How the pit probe works

Here at pitmasterIQ, one the questions we get asked the most is: “Why does my IQ and smoker thermometer differ so much?”. The answer we give most often is: “Well, it could be any number of things!”.

Some reasons include: Your smoker thermometer may be inaccurate. Your IQ probe may be faulty. But most often, you are just witnessing the normal temperature variations that are common in BBQ pits.

Speaking of faulty pit thermometers, check this out. This pit was loaned to us for a show – this thermometer is brand new! [this is dubbed over the video of the primo thermometer]

Pit thermometers are sometimes accurate when new, but after exposure to weather and temperature extremes often loose what accuracy they began life with.

"Bimetal coil thermometers are about as accurate as a sniper scope on a Nerf gun." Alton Brown, Food Network Star, author of multiple cookbooks, God of all things food

Meathead of amazingribs.com says, "Most grills and smokers come with bi-metal dial thermometers, and they're usually crap. It is not unusual for this design from the 1800s to be off as much as 50°F like the one above (on an expensive and otherwise superb grill). You cannot trust them. I have readers tell me that when they bought a good digital from my list below that they learned their grills were off by as much as 100°F! This is a recipe for well done steaks, late meals, cold food, embarrassment, shame, and ostracism."

IQ probes are super accurate when they leave the factory (0.2% of full scale), but can become inaccurate with exposure to moisture, extreme temperature, and mishandling. If you keep your probe out of the dishwasher, don’t subject it to temperatures over 500 degrees, and don’t pinch the cable under the lid, it will retain its accuracy.

More often than not, what customers are seeing is the normal temperature variations or gradients that occur in BBQ pits. It is not uncommon for a thermometer on the lid to read 25 degrees or more different then the temperature at the cooking grate.

How Not to Measure Internal Temperature

Sometimes customers say: “but I set my ET-732 smoker probe next to the IQ’s probe and the temperature was different – why?”. This is where it gets a bit more complicated. Of course, one or both of the probes could have issues. But assuming they’re both accurate, there must be an explanation.

In order to understand temperature measurement, you must first understand how temperature moves from on object to the other, or THERMODYNAMICS!

There are three mechanisms that describe how one object’s temperature affects another’s. They are: conduction (touching), convection (air moving between and around the objects), and radiation (radiant heating; think warm sunlight).

My favorite example of this came about right after we got our first Gateway Drum Smoker. The first thing we did was to install an IQ110 on it and fire it up. Right away I notice the drum’s thermometer running 15 degrees cooler than the IQ. I knew the IQ was accurate, and since I had taped the IQ’s temperature probe to the drum’s thermometer stem, the only conclusion was that the drum’s thermometer was inaccurate.

To prove this, I removed the drum’s thermometer and placed the stem in boiling water (taking care not to touch the bottom of the pot, only the water). To my surprise, the thermometer was dead on. This left me scratching my head, wondering how the IQ could be off. So I took the IQ to our $1000 dry block temperature probe calibrator and discovered it too was dead on. WTF?

As I thought more and more about this, it hit me. Even though the IQ’s probe was in direct contact with the thermometer’s stem, I had wrapped a 2” wide piece of aluminum tape around the thermometer stem to hold the IQ’s probe, blocking the thermometer’s stem from heating through convection and radiation. The IQ’s probe tip stuck out of the tape about 1”, so it was able to be heated through all three mechanisms. I removed the tape, mechanically coupled the two together so that both could be heated by both convection and radiation, and viola, the IQ and drum then read the same from then on.

How to Measure Internal Temperature

My favorite way to measure internal cooking temperature is by clipping the IQ's pit temperature probe 2"-3" away from the food being cooked, right on the cooking grate. If you get too far away from the food, then you may be measuring hot convection currents that come up from the fire along the side of the cooker. This is particularly true when using some sort of diffuser like in a WSM. If you get too close to the food, then the food's temperature will influence the reading taken by the IQ and throw off cooking temperature altogether. If you're short on cooking grate space, try clipping the IQ's probe to a wooden skewer and sticking that in the food, again keeping the sensor 2"-3" from the food.

If you want to compare your Maverick or Thermoworks to your IQ, follow this procedure: The temperature sensing devices are in the tips of the stainless steel housings. The IQ's is just inside the collar of the alligator clip. Hold the Maverick's probe housing side by side with the IQ's so that both probe tips are next to each other, as close as possible. Now, tape them together, at the back of the probe housings, being sure to leave at least 1" of the housing tips sticking out of the tape. Use heat resistant aluminum tape like that which comes with the Standard Adapter kit. Failure to do this may lead you to believe that one unit or the other is faulty, when in reality it may be just different heating mechanisms at play.

Weber Smokey Mountain Experiment

Ok, so now you've heard if from Alton Brown, Meathead, and me, but you still don't believe it. What? You want proof? Whatever.

To prove this for once and all, I did a scientific experiment to analyze the different temperature zones in a 22.5" Weber Smokey Mountain. I used an IQ120 to regulate cooking temperature, set to 250˚. An Omega Engineering (www.omega.com) 8-channel thermocouple temperature logger was used to capture temperature in five zones inside the cooker. A Maverick ET732 was also used to capture and compare temperatures. I knew the lid thermometer was off. I knew there were temperature variations inside the cooker. But I never quantified any of there. Man was I surprised!

The five zones are defined as follows:

1	BOT MID	Lower cooking grate, in the middle.
2	TOP MID	Upper cooking grate, in the middle.
3	TOP FAR MAV	Upper cooking grate handle, placed in rear of cooker. Collocated with ET732 probe and tightly coupled.
4	TOP NEAR IQ	Upper cooking grate handle, placed in front of cooker. Collocated with IQ120 probe and tightly coupled.
5	PIT THERMO	Tightly coupled to the lower 1/3 of the stem of the WSM's thermometer.

The WSM was ran with the water pan in, but dry. We normally run it with a foil covered clay saucer in it to act as a heat diffuser. It was started with a full ring of Ozark Oak lump using the Minion Method. The cooker started out leaning to rear, due to the slope of the pavement. The top damper was to the rear, with the pit thermo in the front.

From 1:55 till 2:40, the cooker behaved as follows:

First off, the pit thermometer ran about 40˚ low. This cooker is only about 4 years old, and has always been stored in a garage. Forget about the pit thermometer!
The top cooking grate runs about 5˚ hotter in the center than the bottom cooking grate in the center. Makes sense - heat rises.
The outside circumference of the top cooking grate runs about 15˚ hotter than the center, on average. Makes sense, heat rises, up the inside walls of the cooker due to the water pan diffuser creating an indirect cooking environment.
The top inside of the dome, where the pit thermo is, runs about 15˚ hotter than the top grate. Again, heat rises. Keep this in mind when cooking tall things like beer can chicken; the breasts will be in a slightly hotter environment than the legs. We should be cooking these upside down!
Finally, the front of the top grate is running about 15˚ hotter than the back. This is presumed to be due to the cooker leaning to the rear. Again, heat rises, straight up.

At 2:40, the lid was rotated 1/2 turn, moving the damper to the front and the pit thermo to the rear. This is what we observed:

The top of the dome, where the pit thermo is, dropped 20˚ and then started recovering. I'm not sure why, and it's been a couple of months, but I'm pretty sure I rotated the lid without lifting it. This may remain a mystery. Note the correlation with the pit thermo - it dropped too.

At 3:00, the cooker was tilted the other way, now leaning forward. These results are interesting:

Before, the front of the top cooking grate was hotter than the rear. Now they are running more closely together.
All temps dropped, but I believe this is due to disturbing the hot embers as the cooker was leaned the other way.

So what have we learned?

I can't say it any better than Meathead: "Most grills and smokers come with bi-metal dial thermometers, and they're usually crap."
Heat rises. In a vertical cooker, you should make sure it is level for the most even heating.
Heat rises. It's hotter the higher up in the WSM you go.
Heat flows up the inside walls of the WSM due to the water pan.
If you think too hard about temperatures inside a cooker, you'll get a headache.

Types of Temperature Probes

There are predominately three types of temperature probes used commercially and in industry. The lowest cost, used most often in commercial products, is the thermistor. While exceptions exist, they are generally regarded as less accurate and as having s smaller useful sensing range than other devices.

By far, the most widely used temperature probe in higher-end commercial and industrial products is the thermocouple. There are many different types of thermocouples, but in general they have a wide sensing range and are only accurate to a few percent. Their main advantage is ease of construction; they are simply two wires made of different metals welded together on one end. Their main disadvantage is they require calibration for high accuracy. If you change a probe in the field, you must recalibrate in order to retain system accuracy.

Gaining ground is the platinum RTD. The platinum RTD is generally regarded as having a medium sensing range, in between thermistors and thermocouples. They are however inherently very accurate. Systems built around RTD probes generally do not require recalibration when a probe is replaced.

Temperature Probe Calibration

RTD probe technology was chosen for the pitmasterIQ product line because its sensing range is more than enough for cooking, and because of its inherent accuracy. All other pit controller manufacturers use thermocouple probes, and if you want maximum accuracy, you must calibrate each probe to your system. With pitmasterIQ's products, you never need to calibrate your probes!

Temperature Probe Validation

Again, stolen from Meathead (amazingribs.com/BBQ_buyers_guide/thermometer_buying_guide.html):

"Boiling water. Bring a pot with about 3" of water to a boil and insert the probe. It should read about 212F. Notice the key word "about". The exact reading can vary slightly with air pressure (factory calibration is based on one atmosphere, about 30" of mercury). Minerals in tap water can cause minor variations, so use distilled water if you want to be absolutely precise. I just use tap water. Remember that water boils at lower temps at high altitudes. The ThermoWorks website has a nifty calculator that helps you determine what your boiling point is."