Background
Thermocouples are a relatively simple device used to measure high temperatures. There are various ranges that thermocouples can measure within, defined by the Calibration Type of the sensor, with 'Base Metal' calibration sensors being the most common (J, K, E, T type). A thermocouple relies on two separate wires of dissimilar metals that are connected at both ends, in the sensor and internally of your measuring device, which is then translated from voltage to temperature when measured at the cold junction. Some ECU's have 1-2 EGT inputs directly on board while most often you'll need an expander of some sort such as the MoTeC E888.
In automotive applications, a K-Type will be the most common sensor type for EGT (exhaust gas temperature) measurement as they are rated from -200º to 1250º C (-328º F to 2282º F) but can also be used for surface temperature measurement. J-Type sensors are more regularly used for higher than usual air temperatures, with charge temperature/compressor outlet temperature being a prime example.
There is a very common misconception that you can't or shouldn't 'extend' the wiring of a thermocouple. If the appropriate wire, terminals, and guidelines are followed you can integrate thermocouple wiring into your harness and greatly consolidate your loom package with zero negative result.
For the purpose of this post and the following wiring guidelines, we will be referencing a K-Type thermocouple which uses an alloy combination of Nickel-Chromium (Chromel) and Nickel-Aluminum (Alumel). These different metals are polarity specific and following the ANSI color code, chromel is the positive yellow wire and alumel is the negative red wire.
The corresponding wire is offered in 'thermocouple grade' and 'extension grade' variations, with the former having a higher accuracy rating and the latter being of lower grade.
We prefer to use thermocouple grade wire for a few key reasons:
- It is sheathed in a FEP outer jacket making it easier to bundle in looms, while adding further temperature insulation for the inner signal wires and separating them from the rest of the loom, if applicable.
- Offers higher accuracy and tighter manufacturing tolerances than extension grade.
- While the cable referenced is shielded, it is not necessary to use in most applications and can be trimmed back. Alternately and if required, you can drain the shield to ground.
- Stranded small gauge wire is less common but a necessity for motorsport and the connections we commonly use.
With this or a comparable wire, you can extend the total wire length from the sensor to your measuring device significantly. A safe reference point is to ensure the resistance of the circuit is <90Ω, which is quite long on a race car!
Connections By Cost
- Low
- The most cost effective way to connect from your sensor to your harness will be using a standard K-Type Connector Kit though these are unsealed and can be bulky when used in high quantities like on 8-12 cylinder applications.
- Medium
- The middle ground is to use a Deutsch DTM connector fitted with specific contacts for the two alloys listed above. These contacts can also be used with the MIL-Spec connector kits we offer!
- Remember, you will need the corresponding contacts of each connection.
For example, if you had one sensor that you put on DTM 2-Position, you need an alumel and chromel pin at the receptacle, and an alumel and chromel socket at the plug. If you had a 12-cylinder, you would need (12) of each!
- Costs can rise quickly here, but the tradeoff is a consolidated solution that offers value in terms of it's packaging and removing the chance of individual cylinder's sensors being left disconnected leaving you missing data.
- High
- The most ideal solution will center around using Deutsch Autosport or Souriau 8STA connectors due to their ruggedized form factor and ability to be fully sealed.
- You must use a connector that features Size 22 contacts for multiple sensor disconnects and again must use specific contacts for the two alloys listed above.
- For individual sensor connections and/or junction boxes with direct to sensor provisions, we do offer the correct terminals for ASL connectors as well. Please contact us via email to purchase.
JV Tip: Building a race car is expensive, as is constructing a motorsport grade wiring harness, as is rebuilding an engine because your EGT compensation didn't work due to janky wiring.
Consider your actual needs, the application, your use case, and your non-negotiables before setting out solely thinking about a low/medium/high cost solution. What is required in professional motorsport is not what a drag race vehicle requires, and both are different than what your weekend warrior may need. There are ways to save here, but it comes down to the planning......it always comes down to the planning.
