OAT indicator reads low

[DaveinOlyWA]

i have had 3 Priuses that all read within 1-2 mph calibrated against a few different GPS systems. but not all that sure about how accurate GPS is either.

i even tracked speed on fairly level ground and saw no difference. as far as my thermometer, mine seems to be relatively close.

[driveleaf]

So back to the OAT topic:

Mine appears to be pretty accurate within 2 degrees. But I run it in centigrade, so..?

My 99 Pathfinder always ran off by about 4 degrees, but I always blamed that on the excess heat of the ICE.

[kolmstead]

Not only low, but unpredictable. On Thursday afternoon car had been out in sun all day, temp in mid-fifties. I drove it a short distance, talked to some guys and jumped back in the car. Indicated OAT was 15 degrees F. It gradually climbed up to somewhere near correct, after 17 miles, when I parked it at home.

Personally, I'd vote to lose the trees and the temp from the speedometer area; they seem equally useless to me. As someone else noted, speedometer reads one mph fast at 55, based on handheld GPS.

700 miles now. First wash yesterday. My "mileage" has dropped quite a bit; I averaged 4.0 mi/kWh until this week. With cold weather, I've been getting 3.3 to 3.5. That heater will draw 4-5 kW if you turn it up high enough!

[DaveinOlyWA]

700 miles now. First wash yesterday. My "mileage" has dropped quite a bit; I averaged 4.0 mi/kWh until this week. With cold weather, I've been getting 3.3 to 3.5. That heater will draw 4-5 kW if you turn it up high enough!

ahh!! the Pac NW chance to penetrate the top 10!!

[tps]

Even the most basic temp sensors have calibration adjustments

Proper thermistors don't need calibration adjustments. I've been working with thermistor temperature sensors for 15 years. $0.50 thermistors will generally give results within a degree or two with no calibration. $4 thermistors will generally give 0.25 or 0.5 at the worst. Apparently the manufacturers trim most of them on the production line.

Now maybe the problem is that the sensor is not in a moving air stream. I wouldn't see how that could be very hard when the car is moving, With no moving air, I'd expect the sensor to read HIGH, because it might be warmed by local sources or indirectly by solar radiation. Another thing one has to do is to "pulse" the excitation to thermistor sensors to reduce "self heating".

In my board test fixture at work, I use 0.1 percent resistors in place of the thermistors which will be connected in the field and reject boards not within 0.2 percent.

The OAT in my Chevy Malibu seems to match other temperature readings within 2 degrees. And it flashes the message "Ice Possible" on the info screen when the OAT is 37 or below.

The dashboard clock in future LEAFs should be set automatically from the GPS NAV system with automatic adjustment for timezone and summer time. That's the "high tech" way!

[TomT]

The brake duct is a common place to place them. Don't think the Leaf has one however.

Now maybe the problem is that the sensor is not in a moving air stream. I wouldn't see how that could be very hard when the car is moving, With no moving air, I'd expect the sensor to read HIGH, because it might be warmed by local sources or indirectly by solar radiation. Another thing one has to do is to "pulse" the excitation to thermistor sensors to reduce "self heating".

[EVDRIVER]

Even the most basic temp sensors have calibration adjustments

Proper thermistors don't need calibration adjustments. I've been working with thermistor temperature sensors for 15 years. $0.50 thermistors will generally give results within a degree or two with no calibration. $4 thermistors will generally give 0.25 or 0.5 at the worst. Apparently the manufacturers trim most of them on the production line.

Now maybe the problem is that the sensor is not in a moving air stream. I wouldn't see how that could be very hard when the car is moving, With no moving air, I'd expect the sensor to read HIGH, because it might be warmed by local sources or indirectly by solar radiation. Another thing one has to do is to "pulse" the excitation to thermistor sensors to reduce "self heating".

In my board test fixture at work, I use 0.1 percent resistors in place of the thermistors which will be connected in the field and reject boards not within 0.2 percent.

The OAT in my Chevy Malibu seems to match other temperature readings within 2 degrees. And it flashes the message "Ice Possible" on the info screen when the OAT is 37 or below.

The dashboard clock in future LEAFs should be set automatically from the GPS NAV system with automatic adjustment for timezone and summer time. That's the "high tech" way!

I don't mean the actual thermistor but a pot that allows the instrumentation to be adjust to match the correct temp. I just calibrated 30 thermostats with remote sensors that varied 5 degrees up and down. Implementing a relatively accurate temp sensor in a car is not that tough. I find it very discouraging how inaccurate many items of the LEAF are being reported, lots of useless readings and generic bars. I really would expect a "high tech" 2011 car to be far better at reporting the most basic functions.

[tps]

I don't mean the actual thermistor but a pot that allows the instrumentation to be adjust to match the correct temp. I just calibrated 30 thermostats with remote sensors that varied 5 degrees up and down.

But my point is that, if the circuitry is designed correctly, an adjustment pot is not only unnecessary but IT usually becomes the largest source of error. IMHO, calibration pots in this type of temperature measurement are a crutch used in poor circuit designs. My latest thermistor circuit design uses a PIC16F88, which only has a 10-bit ADC section, but I anxed over a 0.3 degree error and found the problem was that I didn't correctly understand some undocumented features of the ADC reference voltage input. After I figured out the problem, the circuit measured properly with high accuracy thermistors to better than 0.2F.

One hint I have is to put the reference resistor on the top (connected to Vref) of the voltage divider and the thermistor at the bottom (connected to ground). This puts the non-linear R-T curve for the thermistor in opposition with the non-linear curve for the voltage divider, which makes the temp vs. voltage relationship more linear, rather than less linear as it would be if the thermistor was at the top and the reference resistor was at the bottom.

Most of our users have problems understanding my design philosophy at first, but soon come to appreciate it. "There is no calibration adjustment or procedure. It either measures within specifications or its broken and must be repaired or replaced." We do implement calibration with other types of sensors, but not thermistors...

[EVDRIVER]

I don't mean the actual thermistor but a pot that allows the instrumentation to be adjust to match the correct temp. I just calibrated 30 thermostats with remote sensors that varied 5 degrees up and down.

But my point is that, if the circuitry is designed correctly, an adjustment pot is not only unnecessary but IT usually becomes the largest source of error. IMHO, calibration pots in this type of temperature measurement are a crutch used in poor circuit designs. My latest thermistor circuit design uses a PIC16F88, which only has a 10-bit ADC section, but I anxed over a 0.3 degree error and found the problem was that I didn't correctly understand some undocumented features of the ADC reference voltage input. After I figured out the problem, the circuit measured properly with high accuracy thermistors to better than 0.2F.

One hint I have is to put the reference resistor on the top (connected to Vref) of the voltage divider and the thermistor at the bottom (connected to ground). This puts the non-linear R-T curve for the thermistor in opposition with the non-linear curve for the voltage divider, which makes the temp vs. voltage relationship more linear, rather than less linear as it would be if the thermistor was at the top and the reference resistor was at the bottom.

Most of our users have problems understanding my design philosophy at first, but soon come to appreciate it. "There is no calibration adjustment or procedure. It either measures within specifications or its broken and must be repaired or replaced." We do implement calibration with other types of sensors, but not thermistors...

Well, clearly Nissan can't get it right. I would bet I can get an electronic oven sensor for $20 that will work better than what is in the car. The adjustments I make are more for temp compensation for placement and other placement issues. I will have to see how my car acts. 1-2 degrees F is not a big deal but as people have mentioned if it is more then why have it at all. Perhaps one of the displays can be used for something else with some modification.