Alternative Temperature Sensors

osborni
Posts: 497
Joined: Fri Dec 07, 2007 9:08 pm
Location: USA, Michigan

Alternative Temperature Sensors

Postby osborni » Wed Oct 29, 2008 2:43 pm

I was getting fed up with doing the pull up resistors on the "standard" resistance based temperature probes.

So on a whim, I took a torch to a VDO temp sensor to pull out the guts.

The result:

Image

After cleaning with spray cleaner and a q-tip, the inside is nice a clean.

I've used a "one-wire" sensor (Digikey # LM19CIZ) before. Very small package, 5VDC supply and a voltage output before. Nice little sensor and currently about $1 at Digikey. The package is a TO92 and is about 3/16" or 1/4" inch cube.

After wired up and shrink wrapped:

Image

I plan on potting the sensor in with epoxy and sealing with silicon.

I use the LM19s on my car now - Just epoxied them to the radiator water inlet and to a oil distribution manifold. The measured oil temp is pretty stable, but the water temp swings around a lot (as expected) depending on how fast I'm going (i.e., how much air is going through the radiator) - I knew that would happen, but not to the extent that it did.

faraday
Posts: 267
Joined: Sun Feb 25, 2007 1:18 am

Postby faraday » Thu Oct 30, 2008 3:29 am

Well done :) .
I am not familiar with the component you have chosen but will research it in due course.
Q-tip is also a description I have not come across in my travels. What is it :?:

It seems rude to ask, but are you sure you have the radiator one on the inlet, or do you mean the inlet to the engine? We should expect more stability with airspeed on the radiator inlet than with RPM or throttle.

There will always be heat transfer issues that will delay and sometimes completely mask the true fluid temperatures of interest. I suppose your mention of epoxy indicates that you have some appreciation of these problems and desire to reduce them if practical.
There are special (ceramic, I think) pastes that set hard to pot temperature sensors. They are conductors rather than insulators. Unfortunately they are quite brittle.

Mechanical support is thus important. If the ceramic cracks it can fracture fine leads with it. Your use of shrink helps support, but hinders heat transfer.
:idea: The brass housing should provide sufficient protection to make the plastic sheathing superfluous other than near the outside and on the individual fine soldered leads.

pault
Posts: 64
Joined: Fri Nov 02, 2007 4:53 pm
Location: St Charles, IL

Q-tip

Postby pault » Thu Oct 30, 2008 4:05 pm

A Q-tip is product that has a cotton swab on each end of a short round rod

http://en.wikipedia.org/wiki/Cotton_swab

Paul

osborni
Posts: 497
Joined: Fri Dec 07, 2007 9:08 pm
Location: USA, Michigan

Postby osborni » Thu Oct 30, 2008 4:59 pm

For background - I'm a mechanical engineer and have been practicing for 12 years in a variety of fields, both theoretical and practical and have been around cars since I could walk (dad used to race vintage cars...).

I considered trying to find some adhesive that is more thermally conductive, but none is available as local hardware stores. I made sure that the sensor bottomed out on the bottom of the brass fitting and there is less then a CC of epoxy in there anyway. When I get it mounted and take some data, I'll compare gauge readings against the sensor readings to look at rise and bias. I suspect that the temp changes will be slightly slower then actual, but the sensor I made up will quickly saturate and get to actual fast enough.

As for the location and my commentary of the water temp - it's a 200 BMW M Coupe - essentially an e36 cooling system. I epoxied the sensor to the end tank of the radiator, right next to the top hose feed from the engine. I was just surprised that the temperature wasn't closer to actual water temperature (about ~15-20 degree low) measured on a VDO Vision gauge - and that the temperature of the end tank would vary by 20*F or so down a straight.

faraday
Posts: 267
Joined: Sun Feb 25, 2007 1:18 am

Postby faraday » Thu Oct 30, 2008 9:25 pm

Ah!
Cotton Buds
The ear-cleaners we shouldn't stick in our ears - I find it quite addictive - ah that's better. :oops:

I'm mech eng too. Worked for GM and Ford but that doesn't exactly fill one with inspiration and knowledge. My serious attempts at measurement of water temps have been on engine dynos and in race cars without engine mounted thermostats. As often as possible we try to measure both in and out, but my activity pre-dates in car data logging. Recently I've only collected data from points roughly midway in the engine's cooling system - call it the block temperature.
I had the good fortune to find all the gauge systems - back then I wasn't trying to mix an OTS sensor with an A/D logger - read within +/- 2 deg C of "correct" when I placed them all in boiling water.

To be continued...
Junior is at school now.

I found it harder to get good measurements with oil. Most packaged 'screw in' sensors are large compared to the oil galleries and lines, so placing the sensor body in the flow will block it too much. Eventually I used a series of Earls style size adaptors so I could place the sensor across the flow without fear of reducing the delivery in a short length of Hex bar I bored to an inch .

With regard to water, the typical thermostat on the engine outlet plays havoc on many race tracks with many cars. Often the radiator has very little performance in reserve and the revs used accelerating through the gears to the redline has an engine driven water pump operating on the back side of its efficiency curve most of the time, so temperature is rising the faster you go. With a big stop, the radiator is often efficient enough to take the water temp low enough to shut the thermostat before the braking is over and more often before the following corner is negotiated.
This cools the radiator water even more. When the thermostat opens in response to the next heat load accelerating out of the corner, the engine cops a gob of rather cold water that apart from the thermal shock materials issues can partially put the fire out.
The effect on torque delivery is easy to replicate on an engine dyno. Maintaining gentle temperature gradients is far easier in practice (for racing) without a thermostat. Temperature regulation for different weather conditions or circuit characteristics is achieved by closing the radiator ducting.

Your recent experience sounds more like the sensor is not measuring the water temperature as much as the outside of the radiator tank. If it is old fashioned brass, I 'd not like this explanation much, but if it is plastic :(

For a potting compound: http://www.aremco.com/
:)

osborni
Posts: 497
Joined: Fri Dec 07, 2007 9:08 pm
Location: USA, Michigan

Postby osborni » Fri Oct 31, 2008 2:34 am

Great insights, thanks. One of the reasons I want to get a decent temp probe in there so i can see how much the thermostat is cycling.

The radiator is full AL. I knew it would be off putting the temp sensor where I did. Honestly did it more on a whim then for any real data collection need.

I'll probably end up with a Perterson type probe port for the oil lines. Seams to be the best way to get decent flow and heat transfer to the probe without restricting flow too much.

About the middle of the page:
http://www.petersonfluidsys.com/plumb_bv.html

A bit pricey, so I may machine and weld up one for 1/4 the price.....

faraday
Posts: 267
Joined: Sun Feb 25, 2007 1:18 am

Postby faraday » Fri Oct 31, 2008 3:10 am

Those Peterson ports look just the ticket; I daresay for a big ticket!
How long have they been on the market?
I made mine in 1988. Maybe I should have gone into production :cry:

You'd think an Al rad tank would have good enough conductivity :?:
Is it a standard BMW part?

osborni
Posts: 497
Joined: Fri Dec 07, 2007 9:08 pm
Location: USA, Michigan

Postby osborni » Fri Apr 03, 2009 12:18 am

Just got off my duff and finished off all the other winter mods and got the car on the ground. The sensors pictured above worked perfectly and have no artifacts at all.

They calibrated close enough for race car purposes in a bath of water on the stove.

For oil temp I used a XRP AN-10 pressure fitting plumbed into the outlet to the oil cooler. Water is tapped into the AL thermostat housing (water out to the rad).

About $15 later, I have some well calibrated and accurate temp sensors rather then messing about with pull up resistors and a dodgy calculation. :)


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