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Here you can learn the basics, as well as some more advanced information, about Alcor products and the principles behind them.

Thermocouples (Also called Sensors or Probes)

A thermocouple is a device formed by the junction between two dissimilar metals, which transforms heat energy into electrical energy.  Aviation EGT/TIT and CHT systems use thermocouples, lead wires composed of the same two dissimilar metals, and an indicator, which is basically a millivolt meter. Click here for a graphical explanation of EGT analysis.

Compatibility: Alcor parts are covered under a blanket STC and PMA, making them legal for installation in any aircraft with a reciprocating engine.  Therefore, Alcor products can be used in practically any system of like Type (see below) including digital indicators and Engine Monitoring Systems.  Contact us for details on compatibility.

Grounded Thermocouples

What is a grounded thermocouple?  Wires in the element are welded to each other and the sheath, creating continuity in a grounded thermocouple.  Grounded thermocouples are the most commonly used in reciprocating aircraft.

Why does it matter whether my thermocouple is grounded or ungrounded? Depending on the grounding situation in some digital instruments or aircraft/engine models, grounded probes can lead to electrical “noise” or interference in a digital instrument.  Some digital engine monitors or aircraft models may specify the use of ungrounded probes while others allow grounded.  Ungrounded probes can be used in place of grounded, but grounded should not be used in the place of ungrounded.  All Alcor instruments use grounded probes.

Alcor x-rays and tests 100% of thermocouples individually and offers an industry-leading five year warranty on grounded thermocouples.

Ungrounded Thermocouples

What is an ungrounded thermocouple? In an ungrounded thermocouple, wires in the element are welded to each other, but not to the shaft.  The thermocouple wire junction is isolated from the rest of the outer sheath and wires and must maintain a certain insulation resistance value to work correctly.  This is also referred to as an isolated tip. Therefore, there is no continuity in an ungrounded thermocouple.  This type is more difficult and time-consuming to produce, which can increase the cost.

Why does it matter which kind of thermocouple is used? Depending on the grounding situation in some digital instruments or aircraft/engine models, grounded probes can lead to electrical “noise” or interference in a digital instrument.  Ungrounded thermocouples are easier to design and are used on amplified meters with circuits requiring that style to avoid this issue.  Some digital engine monitors or aircraft models may specify the use of ungrounded probes while others allow grounded.  If an ungrounded probe is required, it should be specified in the aircraft or Engine Monitor manual.  See a list of known ungrounded systems here.  Ungrounded probes can be used in place of grounded, but grounded should not be used in the place of ungrounded.

Alcor x-rays and tests 100% of thermocouples individually and offers a one year warranty on ungrounded thermocouples.

Types of Thermocouples  

Cylinder Head Temerature (CHT)–Type J

What is a Type J sensor?  These are usually identified by yellow (-) and black (+) wires.  A small number of manufacturers use red (-) and white (+) or red (-) and black (+), but yellow and black is considered the industry standard.    Type J thermocouples are made of iron and constantan metals.

When should I use a Type J thermocouple?  Type J thermocouples are more accurate in lower heat range and are used primarily for CHT instruments.  Alcor indicators use exclusively Type J components in CHT systems.  Alcor also manufactures Type K for use in digital and/or amplified CHT indicators.  Amplified meters measure only millivolts, not resistance value, making Type K components sufficient.  Type J and Type K components should not be mixed.  It is important to choose based upon the type of indicator installed in the aircraft.

CHT–Type K

What is a Type K sensor?  Type K thermocouples are identified by red (-) and yellow (+) wires.  Type K thermocouples are made of chromel and alumel metals.

When should I use a Type K thermocouple?  Type K is the industry standard for EGT/TIT, but some manufacturers spec Type K for use in digital and/or amplified CHT indicators.  Amplified meters measure only millivolts and are not affected by resistance value, making Type K components sufficient.  Type J and Type K components should not be mixed, choose based on the type of indicator installed in the aircraft.

Styles of Thermocouples

Alcor CHT thermocouples are available in Bayonet and Gasket style.

Bayonet:  Bayonet style is available in Type K (86252) and Type J (86251) and twists and locks into a screw-in adaptor (28202).  Choose bayonet style if the engine model is equipped with threaded receptor and/or bayonet adaptor in the cylinders.

Gasket:  Gasket style is available in Type J (86202) and no adaptor is required.  The 18mm adaptor fits on many common spark plugs.  Some engine designs, such as radial engines, may prevent the use of bayonet style or may not allow for the use of a bayonet adaptor. Choose gasket style if the engine model is NOT equipped with threaded receptor and/or bayonet adaptor in the cylinders.

RTD (Resistance Thermistor Device): An RTD is a different type of sensor that measures resistance, as opposed to millivolts, and is more of an Ohm Meter.  An RTD may be found in older aircraft.  Alcor stocks RTD 86146 that has identical specifications to AN5546-1.  For more information on RTDs, call Alcor at 800-FLI-SAFE (800-354-7233).

Exhaust Gas Temperature (EGT) and Turbine Inlet Temperature (TIT) Sensors 

EGT–Type K

What is a Type K thermocouple? Type K thermocouples are constructed with with red (-) and yellow (+) wires.  Type K thermocouples are made of chromel and alumel metals.

Why does it matter which Type of thermocouple is used?  Type K is the industry standard for EGT/TIT systems.  Type E and Type K components should not be mixed. Choose based on the type of indicator and leads installed in the aircraft.

EGT–Type E

What is a Type E thermocouple?  Type E thermocouples are constructed with red (-) and brown (+) wires.  The brown (or tan) may start to look silver over time.  Type E thermocouples are made of chromel and constantan metals.

Why does it matter which Type of thermocouple is used?  Type E systems are of an older technology and produce twice the millivolts of a Type K system.  It was later determined that a lesser amount of millivolts was sufficient and the industry switched to Type K.  Type E components are more costly because of rising costs of outdated raw materials and lessening demand.  Alcor still manufactures Type E indicators and thermocouples as replacement parts for older, existing systems, but no longer manufactures Type E leads.  Because it is labor intensive and time consuming to replace leads, some may choose to replace Type E probes and indicators rather than update to a Type K system.  However, if it is determined that a new lead/wire harness is necessary, this provides an opportunity to update since the work must be done anyway.

Type E and Type K components should not be mixed.  Choose based on the type of indicator and leads installed in the aircraft.

Styles of Thermocouples

Screw-in Style

Screw-in style EGT/TIT probes are available in Type E or Type K in grounded and ungrounded options.  A screw-in type may be installed because it provides a neater installation than the clamp style.  Many engines now come with a hole already drilled for a screw-in probe or a threaded weld boss (28113) can be purchased separately.  Alcor screw-in probes are available in 7/16”-20 thread size (common in Continentals) or ¼ inch NPT (common in Lycomings).

Clamp Style

Clamp style is the most common type of EGT/TIT probe, available in Type E or Type K in grounded and ungrounded options.  Clamp style probes offer easy installation with no welding required.  A hole is drilled in the exhaust near the mounting flange and the clamp is tightened to secure the element in place.  For more information on probe mounting location, click here.

Stagger: Normal and Reverse

Stagger refers to the length of the different colored wires at the end of a thermocouple or lead.  “Normal” stagger is considered to be long yellow and short red wires.  “Reverse” stagger refers to long red and short yellow.  If a lead wire has a short yellow and long red, the corresponding “normal” stagger thermocouple should have a long yellow and short red allowing the two wires to match up. If a lead has a long yellow and short red, the corresponding “reverse” stagger thermocouple would be chosen.

Normal and reverse stagger components are compatible.  The only reason to choose one over the other is to create a tidy installation.  Some aircraft manufactures specify normal stagger leads while others specify reverse.  For a neater installation, choose a corresponding stagger when replacing a thermocouple. Alcor also sells a “jumper” that will modify a normal stagger thermocouple to a reverse stagger.  Thermocouples with plug connectors do not have a stagger.

Other things to consider:

  • Styles are available with Omega plug connectors making installation a snap.  Female connectors are also available for retrofitting existing lead wires.
  • Some screw-in probes are available with adjustable depth.  Element should be ¾” into the tube and the adjustable depth allows for weld bosses of different sizes.
  • Styles are available with a 90 degree bend or an 8 degree bend to allow for engine design that will not accommodate the straight-in style.
  • Styles are available with extra-long leads to allow for the design of some engines.

Leads and Wire Harnesses

EGT/TIT

Alcor EGT/TIT leads are Type K (red and yellow) and made of chromel and alumel.  They are available in 4 standard lengths: 90”, 144”, 216” and 244”.  The resistance value of the lead is a function of length and the EGT/TIT is calibrated to that value.  All indicators leave the factory calibrated to a certain lead length based on likely application.  However, many indicators can be calibrated in the field to different lead lengths by following the calibration steps included in the instructions. Custom calibrations can be requested from Alcor.  Instruments calibrated to true temperature (TITs for example) should only be recalibrated by Alcor or an approved Alcal Service Center.  Find one here.

For a new installation, choose the EGT/TIT lead appropriate for your aircraft based on the length required from the probe to the indicator.  Replacement leads should be chosen based on the calibration of the existing indicator.  The leads should not be shortened or spliced if intended for use with analog or unamplified indicators, as this will change the resistance value.  Lighter gauge thermocouple wire can be used in amplified or digital indicators as they measure only millivolts and are not affected by resistance value.  In this case, it is acceptable to cut or splice the wire.

What about Type E leads?

Alcor no longer manufactures Type E leads because Type E is an outdated technology.  If it is determined that existing Type E leads need replacing, it creates an opportunity to update to a Type K system.  Alcor continues to manufacture Type E indicators and probes for those that choose not to go to the expense of replacing existing Type E leads.  When it comes to replacing existing components, the greatest number of labor hours are spent on replacing lead wires.  However, if the leads must be replaced, it is generally more economical to seize the opportunity to update to Type K.  Reminder: All components of an EGT/TIT or CHT system must be of the same Type to function properly.  Click here to view the Alcor EGT system troubleshooting guide.

CHT Leads

Alcor CHT leads are Type J (black and yellow), made of iron and constantan, and all have a resistance value of 8 ohms.  Alcor CHT leads come in 4 standard lengths and a resistor is used to achieve the 8 ohm resistance value, regardless of the length.  The 4 lengths available are 90”, 144”, 216” and 240”.  Choose based on the length required from the CHT probe to the indicator.  The leads should not be shortened or spliced if intended for use with analog or unamplified indicators.  Lighter gauge thermocouple wire can be used in amplified or digital indicators as they measure only millivolts and are not affected by resistance value.  In this case, it is acceptable to cut or splice the wire.

Instruments (Also called indicators, gauges or meters)

Compatibility: Alcor parts are covered under a blanket STC and PMA, making them legal for installation in any aircraft with a reciprocating engine.  Alcor instruments are unamplified and require no electrical connection. The millivolt output created by the thermocouple produces enough energy to power the indicators.  Backlit options are available that require electrical power.

EGT/TIT Indicators

Alcor EGT/TIT gauges are available in Single or Dual configurations, in two sizes, two Types, and are available with several dial options or backlighting.

Single indicators (intended for single engine aircraft) monitor one cylinder for one type of information. (EGT or TIT).

  • Single indicators are only available in 2 ¼” size.  If a customer has an older 3 1/8” single indicator that needs to be replaced, an instrument reducer plate can be purchased to fit a 2 ¼” indicator in a 3 1/8” hole.
  • Single EGT or TIT indicators are only available in Type K.
  • True Temp vs. relative to peak EGT: TIT is always in True Temp.  EGT comes in both styles, the customer can choose based on preference.  EGT is really only relevant in reference to Peak EGT, but some customers prefer True Temp anyway. 
  • A rotary switch (P/N 80825) can be added to allow the customer to click through up to 6 cylinders and a turbocharger.  This is a comparatively inexpensive way to create a full engine monitor.
Dual Indicators EGT/TIT
  • EGT/TIT indicators are intended for twin engine aircraft and monitor one cylinder on each engine.
  • Dual indicators are available in both 2 ¼” and 3 1/8”.   2 ¼” indicators do not have reference needles because there is not enough space.
  • A rotary switch (P/N 80827) can be added to 2 ¼” indicators to allow the customer to click through up to 6 cylinders and a turbocharger on each engine.  This is a comparatively inexpensive way to create a full engine monitor.  The switch only fits 2 ¼” indicators.
  • Dual EGT indicators are available in Type E in 3 1/8” or Type K in both sizes.  TIT indicators are available in Type K in both sizes.
  • NOTE: On a dual 2 ¼” instrument, the pilot uses the knob on the side of the appropriate temperature scale to calibrate the needle.  (Some are tempted to use the side the base of the needle is attached to.)

CHT Indicators, Single and Dual

  • Single indicators (intended for single engine aircraft) monitor one cylinder for one type of information.
  • Single indicators are only available in 2 ¼” size.  If a customer has an old 3 1/8” single indicator that needs to be replaced, they can purchase an instrument reducer plate to fit a 2 ¼” indicator in a 3 1/8” hole.
  • All Alcor CHT indicators are True Temp and Type J.
  • A rotary switch (P/N 80825) can be added to allow the customer to click through up to 6 cylinders and a turbocharger.  This is a comparatively inexpensive way to create a full engine monitor.
  • Dual CHT indicators are intended for twin engine aircraft and monitor one cylinder on each engine.
  • Dual indicators are available in both 2 ¼” and 3 1/8”.   2 ¼” indicators do not have reference needles because there is not enough space.
  • A rotary switch (P/N 80827) can be added to 2 ¼” indicators to allow the customer to click through up to 6 cylinders and a turbocharger on each engine.  This is a comparatively inexpensive way to create a full engine monitor.  The switch only fits 2 ¼” indicators.
  • All Alcor CHT indicators are True Temp and Type J.

Combination

  • Combination indicators are intended to measure two types of information on a single engine. 
  • Combination indicators are only available in Type K EGT and Type J CHT.  TIT is not available in a combination instrument. 
  • Combination indicators are available in both 2 ¼” and 3 1/8”.
  • A rotary switch (P/N 80827) can be added to 2 ¼” indicators to allow the customer to click through up to 6 cylinders and a turbocharger on each engine.  This is a comparatively inexpensive way to create a full engine monitor.  The switch only fits 2 ¼” indicators.

Miscellaneous Tips:

  • Installation time: It takes about 2-3 hours labor to install an EGT/CHT system.  Less time will be required to just replace a lead or probe.
    • Homebuilders can do it themselves.
    • Certified aircraft need to file a Form 337 if not replacing an existing system or component. (See more information on Form 337 below).
  • Finding the leanest cylinder:
    • 2 common ways to find leanest cylinder:
      • Which cylinder has the cleanest plugs on the bottom?
      • Which cylinder is furthest from the carburetor?  Theoretically, this cylinder will get less fuel.
      • FAA Form 337
      •  Some customers ask if installation requires a Field Approval Form 337.  In most cases this is not required.  The FAA only requires this form for what is considered a “major alteration”.  Ultimately, this is up to the discretion of the aircraft owner and mechanic.  Alcor has no authority in determining if  Form 337 is required.

The FAA definition of a “major alteration” means an alteration not listed in the aircraft, aircraft engine, or propeller specifications—

(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or

(2) That is not done according to accepted practices or cannot be done by elementary operations.

Appendix A to Part 43—Major Alterations, Major Repairs, and Preventive Maintenance