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Carb temp sensor

J

justinmg

Active Member
I have a marvel carb, and want to put a temp sensor on it for ice warning.



I think this port is the place to put it. Is this correct? Can anyone suggest a suitable probe?

Thanks
 
Yep, that's the place.

BTW, I found with my installation (RV-9 w/ O-290-D2) that the carb temp actually goes up when you throttle back. The reason being is less air moves through the cowl so it heats up. The carb heat does add a few degrees more but isn't required. I wouldn't have known that if it wasn't for the carb heat sensor.
 
There's a little port on the engine-side of the carb near the outlet that is filled with a soft metal plug. You drill this out and insert your sensor in it; there's one made just for it. I used a solid-state sensor, the National LM135AH-LM235AH in a metal TO-46 package which puts out a voltage proportional to the absolute temperature in degrees Kelvin. Standard sea-level temperature would show a voltage of 2.88V for 288 K. I mounted it in the hole using a thermal insulator so that it doesn't measure the temperature of the carburetor, and with the tip of the package just flush with the inside surface of the carb. It's nice having that temp. displayed so that you can see the temperature change when you do your carb heat check, and the drop you get from evaporative cooling when you do your run-up.
 
Gsuit said:
Elippse -

Do you have a pic of your install?
Click to expand...

Sorry! I didn't take pictures along the way. There's not much to see, though. The little TO-46 metal can fits entirely in the hole with a shielded wire coming from it. This IC just needs a voltage through a resistor and the return. I use a 6.8k coming from +10V, but you could use a 10k from the 13.6V buss. You read the voltage across the IC. There's also a trim terminal on the IC, but the accuracy is sufficient for my needs. I also use this IC on the EFI I designed, and in conjunction with the MAP sensor, I estimate the charge density entering the cylinders to set the fuel flow into the injectors. Seems to work well as Klaus sets the mixture and forgets it during a climb!
 
installation photos

I installed GRT supplied sensor and don't remember the sensor vendor. Here are two photos of my installation.
carbsensor.jpg

carbtemp.jpg

The tip of the sensor sticks out of this hole. In my case the temperature reading is always high. When engine is cold, it reads about the outside temperature. After engine has been running for a few minutes it raises to 127 degrees F (the max temperature my EIS will display). I tried a different prob from GRT and the reading is the same. Last Saturday I was at 9,500' with 2 degree OAT and 64% power, the carb temperature read 119 degree. Later at 74% power, 5 degree OAT the carb temp is shown to be 114 degree. It kind of verify what Bill said.
 
TEMP

sounds to me that your temps over 119 degrees are way high.............temps should be closer to OAT. Might verify you have settings correct in your EMS.

IMHO
DEAN
 
Carb temperature is preset in GRT EIS. I do believe there is a scaling error in my EIS. I should have followed it up.
 
N941WR said:
BTW, I found with my installation (RV-9 w/ O-290-D2) that the carb temp actually goes up when you throttle back.
Click to expand...
Hi Bill - this is interesting info. Can you post some carb temps that you have seen (ideally with OATs)? I've been planning for a carb and after an unpleasent ice incident at 65 OAT I'd like to have a carb temp sensor. I've "heard" that most RV's don't have very effective carb heat, but I cant find anyone who's posted any numbers. Maybe you have them?

Thanks for any info,
 
IowaRV9Dreamer said:
Hi Bill - this is interesting info. Can you post some carb temps that you have seen (ideally with OATs)? I've been planning for a carb and after an unpleasent ice incident at 65 OAT I'd like to have a carb temp sensor. I've "heard" that most RV's don't have very effective carb heat, but I cant find anyone who's posted any numbers. Maybe you have them?

Thanks for any info,
Click to expand...

My RV-6 is equipped with a Rocky Mtn. uMonitor which includes carb temp. Last Saturday on the way to breakfast at 30F OAT, the carb temp was indicating 59F. I have to believe the temperature of the butterfly in the carb was much lower than 59F.

My memory is pretty hazy on this since it happened many years ago, but I think some searching on the Matronics RV list will uncover quite a few threads on this subject back in the late '90's. As I recall, the general consensus from those who had experience with carb temp indicators was that the port we use on the carb for the sensor was essentially useless for an accurate reading of the carb internals where ice forms. The carb body absorbs heat and the sensor reading is badly skewed.

It has been my experience that the carb temp indications on my plane are way too high to accurately reflect what I assume to be the actual temp of the air flow through the venturi. My carb probe reads accurately at ambient temp with the engine cold so I don't think there is any error in the uMonitor itself.

Back when these discussions occurred, it seems (there goes the memory thing again..) that a vendor produced some sort of optical sensor that was supposedly able to produce an accurate temp reading from the temp probe port....or maybe it actually "saw" the ice forming and sounded an alarm. Maybe some Googling would turn up this product if it is still available.

But as far as I'm concerned, the carb temp indicator on my O-320 is pretty much useless.

By the way, I have experienced carb ice three times in 850 RV-6 hours. Twice on the ground during taxi to runup, and once in the air during cruise. In both instances, the standard Vans carb heat setup quickly cleared the ice.

UPDATE: Here it is, the "Iceman" optical carb ice detector from Lamar Technologies, still available at Aircraft Spruce:

http://www.aircraftspruce.com/catalog/inpages/icedetect2.php

And the User Manual for the Iceman:

http://tinyurl.com/3coj7z
 
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czrb heat

I have had carb heat guages on Continentals, Franklins, and Licomings and on the first two they are important. Not so much on the Licoming. Al of my guages were of the steam type and in my opinion were right as far as telling if ice was forming or not. They had a yellow arc that if the needle was in the yellow ice could form. If either above or below the yellow no ice would form. I use it and trust it all the time.
just my two cents.
 
I think I see the same results with my RMI probe. I find the carb temperature guage to be of limited use except that it will indicate the temperature rise when you use carb heat.

Right now, I'm rejigging my carb heat system to enclose the sides of the flange that sits on the top of the FAB. I've already added a Wicks heat muff, and I think the extra work I'm doing now will help with the temp rise.

I'll post pics when complete.

Vern
 
I also have the Rocky Mountain monitor, but instead of buying the sensor Ron recommended, I modified the input circuitry to work with the National solid-state sensor. The RM peaks at 19C, but when OAT is about 50F-60F, the temp will drop to 17C during run-up at 1700 rpm. I've seen carb temps down around 4C, at which temp I have the RM set for a low-temp alarm. If you are getting such high temps, it means your sensor is giving you carb body temp rather than air temp. As I wrote previously, I made sure my sensor was thermally isolated from the carb body to prevent this.
 
vlittle said:
Right now, I'm rejigging my carb heat system to enclose the sides of the flange that sits on the top of the FAB. I've already added a Wicks heat muff, and I think the extra work I'm doing now will help with the temp rise.

I'll post pics when complete.

Vern
Click to expand...

Vern,
I have been thinking of doing the same thing but possibly there is a reason for the gap especially on the side without the activation mechanism. Maybe a gap is need to allow some air circulation from the hot exhaust when the carb heat is off???

Fin
9A
 
Sam Buchanan said:
By the way, I have experienced carb ice three times in 850 RV-6 hours. Twice on the ground during taxi to runup, and once in the air during cruise. In both instances, the standard Vans carb heat setup quickly cleared the ice.
Click to expand...
so, did you notice the reading when it iced or is the probe location to far from where the ice occurs?
 
What about Precision carb & EI

I've got a Precision Airmotive carb for my O-360. I've thought about hooking up a carb temp sensor to my MVP-50. Two questions, where does the carb temp sensor install on Precision MA4-5 carb and does anyone have any words of wisdom regarding installing an EI temp probe in a MA4-5 carb?
 
cytoxin said:
so, did you notice the reading when it iced or is the probe location to far from where the ice occurs?
Click to expand...

Unfortunately, I didn't take note of the carb temp indication. As has been expressed in other posts, even though the probe is near the butterfly, it seems it is recording carb body temp, not the actual fuel/air temp in the venturi and on the butterfly.

Sounds like insulating the probe from the body makes it more accurate, and I look forward to more field reports from that type of installation.
 
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I'm not suggesting anyone do this, nor am I going to do it, but it seems like all that would be needed was to drill the hole in the carb oversize, tap for some type of non-conductive (heat) insert, and screw the probe into that. I also have the GRT setup and can't really imagine this thing giving me anything more than carb body temp. without some type of insulation.
 
vlittle said:
I think I see the same results with my RMI probe. I find the carb temperature guage to be of limited use except that it will indicate the temperature rise when you use carb heat.

Right now, I'm rejigging my carb heat system to enclose the sides of the flange that sits on the top of the FAB. I've already added a Wicks heat muff, and I think the extra work I'm doing now will help with the temp rise.

I'll post pics when complete.

Vern
Click to expand...

Here's the pics. First photo are the bits to seal off the flange (including cardboard templates). Second photo shows the aluminum bits attached and the third pic is the final configuration with the strips of baffle seal to help seal around the actuator.

IMG_0728_1.JPG


IMG_0730_2.JPG


IMG_0732_1.JPG
 
vlittle said:
Here's the pics. First photo are the bits to seal off the flange (including cardboard templates). Second photo shows the aluminum bits attached and the third pic is the final configuration with the strips of baffle seal to help seal around the actuator.
Click to expand...

I'll be interested in seeing how the combo with the Wick's heat muff works out.

If memory serves me right, the open flanges were added to permit additional cowl warmed air, as the 2" scat tube & small muff around the exhaust pipe wasn't enough.

Here is an excerpt from the NTSB report of a RV6A (that I've ridden in, and flown a bit) that most likely suffered from icing problems, and had to make a forced landing on a road. The wings were damaged, but have now been replaced.

According to the pilot, after being airborne for approximately 15 minutes, the aircraft's engine
began to lose power. He stated that he applied carburetor heat and switched the fuel tanks, but
observed no change in power. The loss of engine power was gradual, with no sudden stoppage,
backfiring or surging. Unable to restore power to the engine, he initiated an emergency landing
along a county road 6 miles south of the airport. During landing, he switched to another road to
avoid power lines. Upon touchdown, the aircraft departed the side of the road and struck a fence
post, damaging the nose gear, ailerons and rudder.
At the request and in the presence of a Federal Aviation Administration (FAA) inspector, the engine
was examined and test run at the Spanish Fork Flying Service on April 16, 1999. During the
inspection, the fuel was clear and free of contaminates. The engine ran with and without the fuel
pump turned on, and no discrepancies were noted.
The pilot stated in his accident report that "other pilots at the scene agreed the conditions could
cause carburetor icing." According to the FAA inspector present at the accident scene, weather
conditions were partly cloudy and cold with calm wind. According to the inspector, "the conditions
were a classic setup for carburetor icing."
According to the Van's Aircraft construction and operating manual, one method of building the
internal carburetor heat system is to "run a 2-inch air hose from a heat muff and position it to
feed into the alternative air inlet of the carb[uretor] air box without being attached and closed."
According to the FAA inspector who examined the aircraft following the accident, this is the
method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.
The aircraft was issued an airworthiness certificate by the FAA on August 24, 1995.

L.Adamson --- RV6A
 
L.Adamson said:
...
According to the Van's Aircraft construction and operating manual, one method of building the
internal carburetor heat system is to "run a 2-inch air hose from a heat muff and position it to
feed into the alternative air inlet of the carb[uretor] air box without being attached and closed."
According to the FAA inspector who examined the aircraft following the accident, this is the
method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.
The aircraft was issued an airworthiness certificate by the FAA on August 24, 1995. [/i]

L.Adamson --- RV6A
Click to expand...

I'm not sure it's the hose that's the limiting factor during an icing event. It seems to me that the temperature of the air is more critical.

We'll see... it will be a couple of weeks before I'm back in the air and can determine the temperature rise. My setup may show a more noticable rpm drop when carb heat applied, just because the 2" hose is more restrictive.

Carb icing is more prevalent at reduced power settings, however, so the restriction of the 2" hose may not be a factor in practice.

V
 
GRT carb sensor

I took a picture of my GRT carb sensor. When measuring the resistance value of the sensor, it does not matter where I touch with a hot object, the resistance reduces. I was naive to think that somehow the tip of the snesor is insulted from the rest of the sensor. It is not. Well, it proven that the sensor is measuring the carb body temperature. Even if I enlarge the hole and insert insulation in between, the large sensor body outside of the carb will still be reading the temperature in the lower cowl. As several of you said before, the carb tempertaure reading should not be used for carb icing indication.

carbsensor.jpg
 
IowaRV9Dreamer said:
Hi Bill - this is interesting info. Can you post some carb temps that you have seen (ideally with OATs)? I've been planning for a carb and after an unpleasent ice incident at 65 OAT I'd like to have a carb temp sensor. I've "heard" that most RV's don't have very effective carb heat, but I cant find anyone who's posted any numbers. Maybe you have them?

Thanks for any info,
Click to expand...
Ok, we took note of what was going on today. OAT's at 10,500 were in the 20's. Carb temps were around 50. Slowing down and entering the pattern the OAT's were up to 44 and the carb temp was at 70. Pulling the carb temp raised it to 74 degrees.

I still use carb heat and what I want to do is check the temps in the hot humid days of summer.
 
GRT EIS Carb Temp solution!

:)I have figured out the problem with the GRT carb temp sensor. I actually dissassembled one and found that they had only potted the sensor portion into the main body of the sensor. It did not extend all the way into the tip as it should. That's why we are always reading carb body temp, which max's out at 127 degrees in the GRT EIS software. You can get a "Naked" sensor from GRT (talk to Sandy) and make your own probe which DOES work. I just took a 1/4-28 bolt and drilled it out for the sensor, and I tapered it a little at the end, and pushed the sensor all the way almost to the bottom of the bolt, such that it is now protruding into the throat of the carb. I am now reading actual airflow temps, which seem to be right around OAT or a tad higher, and increase when carb heat is applied.
We probably don't need the temp gauge, but I can't stand it when something is not working as it should, so I set about to fix it. I am pleased with the results, I plan to call Sandy at GRT on Monday and explain to them what they need to do. Hopefully they will be interested.

Vic
 
Vic,

The question is, what are you measuring and why?

If you are measuring OAT at the carb, then when you did was good but if you are trying to find out if ice will form on the carb, then measuring the temperature of the carb body is the way to go.

I could be wrong here but my thinking is, if the carb is warm, there is little to no chance of ice forming on it to cause a loss of power.

Thoughts?
 
Bill, the carb body temp could be a little warmer than freezing, but due to the thermodynamics of the way carb works, you will actually get a drop in temperature inside the throat of the carb. The air is actually flowing through a venturi, and as it passes the venturi it expands as the fuel is drawn into it, thus the temp drop.
A temp gauge is not all that critical for these lycomings, as long as one prudently uses the carb heat when the potential for carb icing exists. Mine was installed on the Chelton, and it's not unconfigurable, so I wanted it to work.

Vic
 
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