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Old 24th August 2010, 10:01 PM   #1
Tenson is offline Tenson  United Kingdom
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Default Thermocouple - fast response, high temperature?

Hi,

It's not hi-fi related but I need some of the electronics genius on this forum!

I am installing water injection on my turbo diesel car and I need to be able to measure the temperature of the air after the turbo going in to the engine.

I'm told this can reach a few hundred degrees at high boost. So I need a thermistor or thermocouple that can cope with that sort of temperature, but the thing I am not sure about is what sort will react the fastest?

Someone told me steel seethed ones don't react fast enough to detect quick air temp rises from a short blast on the turbo. I don't know exactly how quick it needs to be, but I guess it will need to react within a second or so.

So what do I need?

P.S. Having just read Wiki about thermocouples,, is it possible to just stick my meter on the wires and convert the voltage to a temperature? This stuff about cold junctions has confused me. I thought it was as simple as using a thermistor?
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Old 24th August 2010, 10:34 PM   #2
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Quote:
Originally Posted by Tenson View Post
Hi,

It's not hi-fi related but I need some of the electronics genius on this forum!

I am installing water injection on my turbo diesel car and I need to be able to measure the temperature of the air after the turbo going in to the engine.

I'm told this can reach a few hundred degrees at high boost. So I need a thermistor or thermocouple that can cope with that sort of temperature, but the thing I am not sure about is what sort will react the fastest?

Someone told me steel seethed ones don't react fast enough to detect quick air temp rises from a short blast on the turbo. I don't know exactly how quick it needs to be, but I guess it will need to react within a second or so.

So what do I need?

P.S. Having just read Wiki about thermocouples,, is it possible to just stick my meter on the wires and convert the voltage to a temperature? This stuff about cold junctions has confused me. I thought it was as simple as using a thermistor?
Thermocouples are very well suited for harsh environments and car engines indeed are.

The speed of response of a sensor mainly depends on his mass and thermocouples can be very thin so fast response

Look at Linear Technology web site for a huge info about thermocuples and ICs to "control" them:

Search Results: thermocouple

Last edited by diy_audio_fo; 24th August 2010 at 10:39 PM.
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Old 25th August 2010, 04:08 AM   #3
eyoung is offline eyoung  United States
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many auto manufacturers use thermister devices as intake air temp sensors one that comes to mind is early 90's ford IAT sensors, simple 2 wire connection the hard part is plotting resistance vs temp and translating correction. then trimming the value as they call it in home heating and cooling as an anticipator...

Just a thought, Elwood
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Old 25th August 2010, 04:12 AM   #4
eyoung is offline eyoung  United States
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It also comes to mind that many air density (mass air flow)meters use a thin platinum wire heated of course and monitor resistance variance (voltage drop across wire)to interpolate air temp as well.

and that is why they call it rocket science

Just another thought...Elwood
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Last edited by eyoung; 25th August 2010 at 04:15 AM.
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Old 26th August 2010, 08:14 PM   #5
KBK is offline KBK  Canada
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The basic MAF for any VW 2.0 (cheap on ebay) should work just fine. With some important caveats and potential issues.

Your biggest issue will be one of calibration and of potentially being out of the sensor's range with your basic (low range) temps. Meaning, your lowest temps will be to high for the top temps for the sensors.

The MAF's detect via cooling on flow aspects and temperature correction according to a temp sensor external to the MAF, in most high end cars. Thus they can change their air/fuel ratio's on the fly according to localized considerations.

Whereas just using an MAF alone is only good when the flow rate possibilities down that particular throat, tube, or channel assembly are known and are part of the mapping equation.

You've got some studyin' to do.

You may have to go with a low end cheap thermocouple and then calibrate against the measurements you make and slowly work your way to accuracy via altering the map of the thermocouple measurments.

It's a catch 22, I know, but that is where you are right now. The reality is that -exactly, for the most part, this is how they are calibrated in the real world. A given situation has many differing aspects and thus no single solution exists.

Last edited by KBK; 26th August 2010 at 08:18 PM.
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Old 26th August 2010, 08:22 PM   #6
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First thing I'd do would be make sure the car didn't already have instake manifold air temp data available. A late model turbo car surely would.
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Old 26th August 2010, 08:31 PM   #7
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Quote:
Originally Posted by Tenson View Post
Hi,

It's not hi-fi related but I need some of the electronics genius on this forum!

I am installing water injection on my turbo diesel car and I need to be able to measure the temperature of the air after the turbo going in to the engine.

I'm told this can reach a few hundred degrees at high boost. So I need a thermistor or thermocouple that can cope with that sort of temperature, but the thing I am not sure about is what sort will react the fastest?

Someone told me steel seethed ones don't react fast enough to detect quick air temp rises from a short blast on the turbo. I don't know exactly how quick it needs to be, but I guess it will need to react within a second or so.

So what do I need?

P.S. Having just read Wiki about thermocouples,, is it possible to just stick my meter on the wires and convert the voltage to a temperature? This stuff about cold junctions has confused me. I thought it was as simple as using a thermistor?
A 5 mil type J is probably good enough. In 60 ips air, it responds in 80 milliseconds. The 1 mil is good in still air at 50 milliseconds. I don't think your water injection system can react faster than that..

Thermocouples, J, K, T, E, R &S

Cheers, John
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