the two Arduinos use different variables in their code then they will produce different results. So to recap. The thermistor readings taken by my Arduino match those from the DS1820 sensor and the infrared thermometer but are significantly different, by up to 17%, from those read by the Rasie3D panel. I have ordered some thermistors and intend to carry out some further tests. I will place one on the bottom of the heat bed as close to the existing thermistor as possible and place several on different areas on the top of the plate. That should provide some interesting results.

As for sticking.
The best results I have found for nylon is to turn the BuildTak plate over and fit Plumbers blue tape to the glass surface. I also used pva glue on top of the blue tape. Really works very well. Now I'm trying to resolve the warping problem.

firesped
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Re: Heat bed temperature anomally

Postby firesped » Wed Dec 20, 2017 2:32 pm

the firmware is using 4700 pull up. If you are using 4500 pull up, your readings will not match.

The following is the thermistor types for marlin. Note the first thing that is stated, NORMAL IS 4.7kohm PULLUP! The smallest anomaly does not matter. also the smallest anomaly between what? the readings of your other sensor?

Code: Select all

//===========================================================================
//============================= Thermal Settings ============================
//===========================================================================

/**
 * --NORMAL IS 4.7kohm PULLUP!-- 1kohm pullup can be used on hotend sensor, using correct resistor and table
 *
 * Temperature sensors available:
 *
 *    -3 : thermocouple with MAX31855 (only for sensor 0)
 *    -2 : thermocouple with MAX6675 (only for sensor 0)
 *    -1 : thermocouple with AD595
 *     0 : not used
 *     1 : 100k thermistor - best choice for EPCOS 100k (4.7k pullup)
 *     2 : 200k thermistor - ATC Semitec 204GT-2 (4.7k pullup)
 *     3 : Mendel-parts thermistor (4.7k pullup)
 *     4 : 10k thermistor !! do not use it for a hotend. It gives bad resolution at high temp. !!
 *     5 : 100K thermistor - ATC Semitec 104GT-2 (Used in ParCan & J-Head) (4.7k pullup)
 *     6 : 100k EPCOS - Not as accurate as table 1 (created using a fluke thermocouple) (4.7k pullup)
 *     7 : 100k Honeywell thermistor 135-104LAG-J01 (4.7k pullup)
 *    71 : 100k Honeywell thermistor 135-104LAF-J01 (4.7k pullup)
 *     8 : 100k 0603 SMD Vishay NTCS0603E3104FXT (4.7k pullup)
 *     9 : 100k GE Sensing AL03006-58.2K-97-G1 (4.7k pullup)
 *    10 : 100k RS thermistor 198-961 (4.7k pullup)
 *    11 : 100k beta 3950 1% thermistor (4.7k pullup)
 *    12 : 100k 0603 SMD Vishay NTCS0603E3104FXT (4.7k pullup) (calibrated for Makibox hot bed)
 *    13 : 100k Hisens 3950  1% up to 300°C for hotend "Simple ONE " & "Hotend "All In ONE"
 *    20 : the PT100 circuit found in the Ultimainboard V2.x
 *    60 : 100k Maker's Tool Works Kapton Bed Thermistor beta=3950
 *    66 : 4.7M High Temperature thermistor from Dyze Design
 *    70 : the 100K thermistor found in the bq Hephestos 2
 *    75 : 100k Generic Silicon Heat Pad with NTC 100K MGB18-104F39050L32 thermistor
 *
 *       1k ohm pullup tables - This is atypical, and requires changing out the 4.7k pullup for 1k.
 *                              (but gives greater accuracy and more stable PID)
 *    51 : 100k thermistor - EPCOS (1k pullup)
 *    52 : 200k thermistor - ATC Semitec 204GT-2 (1k pullup)
 *    55 : 100k thermistor - ATC Semitec 104GT-2 (Used in ParCan & J-Head) (1k pullup)
 *
 *  1047 : Pt1000 with 4k7 pullup
 *  1010 : Pt1000 with 1k pullup (non standard)
 *   147 : Pt100 with 4k7 pullup
 *   110 : Pt100 with 1k pullup (non standard)
 *
 *         Use these for Testing or Development purposes. NEVER for production machine.
 *   998 : Dummy Table that ALWAYS reads 25°C or the temperature defined below.
 *   999 : Dummy Table that ALWAYS reads 100°C or the temperature defined below.
 *
 * :{ '0': "Not used", '1':"100k / 4.7k - EPCOS", '2':"200k / 4.7k - ATC Semitec 204GT-2", '3':"Mendel-parts / 4.7k", '4':"10k !! do not use for a hotend. Bad resolution at high temp. !!", '5':"100K / 4.7k - ATC Semitec 104GT-2 (Used in ParCan & J-Head)", '6':"100k / 4.7k EPCOS - Not as accurate as Table 1", '7':"100k / 4.7k Honeywell 135-104LAG-J01", '8':"100k / 4.7k 0603 SMD Vishay NTCS0603E3104FXT", '9':"100k / 4.7k GE Sensing AL03006-58.2K-97-G1", '10':"100k / 4.7k RS 198-961", '11':"100k / 4.7k beta 3950 1%", '12':"100k / 4.7k 0603 SMD Vishay NTCS0603E3104FXT (calibrated for Makibox hot bed)", '13':"100k Hisens 3950  1% up to 300°C for hotend 'Simple ONE ' & hotend 'All In ONE'", '20':"PT100 (Ultimainboard V2.x)", '51':"100k / 1k - EPCOS", '52':"200k / 1k - ATC Semitec 204GT-2", '55':"100k / 1k - ATC Semitec 104GT-2 (Used in ParCan & J-Head)", '60':"100k Maker's Tool Works Kapton Bed Thermistor beta=3950", '66':"Dyze Design 4.7M High Temperature thermistor", '70':"the 100K thermistor found in the bq Hephestos 2", '71':"100k / 4.7k Honeywell 135-104LAF-J01", '147':"Pt100 / 4.7k", '1047':"Pt1000 / 4.7k", '110':"Pt100 / 1k (non-standard)", '1010':"Pt1000 / 1k (non standard)", '-3':"Thermocouple + MAX31855 (only for sensor 0)", '-2':"Thermocouple + MAX6675 (only for sensor 0)", '-1':"Thermocouple + AD595",'998':"Dummy 1", '999':"Dummy 2" }
 */


As far as the DS1820 sensor is concerned. maybe you should look at your datasheet for it.

I don't know if this is yours but here is a datasheet from one of them. https://datasheets.maximintegrated.com/ ... S18S20.pdf

Note this.
Maximize System Accuracy in Broad Range of
Thermal Management Applications
• Measures Temperatures from -55°C to +125°C
(-67°F to +257°F)
±0.5°C Accuracy from -10°C to +85°C
• 9-Bit Resolution
• No External Components Required


Also NOTE FIGURE 3 on page 5. 4.7kohms. If you are reading the DS1820 at 4.5kohms, you will get bad data from it as well.

at 110°C you are outside of the accuracy range of the DS1820 sensor. You may even be damaging the sensor by putting it on the heated build plate at that temperature.

If you think your controller board is damaged and giving the wrong data, maybe you should put in a ticket to get it replaced or order a new one, they run around $100 before shipping. Otherwise you need to go over to the marlin firmware and complain to them and explain why the pull up should be 4.5k instead of 4.7k when they said 4.7k is normal. Here is the link for you, so you don't even have to look it up. https://github.com/MarlinFirmware/Marlin

On top of this. just because the thermistor is stabilized in temperature does not mean the heated build plate is stabilized in temperature. It is a big piece of metal and takes time to heat up. the thermistor is located to get the temperature of the heater element not the heated build plate. the heating element will eventually heat the build plate up to the temperature and then the heat will transfer to the glass.

as far as adhesion goes, this is far more complex then just heating the build plate to the right temperature. Heating the build plate actually causes it to warp. you should not need to heat it to 110°C. That is actually likely part of your issue right there. The bed being level, and flat. with the default system for mounting the glass, the glass will warp with the bed as well. Different materials also stick to various methods to assist them. I was using glue stick on buildtek for a while but eventually dropped the buildtek because it does not produce a smooth finish. Now I just use glass and glue stick, preferably extreme glue stick. purple glue stick did not work for me.
RL name: Michael Nolen
printers:
raise3D N2 kickstarter Early Bird
Trinus Deluxe (running smoothieware on Azteeg X5 GT board)
Monoprice Maker Select v2

ABH
Posts: 182
Joined: Sun Aug 14, 2016 7:31 pm

Re: Heat bed temperature anomally

Postby ABH » Wed Dec 20, 2017 3:42 pm

Well, as I wrote in my posting, the beta_25_85 value for the thermistor used in the printer is 4100K and it has 100 kOhm at 25°C.
This is basically all you need to know.

If you have an ohmmeter, then you can measure the resistance of the thermistor and immediatly convert that to a temperature using my formula, or any "online service" you can find.
You don't need to use another Arduino board or care about what pull-up resistor is used in your printer.

If you want to check that the printer is converting the thermistor resistance correctly to temperature, then this is easily done by replacing the printer thermistor with a fixed resistor of known value. You can get 0.1% resistors for no money.
If you want, then you can test the printers temperature measurement using a couple of different resistor values.
Here are a couple of suggestions for resistor values:
100 kOhm -> 25 °C
10 kOhm -> 85.0 °C
4.99 kOhm -> 108.1 °C

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Wed Dec 20, 2017 8:03 pm

I'm not sure I have explained myself correctly regarding the pull up resistor of the thermistor. I only have limited experience in electronics so I might be using the wrong terminology. I understand the 'pull up' resister is the fixed resistor that is attached to the thermistor. So when I am measuring the resistance across the blue to black wires on the thermistor cables then I am reading this fixed resistor (pull up resister). Is that correct? You say it should 4.7K but I measure the resistance as 10k. And for completeness, at 25 degC, I measure the resistance across the black to blue wires as 100K. The beta value I used in my arduino was 4500K. Compare that to the beta value used in the Raise 3d panel of 4100K. Not a significant difference really.
As stated befoe, I intend to fit another thermistor alongside the existing thermistor, that should throw some more light on the problem. The DS1820 sensor I used has an acurraccy of +- 0.5 degress up to 85deg C and then +- 2 deg C up to 125 deg C.

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Wed Dec 20, 2017 8:50 pm

Actually I take back my statement about using the correct beta value of 4100K versus 4500K. That could account for about an 8 degC error.

Can someone try reading the resistance across the blue to black wires on their thermistor fitted to a Rasie3d hot bed. Is the resistance 10k or 4K7K. What should it be? If mine is 10k then should the software not state 'NORMAL IS 10kohm PULLUP' instead of 'NORMAL IS 4.7kohm PULLUP'.
From what I gather so far from all the advice I have received is that either the wrong thermistor is fitted to my hot bed or the software does not reflect the thermistor fitted. I'm probably missing something here.

ABH
Posts: 182
Joined: Sun Aug 14, 2016 7:31 pm

Re: Heat bed temperature anomally

Postby ABH » Wed Dec 20, 2017 9:09 pm

The pull-up resistor needs to be 4.7 kOhm. It is placed on the motion controller board. It normally is.
I know there is also a 10 kOhm resistor sitting on the heatbed. I'm not sure about the purpose of that resistor, maybe it is some check for disconnected cable, but I don't think it is included in the temperature measurement circuit.

The difference between using a beta = 4100K and a beta = 4500K is a 9 degree error at 100 degree.
(assuming that it is a 100 kOhm thermistor and the lower ref. temp. is 25 deg. also for the beta = 4100K thermistor).

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Thu Dec 21, 2017 8:00 am

This post by Jetguy
Re: Heatbed problems
Postby Jetguy » Fri Jul 14, 2017 1:08 pm

supports the assertion/assumption that the 10kohm resistor on the heatbed is the pull resistor.

ABH
Posts: 182
Joined: Sun Aug 14, 2016 7:31 pm

Re: Heat bed temperature anomally

Postby ABH » Thu Dec 21, 2017 10:40 pm

I'm sorry that I seem to have posted wrong information here.

I assumed that the thermistor table given in the Marlin SW was correct.
In the firmware source code it is stated that Raise3D printers are using a linearization table number 8.
It is stated that this table is for a thermistor of type Vishay NTCS0603E3104FXT, using a 4.7 kOhm pullup resistor.

NTCS0603E3104FXT is a 100 kOhm thermistor with beta_25/85 = 4100K. This is without any doubt correct, stated in the datasheet for this component.

I have now checked the linearization table number 8 in the firmware, and it doesn't match a thermistor with these specifications using 4.7 kOhm pull-up.
Instead table 8 in the firmware matches the more common 100 kOhm thermistor with beta_25/85 = 3950K and using 10 kOhm pull-up.
So, this is a clear mistake in the Marlin firmware (comments).
Table 8 cannot be used for Vishay NTCS0603E3104FXT, with 4.7 kOhm pullup.
We must assume that the Raise3D Heatbed uses a 100 kOhm thermistor with beta_25/85 = 3950K and 10 kOhm pull-up,
and the 10 kOhm resistor mounted on the heatbed is indeed the pull-up resistor
and there are no further pull-up resistors present on the motion controller board.

ABH
Posts: 182
Joined: Sun Aug 14, 2016 7:31 pm

Re: Heat bed temperature anomally

Postby ABH » Thu Dec 21, 2017 11:19 pm

Here are some plots showing how well the used linearization table number 8 matches different combinations of thermistor type and pull-up resistor value.
Table8.png

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Fri Dec 22, 2017 3:38 pm

I completed the latest tests with thermistors fitted to the top an bottom of the heat bed. I really can't figure out how to show the table and graph on this forum page so I have attached it as a file.
Attachments
Heat bed test with thermistor.ods
(20.33 KiB) Downloaded 25 times

brid971
Posts: 13
Joined: Tue Dec 12, 2017 7:02 pm

Re: Heat bed temperature anomally

Postby brid971 » Fri Dec 22, 2017 3:45 pm

Sorry, I'm new to forums. I attached the wrong file previously. Think I have the correct one now.
Attachments
Heat bed test with thermistors.odt
(26.56 KiB) Downloaded 32 times

ABH
Posts: 182
Joined: Sun Aug 14, 2016 7:31 pm

Re: Heat bed temperature anomally

Postby ABH » Sat Dec 23, 2017 12:38 pm

A couple of comments to your write-up:

1.
I agree that the thermal conductivity of aluminum is so high that the temperature gradient across the aluminum part of the bed is insignificant.

2.
The Marlin linearization table #11 assumes a pull-up resistor of 4.7 kOhm. You don't mention the size of the resistor you used with the Arduino. It's not likely however that you used 10 kOhm, as you would then measure too high temperatures with your Arduino.

3.
Your observed temperature difference (between Raise3D and Arduino) seems to be proportional to the temperature deviation from the ambient temperature (assumed to be around 22 degC). This doesn't point towards an electronic error.

The RaiseTouch computer is just displaying the temperature numbers it receives from the motion controller board.
The motion controller MCU is measuring the voltage from the voltage divider formed by the thermistor and the pull-up resitor (to +5V).
The measured voltage is nonlinear. The motion controller firmware converts the measured voltage to a temperature value, using the linearization table.
It is extremely unlikely that an electronic error or an error with the Raise3D thermistor can cause a temperature error like the one you claim to have found. All electronic errors (offsets or scaling) would manifest themselves as a nonlinear deviation in the measured temperature, because of the nonlinear relationship between voltage and temperature.

4.
A likely explanation for the lower temperature readings in your Arduino is that your wires connected to your thermistors are not in contact with the heated bed. Not only the thermistor itself, but also the connecting wires needs to be in contact with the surface you want to measure. Otherwise the copper wires will transport heat from the thermistor to the ambient. This effect, which can be very significant, will create an error proportional to the difference between bed and ambient, like the error you have measured.


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