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Question: How does the 914's airbox temperature sensor work?

The Rotax 914 airbox temperature sensor is a JUMO manufactured Pt-100 RTD type (Resistance Temperature Detector) and requires 5 VDC provided by the TCU to measure temperature as a function of resistance. The resistance increases with temperature and the TCU converts that to temperature for display on the Rotax TLR 4.6 software "MONITOR" page on a MSDOS laptop computer. “Pt” is the chemical element symbol for the probe's internal platinum coil and “100” is the ohms resistance at 0ºC. When inserted into a cup of melting ice at 0ºC, the 914’s Pt-100 RTD should read 100 ohms. A specification sheet on the JUMO probe is attached with highlights on the 914's version.

Resistance increases with temperature and the TCU converts that resistance to temperature on the TLR 4.6 software for display on a laptop computer running MSDOS. If there is increased resistance due to corrosion or a poor connection on either end at the probe or TCU, it will see that as a rise in temperature. At 88ºC the TCU will command a reduction of turbo boost with no warning in either the red or yellow TCU lights.

As RTD temperature sensors have a “self-heating” error due to the resistance current, the Rotax sensor shows an 8.4ºC high temperature of 34.6º after about 3 minutes of being powered as compared to all ten of the other temperature sensors on the engine display showing actual ambient 26ºC. Note the circled builder installed J-type thermocouple probe showing 26ºC with the engine is not running. J-Type CHT probes having 1/8 NPT male mounting threads fit into the the mount adjacent to the Rotax sensor and are supported by most engine monitors. This builder used a Westach 712-7DJK for $64.

The 5 VDC from the TCU can be measured at the airbox temperature sensor connection.

 

8332_1_100 Ohms.png (You do not have access to download this file.)
8332_1_Jumo Probe.pdf (You do not have access to download this file.)
8332_1_34 to 26.png (You do not have access to download this file.)
8332_1_5 VDC.png (You do not have access to download this file.)
8332_1_Pt-100.png (You do not have access to download this file.)

John Spratt

  • Re: 914 Airbox Pt-100 RTD Temperature Sensor

    by » 3 months ago


    I am curious as to the reason for this post. I installed a second Jumo temperature sensor for the purpose of monitoring airbox temperature on Dynon Skyview. The Dynon instructions are completely wrong in every respect, and are of no help getting airbox temperature monitoring on the Skyview. I notifide them of this five years ago, and a couple of times since. They have done nothing to correct their documentation. I did get a Dynon tech to work with me on a custom configuration file, and we have made some progress. Anyway, I don't see a question in your post, and just wondering what prompted it.


  • Re: 914 Airbox Pt-100 RTD Temperature Sensor

    by » 3 months ago


    Ken,

    The question was added to the "914 Airbox Pt-100 RTD" Sensor" posting and mention of Dynon removed. Dynon and others engine monitors do  support  J-Type THERMOCOUPLE (CHT) probes. Here is a screenshot of my "Sensor Input Mapping" page. It works well.

     

    Over & out,

    John Spratt

    28072_2_DynonSensorInputMapping.png (You do not have access to download this file.)

    John Spratt


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