The readings from different probes and roasters will always vary. However, learning the variables which affect individual roaster probe readings and how to control these can help you with consistency in roasting, data, and quality.

Join us as we look at how to better understand roaster probe data, how different probes work, and how to help ensure your readings are consistent.

Lee este artículo en español Consejos Para Usar Las Sondas de Las Tostadoras de Café

Roaster Probe Position And How to Understand Their Data

A probe is a small electric device that is placed inside a coffee roaster to measure temperatures. The information is then transformed into data and graphs which help roasters to track and document their roasts.

Coffee roasters can have a number of different probes fitted to record different temperatures.

First, you may find a probe that monitors the environmental temperature, which is the air temperature inside the drum. This probe is usually located in the front and centre, on the face of the drum. A lot of the heat transfer in roasting is through the air in the drum, so, following environmental temperature is key to controlling a roast. 

Another important probe measures the bean temperature. Although roasters cannot know the exact temperature of each individual bean, placing a probe that is immersed in the bean mass will help provide the most accurate data. The probe should be placed lower down in the roaster, either on the bottom right or left depending on which direction your drum spins. 

Bean temperature is used to record and analyse Rate of Rise (RoR), which measures the speed at which the temperature of beans is changing over time. Roasters need to find the right ROR to ensure that they get their desired flavour profile without letting the bean heat up or cool down at the wrong point. 

A third probe can be used to measure the exhaust temperature, the temperature of the air as it leaves the drum. This probe is placed at the point in which air is streamed out of the roasting machine. Fay Kamanis, director of the coffee roastery Padre Coffee in Australia, tells me that reading this data along with the input of air “can determine how much heat/air you choose to apply or take away.” 

Probes help with reading data as you roast as well as providing roasters with means to replicate roasts that have worked well. Data analysis has become a tool in replicating great quality coffee.

You may also like: A Guide to Bean Temperature & The Coffee Roasting Curve

The left probe will read the environment temperature, whilst the right will give us the bean temperature. Credit: Chris Nance of Coffee Tech.

Which Probes Should You Use?

You can choose a number of different types of probes which can help provide different readings. 

The Resistance Temperature Detector (RTD) probe measures electrical resistance and uses that to determine the temperature. The higher the temperature, the higher the resistance of the flow of electricity, which is then measured on a sensor and converted into a temperature reading. 

RTDs are considered a great option for recording bean temperatures as they’re generally responsive and accurate. They operate well between 0°C/32°F–250°C/482°F, so may not be a good option for recording exhaust temperature.

Another popular probe is the thermocouple. Thermocouples are made up of two different metals within the probe. They work by measuring how the two different metals respond when the temperature changes. These create a temperature-dependent voltage as a result of heating – this voltage is then interpreted to measure temperature. 

Type J or K thermocouples are considered popular among roasters and are also recommended by Scott Rao. Type K thermocouples can work well at higher temperatures and can, therefore, be used as a probe throughout the entire roaster.

Fay explains that their best results are using J Type 3mm thermocouples: “They are extremely durable and give fast and accurate readings. There’s no right or wrong though – it all depends on your machine, roasting style, and personal preferences.”

There are different considerations to make when choosing a probe for your machine, such as the diameter which affects its response time; how well it records at different temperatures, and the cost of the probe, too. Each probe will work differently and will suit different roasters and methods. So, research into probes is a valuable consideration for roasters.

Top Tips For Using Roaster Probes

You may find that roaster probes seem inconsistent or aren’t measuring expected temperatures. Here are some top tips for using and reading temperatures from roaster probes.

Understanding The Roaster & Probe

Consistency is critical in roasting to achieve the desired roast profile and ensure batches of coffee are not wasted. However, varying readings from similar machines will always happen.

Fay, who has roasted on a number of different machines including Has Garanti, RoastMax, Pertroncini, Probat, and Loring, tells me, “No two roasting machines are the same. The important thing to know is that you have consistency in temperature readings with the machine you’re using as opposed to what you previously roasted on and expect that they will most likely be different.” 

When using a new machine, a roaster should use their senses as well as temperature probes to help guide the roast and adjust the curve. Sometimes roasts will follow data perfectly and not turn out as expected, so always trust your own instincts.

Fay says, “Taste is ultimately the best way to assess your roast. The profile will act as your first quality indicator, then confirming via taste is our protocol – especially when you make adjustments to an existing profile.” 

Different probes types and diameters will also result in different readings. Ram Evgi, the founder of roaster manufacturer Coffee-Tech Engineering, based in Israel, tells me, “Every change in the diameter of the measuring probe will affect the reading and the speed of response, a smaller diameter probe will read higher and react faster.” 

Scott Rao points this out on his blog. Using the example of a 1.5mm probe placed next to a 3mm probe during the same roast, Scott Rao explains how bean probe diameter and responsiveness affect the time and temperature data of a roast. As a roaster, you should pick the most appropriate probe for your machine, and use this consistently.

Installing Your Probe

Where your probe is positioned, and how far into the roaster it’s placed, will also affect the reading. 

Ram says, “Every change to how deep the measuring probe is inserted will tremendously affect the reading result. The deeper the probe goes, the higher the temperature it will read at that given moment.”

This can also explain why the same machine may have different results using the same probe. Ram tells me, “Every installation, even with similar machines, will behave slightly differently.”

Maintenance of Roast Probes

Measures in cleaning and maintenance are integral to ensure every part of a roaster is in good working order. 

Fay tells me, “Longevity and accuracy of your probes will rely on regular maintenance – how often will depend on how much volume you roast. You will see ‘roast’ build-up. Cleaning with a cloth or gentle scourer and some warm water will usually do the job.”

Replacing probes is also an option. If you’re after the same consistency, opt for the same type of probe with the same diameters. Remember to install the probe with the same immersion inside the roaster to ensure consistency.

Older roasters that were originally designed without the function of using probes do not have to go without either. Ram tells me that the process of adding roasting probes retroactively is relatively easy. 

You may also like: How And When to Clean Your Coffee Roaster

Ram Evgi calibrating one of the probes installed on a Ghibli roast machine. Credit: Chris Nance

Roaster probes provide valuable information that is represented in roast data. However, each probe will read temperatures in different ways and at different speeds depending on its design or how it’s placed in the roaster.

So, familiarise yourself with the machine, probe, and potential variables to ensure consistency in temperature readings and data. But don’t forget, trust your instincts too, and use your senses alongside data to roast batch after batch of high-quality coffee.

Enjoyed this? Check out A Guide to Achieving Consistency in Coffee Roasting

Photo credit: Chris Nance, Georgia Folker, Padre Coffee Roasters.

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