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To begin with, any temperature probe (such as the CHT probe), is a thermocouple type probe.

For the uninitiated, a thermocouple is formed at the joint between two metals. When the joint is heated, a very small voltage measured in millivolts is generated. Higher the temperature, higher the voltage. Also, each metal combination is best suited for a specific temperature range. The type of thermocouple indicates the metal combination. Three types of thermocouple probes are used in aviation EGT, CHT and TIT.

Types of CHT probes:

There are three basic types of CHT probes; J, K and E

J-type OAT Probes are very sensitive and therefore limited to use in low temperature ranges. While they can be used as CHT probes, they are not suitable for use as EGT or TIT probes. The temperature measuring range is 40°F to +1382°F (-40°C to +750°C) and the sensitivity is approximately 55 µV/°C. J-type CHT probes are typically made using Constanton and Iron.

The J-type probe is used in the manufacture of gasket probe. This probe replaces the gasket on 18mm spark plugs and do not need a screw-in adapter. The 18mm gasket fits on many common spark plugs. Some engine designs, such as radial engines, the O-235 and O-200, may prevent the use of bayonet style or may not allow for the use of a bayonet adapter. Choose gasket style if the engine model does not have threaded thermowell receptor and or bayonet adapter in the cylinders.

K-type CHT probes have sensitivity over a wide temperature range and are the preferred type for use as EGT or TIT probes. The temperature measuring range is -330°F to +2460°F (-200°C to +1250°C) and sensitivity is approximately 41 µV/°C. The k-type probe is usually made from Alumel and Chromel.

K-types are usually used in the form of spring loaded bayonet mount CHT probes. They screw into the cylinder head thermo well. These twist and lock into a screw-in adapter. The probe is spring helps ensure contact with metal instead of air. A bayonet style k-probe is used if the engine model is equipped with threaded thermowell receptor or, it has a bayonet adapter in the cylinders.

E-Type probes are now obsolete. They were used in older EDM Gauges and therefore commonly found in older aircraft. The E-type probe is usually made from Constanton and Chromel.

You might also want to know that there is something called the ‘CHT adapter probe’ and they are used in conjunction with the original manufactures CHT probes. These threaded probes fit into the cylinder heads thermo well. They are available in two options – to accept bayonet type or threaded type OEM probes.

While on the topic of CDT Probes, you might also want to know that there are grounded and non-grounded probes.

Grounded probes have the thermocouple junction welded directly to the probe shaft. They have faster response time, longer life and lower cost, but since they have continuity with the engine ground, they are susceptible to more electrical noise.

Ungrounded probes have the thermocouple junction isolated from the probe shaft so there is no continuity between the thermocouple.

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