When it comes to the welding preheat measurement or interpass temperatures, you need an accurate way to measure the temperature.

Efficient preheat and interpass measurements can be achieved with a variety of high-tech and old-school devices. Do you use the latest digital thermometers or the classic thermo crayons, it’s difficult to choose.

What Are Thermo Crayons?

Thermo crayons are temperature indicating crayons that are used to get an accurate reading of a specified temperature.

First, you need to select the exact temperature desired. While you can purchase a thermo crayon in a wide variety of temperatures, each individual crayon is useful only for an exact temperature reading.

Once the minimum temperature is reached, or exceeded, a thermo crayon will melt. This allows you to get a very quick and convenient visual cue that the metal has reached the desired or rated temperature.

You can hold the crayon to the metal to check immediately, or you can make a chalk-like mark across the base material in order to discover when the entire length of the material reaches the ideal preheat temperature. This allows you to get a hands-free reading of surface temperatures while you are working with a heating torch or other device.

two thermal crayons

Thermo Crayon Uses

Although there are many other uses for measuring high temperatures, it’s critical to get accurate temperature readings during the preheat stage of welding.

Whether you are using TIG (GTAW) welding, automatic welding or gas metal arc welding (MIG), many base materials, like certain Quench and Tempered Steels, require you to raise the temperature in order to prevent hardness in the heat affected zone and prevent hydrogen assisted cold cracking in joints of larger thicknesses. With Q&T materials the maximum interpass temperature must be monitored to ensure it is not over-tempered (softened) this is result in loss of tensile strength. Otherwise, your welding joint may not have the required mechanical properties.

There are other critical factors to achieving mechanical properties, such as the consumable electrode type. Thermo crayons, however, are designed to help with temperatures indiaction and can be purchased for a specific temperature. Normally you need a series of temperatures 50C, 75C and 100C. The temperature you need depends on the base material type, thickness, welding process, and consumables.

They mark almost any base material, from steel, brass and aluminium. This temperature indication technology is clearly useful when welding.

Discover some of the pros and cons and determine for yourself whether thermo crayons are outdated or still useful for modern welding.

Advantages of Thermo Crayons

There are a number of positive benefits to using thermo crayons instead of other temperature reading alternatives.
Some of the advantages of using a thermo crayon include:

  •      Affordable: Premium digital thermometers can easily cost up to ten times the amount of a thermo crayon. If you are looking for a cost-effective way to reach a set temperature point, thermo crayons are the most affordable option.
  •      Hands-Free: Simply make a mark and begin heating the material. Because you don’t have to constantly hold a thermometer to the material, you’ll be able to work on another project or use both hands to operate a heating torch while you heat the project.
  •      Accurate: Thermo crayons are typically rated to be accurate within 3 degrees Celsius.

Blue thermo crayons

Disadvantages of Thermo Crayons

There are, however, reasons why companies continue to innovate and create new types of welding thermometers.

Thermo crayons can be:

  • Inflexible: You need to purchase a thermo crayon that is designed for a single temperature. Because of this, many welders will end up with a wide range of crayons for various materials and welding projects.
  • Uninformative: While a thermo crayon melts easily when it reaches the preset temperature, it won’t give you any other information about the temperature only an indication. It can be difficult or impossible to determine how close you are to the desired temperature, or how much higher than the desired temperature you’ve reached. This can be detrimental when welding certain types of base metals that suffer loss of mechanical properties when overheated.
  • Accuracy: Other thermometer options, like infrared thermometers or contact thermometers, allow you to take temperatures more accurately. There are some accuracy issues that you need to be mindful of with infrared thermometers, such as emissivity and standoff.
  • Fragile: Often thermo crayons break from rolling around in a tool box or bag or from being dropped.

Learn More at Technoweld

Many professional welders and other industry professionals still utilise thermo crayons for accurate temperature measurements because they are convenient. However, there are many exciting thermometer designs that make compelling alternatives.

For more information about different welding tips, tricks and tools, contact Technoweld. Our training and consultations programs can help you improve your welding technique.

Whether you choose thermo crayons or another thermometer option, you’ll be able to implement all the skills, training and supervision of our complete courses for your welding career or company.