Thermal Couple Vs Thermal Probes (thermistors)


 

Nathan Stock

New member
I have heater meter 4.3 with thermalcouple for the pit probe. I have noticed that when I put one of my extra thermal probes (thermistors) in a similar location to the pit probe it will read about 15 degrees cooler.

As a test I put the 3 - Thremalprobes ( thermalworks pro series) and the two thermalcouples into the oven and set it to 225 with the heatermeter & multi meter for the other themalcouple and all the temperatures tracked with a few degrees. The is also true when they are at ambient temperatures.


I am wondering if the difference in temperature I observe in my smoker are do to the air current and the responsive difference between a thermistor and a thermal couple?
 

Bryan Mayland

TVWBB Hall of Fame
I would try wrapping the two in aluminum foil together (with paper between them if the thermocouple is exposed or the metal shield is connected to one of the two pins) so they experience closer temperatures. If they track together closely in the oven though, I'm not sure why they would read differently anywhere else. If the thermocouple was more sensitive, you'd see on BOTH sides of the thermistor probe, bouncing back and forth where the thermistor is averaging the temperatures due to its mass.
 

Nathan Stock

New member
I collected a little bit more data and have attached the log below. I put the thermalcouple & two thermal probes in a cold oven and set it to 225. They started within a couple of degrees of each other but during the warm up they diverged (thermalcouple and thermal probes by ~14 degrees). From the data you can see that any time there is a temperature change they diverge. Late in the log I turned on the convection and then I wrapped the thermal couple and probe #2 together in a tin foil packet.

I have surmised that the thermal couple is slower to respond then the probes. This can be noted both during the warm up and when I opened the oven to wrap 1 thermal probe and the thermal couple together.

I suspect that the temperature would diverge more in my UDS as that is less table then my electric oven.

Oven.png
 
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Bryan Mayland

TVWBB Hall of Fame
I would reach the same conclusion from that graph, that the thermocouple is reacting more slowly and that it reads slightly higher than the thermistor. You can adjust the gain of the thermocouple by changing the mv/C value in the webui. Standard is 5, higher values will lower your temperature, lower values will raise it, and the effect is amplified the higher the temperature is.

I'd be curious to know what your 3.3V voltage is though if you have a chance to measure it (just measure on the probe expansion header on the right side where it says 3V3 and GND). HeaterMeter assumes that the 3.3V power is exactly 3.300V but it has no way to measure it. The voltage regulator has a pretty wide tolerance though so it could be as low as 3.135V or as high as 3.465V, and these could generate much higher or much lower thermocouple temperatures. This is somewhat mitigated by the fact that the bandgap measurement is lower when the 3.3V power is higher, but the bandgap has an even larger tolerance for its voltage, so it isn't reliable.

If you could measure the 3.3V power voltage, it would provide me another data point for trying to figure out a better correlation. I'd also need the bandgap measurement. You can find this by using the device's LCD menu and going to the "Manual Mode? Yes/No" menu, then hold the LEFT button until the display switches to the diagnostic screen, displaying the Pit temperature dump. Short press left again to get to the next diagnostic display and the bandgap is displayed on the bottom left like "BG332".

The easiest way to get a temperature reading that you can have more confidence in from the thermocouple is to put it in boiling water and then adjust the mV/C up or down slightly until the thermocouple reads the correct temperature, somewhere between 4.75 and 5.25 most likely.
 

Nathan Stock

New member
I measured the 3.3V from the pin labeled 3.3 to the ground connection on the board and got ~2.99volts

I wasn't able to figure out how to get to "manual Mode"... I scrolled thought all the option and can not find it.

I did calibrate the thermacouple with boil water. I spot checked the boil temperature vs altitude and it should 208.5 - 209.5 ( based on my google earth altitude) The thermistor read 209, but the thermal couple was higher so I tweaked the value to 5.09 and now everything lines up at boiling temp.
 

Bryan Mayland

TVWBB Hall of Fame
Thanks for the info on the voltages. "Manual Mode? Yes/No" is on the device itself, using the buttons press RIGHT twice. It goes Setpoint -> Manual Mode. On that screen hold the LEFT button until it changes to a screen filled with numbers, then (short) press LEFT again to see the BGXXX display. This is on the firmware from within a year or so ago, so if you're on "v14 Release" it won't be there.

Wow 2.99V on the 3.3V seems a little low, it should be 3.125V or higher but I'm not sure what would cause it to be lower.
 

Nathan Stock

New member
Thanks for the info on the voltages. "Manual Mode? Yes/No" is on the device itself, using the buttons press RIGHT twice. It goes Setpoint -> Manual Mode. On that screen hold the LEFT button until it changes to a screen filled with numbers, then (short) press LEFT again to see the BGXXX display. This is on the firmware from within a year or so ago, so if you're on "v14 Release" it won't be there.

Wow 2.99V on the 3.3V seems a little low, it should be 3.125V or higher but I'm not sure what would cause it to be lower.
Bryan- That was a typo on my part... it should have said 3.299 so it is right on the money
 

Nathan Stock

New member
Is manual mode the same as "manual fan mode"
- this is what I get when i press the right button twice. If I hold the left button for a long press it just takes me back to home screen with Pit temp & fan %

20201006_224842.jpg
 

Bryan Mayland

TVWBB Hall of Fame
Oh yes, "Manual FAN Mode?" my bad. If it doesn't go to the diagnostic screen then it is an older firmware since that was added in 20180718 firmware (haha wow *two* years ago? How old am I?)

Are you not on the Snapshot firmware? Everyone should be on Snapshot, it's the best!
 

JohnR

New member
I hope I'm not completely hijacking this thread, but it is discussing thermocouple accuracy which I have been trying to improve on my unit.

When testing my unit, a Thermoworks Thermapen is used as a control along with 3 Thermoworks Pro Series probes plugged into the Heatermeter for extra comparison.

All Tests are done with the probes submerged in a large pot of water

At "room" temperature - the Thermapen reads 60.0
All 3 Pro Series Probes all agree at 59.75 - 60.00
The thermocouple (also from Thermoworks) reads 59.0 (slightly low)

Heating up to boiling temperature (211.0 at my elevation)
The Thermapen reads 211.0 (correct)
3 Pro Series Probes all register at 211.0 - 211.4
But - the thermocouple is high here at 214.7

I could modify the mv/C for the thermocouple at boiling temperature, but this then increases the error at other temperatures.

My "3.3 voltage" provided by MCP1700-33 on the heatermeter at the probe contact points measures 3.275v (a little low) measured with a Fluke Multimeter. The Heatermeter states my Bandgap referance is (BG349). I have no noise problems as this test was measured with a 12v battery source.

Where are the most probable sources of error that I might be able to tract down at this point. Is the fact that the 3.3 volt reference is little low a possible source? Is there a reason why a MCP1702-3302E was not used in design as it has a better voltage regulation accuracy of 0.4%?

Adding to the info, the SMD part of this board was prebuilt (purchased) from Bryan Mayland so it should be good. I have recleaned the whole board multiple times with 99% isopropyl and used MG Chemicals 4140 flux remover.

Am I just crazy in looking for this much accuracy? It just appears that the Pro Series Thermistors are more accurate so far in my case. The type K Probes "should" be more accurate though and benefit from being more robust. Its just the circuitry that drives the thermocouple is more complicated and temperamental.

Cheers,
John
 

Tom O.

TVWBB Member
"Am I just crazy in looking for this much accuracy?..."

I am not a clinical phycologist nor do I play one on TV nor did I sleep at a Holiday Inn Express last night, but I would have to say yes... you are crazy. The BBQ doesn't care.

Hope that helped.

Tom
 

RalphTrimble

TVWBB Diamond Member
If we were talking about meat probes being off 3-5 degrees in 100-200F range that may be something to be concerned about, 'cause you're relying on that temp to estimate doneness.... but the pit temp being off a couple degrees is not a big deal from my point of view. The pit temp varies quite a bit anyways, depending on where the probe is placed in the grill, so it's all a bit subjective.
 

Bryan Mayland

TVWBB Hall of Fame
My "3.3 voltage" provided by MCP1700-33 on the heatermeter at the probe contact points measures 3.275v (a little low) measured with a Fluke Multimeter. The Heatermeter states my Bandgap referance is (BG349). I have no noise problems as this test was measured with a 12v battery source.

Where are the most probable sources of error that I might be able to tract down at this point. Is the fact that the 3.3 volt reference is little low a possible source? Is there a reason why a MCP1702-3302E was not used in design as it has a better voltage regulation accuracy of 0.4%?
The MCP1700 variant wasn't exactly selected for this application specifically, but came from being the part I had on hand because it had lower dropout voltage. It was also a part you could find at a variety of places, which was important because the design pre-dates the ubiquity of electronics components we're blessed with today. I'm sure there are much better linear regulators in the same price ballpark, as these parts are optimized for low quiescent current draw which is something we don't really care about in this always-powered application. I do think that the output voltage is good enough for our application, but if I were going to design the whole thing from scratch again I would definitely look at other options.

The calculated thermocouple output is dependent on the 3.3V regulator and the code expects the AVCC voltage to be exactly 3300mV, so any variation from that is going to cause a little error. The bandgap voltage is measured with AVCC as the reference, and then the bandgap is used as the reference when measuring the thermocouple amplifier output. Your BG349 means the bandgap is calculated to be 349/1023 * 3300 = 1125mV. A temperature of 25C is 125mV and we'd get the expected ADC reading of 113 (113.77). Because the 3.3V AVCC is actually 3275mV, the actual bandgap voltage is 349/1023 * 3275 = 1117mV and that same 25C reading is now 114 (114.59) leading to a temperature error of around 0.2C. The error can be corrected by changing the mV/C to 3300/3275 * 5.000 = 5.038mV/C.

All that said, the precision of the temperature measurement isn't too critical given all the other uncertainties in the system. Being consistently off by 0.2C isn't a big deal when the temperature inside the smoker can vary by 10-20C depending on where you measure the temperature, the state of the meat, if you use a water pan, etc.

The more data I have to look at the more upset I get about why we can't get better temperature control with all this technology. I used to advocate for the dome being the best place to measure the temperature but the testing was done without meat involved. I've since found that if I put a single rack of ribs (St Louis style) on my Big Green Egg, 225F at the dome can be as low as 180F an inch away from the ribs at grate level (45F error). I bumped up to 240F and later 250F the error was just 35F. As the ribs approached done-ness (about 1.5 hours left), the error inverted and now the temperature at the grate was 15F above the dome temperature. At one point, one grate-level temperature was 8F above and another was 7F below the dome temperature-- an error of 15F between two things you'd expect to be the same. The whole time, HeaterMeter kept the temperature within 5F of the setpoint, but the meat experienced a range of temperatures spanning 60F! The ribs were delicious, smoked and cooked perfectly, but the question is What was the actual temperature?
pdiff-ribs65.png
 
I've wrestled with this as well, actually I use 3 of the probes to measure internal temp and one for the meat just to feel better about accurate results. There was a good scientific analysis on evaporation from the meat and the mass as a heat sink and the effect that would have on temps inside a smoke chamber. At the end of the day, I don't think all this tech is that critical. I've had ribs done at 275 that were the same to me as ones done at 225.
 

Darren L

TVWBB Member
FWIW, I believe thermocouples encased in a housing are slower to respond because the junction isn't in direct contact with the heat. The housing acts like a thermal mass. Try an exposed bead TC.
 

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