All About Tolerances In Conductive Heaters

There are normal variations in the dimensions and electrical properties of heaters we build, and it’s good to know what they are so there are no surprises.

First, a few words about tolerances in general. Tolerances are deviations from nominal values, be they lengths or resistance values, in the case of our heaters, that are deemed acceptable. Tolerances exist because of variations in manufacturing processes. Nothing can be manufactured perfectly the same way every time. If you measure closely enough (to enough decimal places), you’ll always find a difference between two manufactured parts. Some tolerances are present because of things that we do in the process, like swaging, and others are passed on to us from suppliers that we have to live with, like the resistance tolerance of a spool of resistance wire, measured in Ω/ft.

Varieties of Tolerances we’ll cover in this article are: Length, Wattage, Resistance, Diameter, Width, Leadwire Lengths, Protection Lengths, and Camber.

Length: Starting off with Hi-Temp Cartridge Heater lengths, the tolerance of a swaged cartridge heater length is ±2% of its length or 1/8”, whichever is larger. For example, the length tolerance of a 12” long heater is 12” x 0.02 = 0.24” or about 1/4”. For a 3” long heater, it would be 3 x 0.02 = 0.06”. Since 1/8” is larger than 0.06”, the tolerance on that heater is 1/8”. The reason for that tolerance being length-dependent is because of the swaging operation we perform on every heater. There’s a small amount of uncertainty as to the exact length heaters will swage to due to there being so many variables in the components of what is being swaged. That variability results in heaters that, when built and assembled identically, result in slightly different final lengths. That tolerance of ±2% of the length accounts for that variability.

For Standard Cartridge Heaters (unswaged, loose fill heaters), it’s a lot easier. The tolerance of the length of those heaters is ±1/16” regardless of the heater length. The reason for that is because we have a lot more control over the heater length. Since the heaters aren’t swaged, there’s no change in their length during our processing and no predictions of the final length are required. What you cut (to length) is what you get, and we can cut tubing to a precision of 1/16” over a heater of any length. The same applies to Ceramic Strip Heaters, Ultima Strip Heaters, and Permaheat Strip Heaters.

For some band heater and other strip heater dimensions, we have similar control as with Standard Cartridge Heaters. What you cut is what you get. For heaters with widths <=4”, the tolerance of that width is ±1/16”. For heaters over a 4” width, you’re looking at a tolerance of ±1/8”. That applies to HT Mica Bands, Better Bands, HT Mica Strip, and Better Strips.

The ID tolerance on band heaters, in general, isn’t as much of a concern, nor is it as easy to specify because there’s spring-back when it comes to forming the ID of a band heater. If your heater clamps around your object properly and clamps tightly in place without the gap shrinking to 0”, then you’ve got a good fit.

Wattage and Resistance: Now we move to an electric property of all heaters. When you purchase a heater at a certain wattage, say 1000 watts at 120 volts, there’s a variance on that wattage of +5%/-10%, meaning the wattage could vary from 1050 watts down to 900 watts. The variance actually resides with the resistance where the resistance tolerance is +10%/-5%, which translates to the aforementioned wattage tolerance when a known voltage is applied. The reason for the overall variance is because of the variance in many of the materials we use, along with the processes performed to make your heaters. There’s a tolerance of ±5% of the resistance (in Ω/ft) in the plain resistance wire we purchase. There’s a variance in the winding process that puts wire around a ceramic core or piece of flat mica. There’s a variance in the amount of resistance change when a heater is curved to shape or swaged to diameter. Variances stack up, so we end up with a final resistance range of 15% around the nominal value, whether you’re talking about wattage or resistance. We really build resistance devices where we can tell you the wattage you’ll get when a certain voltage is applied. We control the resistance as best we can, and knowing the voltage you’ll be applying, we can determine the wattage you’ll get out of the heater, along with the tolerance of that wattage. Why is the wattage tolerance +5/-10% instead of ±7.5% if the tolerance range is 15%? We do that to be on the safe side. We would rather have a heater operate with a lower wattage than one that’s much too high.

Can we do anything to make the tolerance smaller? Yes, but it’s not inexpensive. Sometimes the only way we can do it is by sorting. Finished heaters that do not conform to the tighter tolerance are simply scrapped and built again. That drives up costs, which are then passed on to the end customer. We could also attack the problem further up the production stream by asking our suppliers to provide parts built to a tighter tolerance. Guess what happens? The costs go up. There’s always a trade-off, and no such thing as a free lunch.

Diameter: Changing gears again, when it comes to cartridge heater diameters, we size our heaters to an ideal heater size that’s slightly undersized to fit into a hole it’s intended for, along with a tolerance. For example, a 1/2” diameter hi-temp cartridge heater is actually provided at a 0.495” diameter with a tolerance of 0.002”. The heater is named after the hole it fits into, not its actual size. Due to tool availability, it’s a lot easier for a machinist to drill a 0.500” diameter hole than a 0.505” diameter hole, which would be required if we provided a heater at 0.500” in diameter. And besides, “a 1/2” diameter heater” rolls off the tongue a lot more easily than “a 0.495” diameter heater.

Here are lists of diameters for the two varieties of cartridge heaters. 

The number in parentheses is the decimal equivalent of the imperial fractional diameter, and the number after the dash is the true size we furnish the heaters, followed by the tolerance. With little exception, we can control the swaged diameter of a hi-temp cartridge heater to ±0.002”. For the standard cartridge heaters, we don’t swage those heaters, so we’re at the mercy of the tubing tolerance as we receive the tubing from the supplier. Those tolerances are typically a bit bigger.

Leadwire: Next is an easy one; Leadwire and Protection Length.

Across all product lines, leadwire lengths may vary as much as +1/-0”. Leadwire Protection, like braid and armor, has the same tolerance. We typically provide 2 to 4” more leadwire than the protection length when a difference is not specified, and those tolerances may only become apparent when things get long (say over 3 feet) and stretch of the items (protection, not leadwires) becomes a factor.

Camber: The last item to mention that has a tolerance is something called camber. It pertains only to hi-temp cartridge heaters and is the measurement that the heater is away from being perfectly straight. It’s the bow in the heater length that is imparted during the swaging process. If present, it starts becoming noticeable in heaters that are at least about 2 feet long. If you were to roll the heater along a perfectly smooth surface, like a granite surface plate table, you might see a point along the rotation where the gap between the center of the heater length and the table is at a maximum. That gap is the camber and can be measured with a feeler gauge. It should not exceed 0.020” per foot of heater length. There is no need to worry about installing a heater with a slight amount of camber as the heater will flex and straighten as it is inserted into its hole.

As you can see, there are several tolerances to consider when specifying and sizing conductive heating elements. Manufacturing tolerances should be considered to ensure parts fit together properly and behave as expected. If you ever have questions about how heaters will fit into your application properly and if any of the tolerances mentioned here will have an adverse effect on heater performance or life, please contact one of the members of engineering at TUTCO, and we’ll get you the information that you need.