CHT Probes Used In Aircraft
A CHT probe is used to obtain temperature data of the part of the engine in which the CHT probe is installed. A CHT probe is a thermocouple type probe – meaning, two different metals that react to heat, are joint together. When heated, a tiny voltage (millivolts), is generated.
As the heat increases, so does the voltage. The type of metals used depends on intended use of the probe. Essentially, there are three types of thermocouple probes; CHT, EGT and TIT.
CHT Probes in turn, are divided into J, K and E type probes. Of these, the E-Type probes are obsolete and no longer used. Though, you might still find them in older EGT systems as in vintage aircraft. The E-type probe was usually made using Constanton and Chromel metals.
The J-type probes on the other hand, due to the metals used, are a type of CHT probes that are used in low temperature areas. However, the J-type CHT probe has a higher sensitivity and range. In fact, the range is 40°F to +1382°F (-40°C to +750°C). The J-type CHT probe has of approximately 55 µV/°C. These probes are typically made using Constanton and Iron.
Typically, the J-type probes are used in the manufacture of gasket probes. These probes are used on 18mm spark plugs. The 18mm gasket probe fits on many common spark plugs. It may be noted that radial engines such as the O-235 and O-200, may make it difficult to use the bayonet style adopter or may altogether prevent the use of a bayonet adapter. You should opt for the gasket style only if the engine model does not have a threaded thermowell receptor and or bayonet adapter in the cylinders.
Spring loaded bayonet mounted CHT probes are usually K-type probes. These twist and lock into a screw-in adapter and screw into the cylinder head thermo well. The probe is spring-loaded and helps ensure contact with metal instead of air. You use a k-probe if the engine has a bayonet adapter in the cylinders or, is equipped with threaded thermowell receptor.
The preferred CHT probes are the K-type probes because they have sensitivity over a wider temperature range. The K-type probes temperature range is -200°C to +1250°C (-330°F to +2460°F), and sensitivity is approximately 41 µV/°C. The K-type probes are usually used as TIT or EGT Probe. K-type probes are typically made using Alumel and Chromel.
You can also use the probes via the ‘CHT adapter probe’. These adopters are used in conjunction with the OEM CHT probes and helps to fit the probe into the cylinder heads thermo well. These adopters are available in two options – to accept threaded type OEM probes or, to accept bayonet type probes.
CHT probes are available in non-grounded and grounded options. Ungrounded probes have the thermocouple junction isolated from the probe shaft so there is no continuity between the thermocouple.
The grounded probes on the other hand, have the thermocouple junction welded directly to the probe shaft and therefore, have longer life and lower cost and faster response time. But, since they have continuity with the engine ground, they are susceptible to more electrical noise.
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