Small Diameter Miniature Pencil Heaters

Small Diameter Pencil/Cartridge Heaters–Description

Split-Sheath Design
Uniform Heating
Lengths From ½ Inch to 6 Inches
1/8" and 4mm Diameters
Unique Hot Tip

Patented high-temperature design provides these advantages:

  • When energized, the patented split-sheath expands into contact with surrounding bore for maximum heat transfer, fast response.
  • When de-energized, the split-sheath contracts for easy, slide-out removal. Sunrods are guaranteed never to sieze in the bore.
  • Uninterruped hot zone and fully heated tip provides unmatched heat distribution without cold spots. (See upper photo).
  • Slender continuous leads simplify wire management. High flexibility allows free motion when heating "floating" components.
  • The result is a miniature cartridge heater offering longer life, uniform temperatures, and easy installation in the smallest devices.

    NPH’s 1/8" Cartridge Heater with Swaged-In Leads-Simplifies Wiring

Extending Miniature Heater Life:
Designing cartridge heaters with reduced watt-density multiplies service life.
The industry-standard warrantee for electric cartridge heaters is 2000 hours, or one year on single shift. This is a reasonable life expectancy for many applications. But some applications demand much more. Life expectancies of five, seven or even ten years are not unreasonable for some mission-critical applications:

  • Blood warmers used in medical theaters must never fail
  • Battery conditioners for EVA spacesuits on the International Space Station must last ten years before replacement, due to payload costs
  • Here on earth, high throughput semiconductor chip testers must operate without significant downtime for the 5 year life of the equipment. For these applications, a service life of 50,000 to 90,000 hours is a requirement that is both reasonable and attainable.


How can it be done?
For every heater power loading (See "Calculating Watt-Density”, at bottom) there is a maximum operating temperature that will guarantee 2000 hours life. This is the "Critical Temperature” for that power loading. (See chart below).

Power Chart – System Temperature versus Maximum Watt-Density

Critical Temperature (F): 200 300 400 500 600 700 800 900 1000
Maximum Watt-Density : 365 345 310 275 245 225 170 150 130

Go above the Critical Temperature by 100 degrees and life will be cut to a third, to 666 hours. But go below it by 100 degrees and heater life will be tripled, to 6000 hours! We can use this relationship to determine the watt-density at a given system temperature that will yield thousands of hours of extra life.


For Example…
Let’s suppose that your system requires a processing temperature of 500 degrees F with an input power of 80 watts. A 1/8” by 1” cartridge heater could provide the necessary wattage, and would have a power density of 270 watts-per-square-inch. As can be seen from the power chart, a heater with a power loading up to 275 watts per square inch would be acceptable. Heater life would be a respectable 2000 hours.

But what happens if we use a utilize a 1 ¼” heater, just a ¼” longer? The heater’s power loading is lowered to 210 watts-per-square-inch. The Critical Temperature is raised by 200 degrees. The heater is now operating 200 degrees below the new Critical Temperature and heater life is increased to 18000 hours (2000 hours x 3 to the 2nd)!

Maximum Watt-Density Lowered Watt-Density
How Do You Reduce Watt-Density?
Your heater watt density may not have to be reduced. Many miniature heating applications inherently require relatively little wattage, due to their small mass. This often results in a minimal power loading on a 1/8” miniature heater. In these cases, miniature cartridge heaters are often under-loaded by 200 to 400 degrees, relative to the Critical Temperature. Their 2000 hour life expectancy* may be multiplied up to 81 times! Increasing Surface Area to Reduce Watt-Density
1. Maximize your heater length. Increasing the length of a 1” heater to 1 ¼” (just a ¼”) can increase the active area* by 33%, reducing power loading by 25%.
2. Consider using more heaters. Two heaters dividing the load will reduce watt-density by 50%.
3. Take advantage of any available space to install a supplemental heater. Adding a ½” long
heater to an existing 1” heater can reduce power loading by 25%.
Calculating Watt-Density
Watt-density is the power loading of the heater, expressed in watts-per-square-inch of active heater surface.
The formula for active heater surface is: (Heater Length – Cold End Length) x Diameter x Pi. For a 1/8” diameter by 1” long heater it is: (1.00 - 0.25) x .38, or .285 square inches.
The formula for watt-density is: Wattage / Active Heater Surface For 1/8” diameter by 1” heater at 80 watts, this is: 80 watts / .285 in sq or 270 watts/sq inch.
Small Diameter, Miniature Cartridge Heaters Selection Guide
The key questions which need to be answered before selecting a cartridge heater for your application are:

  • How much wattage is required?
  • What voltage will be used?
  • What is the heated length? What is the overall length?
  • How long are the heater leads or what is the lead length required?
  • What is the dimentions of the cartridge heater?
  • What is the ambient temperature the cartridge heater will see?
  • What is the maximum temperature required from the heater?
  • How quickly do you want to reach your operating temperature or set point in hour/minutes?
  • What is the type and weight (in lbs.) of the material you want to heat?
  • Special Modifications


  NPH Split Sheath Expandable Cartridge Heater

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