Chris,
That sounds good. If another sensor didn't match physically but did functionally; is there any reason that a mount couldn't be made for the thermistor if it was too long, etc?
1. The mechanical envelope is critical as we want to get a reading at or, as close as possible to the center of the EVAP CORE as possible.
2. Thermistors, typically by design, are not intended to be subjected to moisture and / or direct contact for HVAC systems. While certainly other types are intended for direct contact, those involve other applications.
3. The thermistor is typically installed inside of a probe at the very tip and environmentally sealed to prevent what is called intrusion.
4. These components typically need to be mechanically and electrically isolated and are quite prone to mechanical as well as thermal shock and electrical-noise. Dropping or subject to impact shock, thermal shock, electrical shock = bad news thus warrants R&R 100%.
5. Thermal shock = bad news. Subjecting thermistors directly to say, open flame = BAD / Catastrophic Failure will ensue. Exceeding upper and lower t-rating = bad news. Typically destroys them depending on how far exceeded.
6. Mechanical contact is critical in thermal-transfer rate to ensure temperature is transferring optimally as possible. Poor transfer rates equates to higher lag time or, what is called Response Time. This can lead to serious consequences at the EVAP CORE. If one were to select a shallow depth compounded with a smaller diameter, one is going to experience inaccurate readings fed out to the controller - late readings or, readings that may end up being erratic and sporadic (jumpy) thus, the controller will struggle in general. If one chooses a probe diameter that is grossly larger than the allotted spacing in the EVAP CORE fins allow, mechanically, displacement and / or possibly, inducing mechanical damage to the core plumbing. Slightly larger dia is not be an issue as the fins are somewhat compliant. This is the OEM design intent - snug enough but, not loading stress into the fins or, the probe body.
7. If one were to exceed the specified length, you're going to move away from the ideal mount location thus, you're going to have the same problem as a sensor body that is too short. Six-of-One, Half-Dozen-of-The Other scenario. Moving the probe body in or out from the centerline in either direction = negative results.
8. Electrically - the type, the profile, and many other variable must all match to the OEM component. This system as are most, are not designed to be plug and play like light bulbs. Each component is designed around a controller circuit - and, each controller circuit, from year to year, varies as does the thermistor used. This is why folks on this site have experienced what I had described would occur when using other thermistor assemblies from other OEMs - the scale is way off and not a match at all thus, it would never function properly.
Thermistors have both, power and current ratings for a specific circuit along with dissipation curves and all sorts of impedance characteristics not too mention the upper and lower limit intended to match the end-use-environment application. You've got dissipation constant, thermal time constant, max power at ambient to the type used - negative or positive pending controller design (typically negative associated with HVAC). As one can see, there's a lot more than meets the eye regarding physical envelope. Everything must match as close as possible to ensure the controller will not only function circuit wise but, be fed accurate readings to minimize risk of mechanical damage from events like ice-over events. Those are really hard on the EVAP CORE and can lead to serious and very costly repair bills when one blows and contaminates the entire system.
Summary:
Ideal location in this type of application: close to center as possible = optimum results
Regards,
CM
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