accuracy of the temperature gauge? - Toyota Prius Forum : Prius Online Toyota Forums
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#1 (permalink) Old 06-27-2006, 11:49 PM
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Default accuracy of the temperature gauge?

I've found that my MFD thermometer does not display the same temperature as, say, a thermometer displayed out front of a bank. The difference has been as much as 9 degrees when compared to several different banks. How accurate are these bank thermometers and how accurate is the MFD thermometer? Anyone else experiencing this?

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#2 (permalink) Old 06-28-2006, 01:11 AM
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The Prius thermometer is in the bottom of the driver's side mirror. If that's the sunny side of the car, it can read higher than you expect.

As for whether a temperature sign reads correctly, those can vary widely. I seldom trust them.

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#3 (permalink) Old 06-28-2006, 09:50 AM
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Are you sure about that? Repair manual seems to show that it's behind the front bumper towards the passenger side.

Bank thermometers can be off especially if they are sitting in the sun. I bet they are in or near a black container too. I find my MFD to be fairly accurate even with most temp/clock displays out there unless the sun has been beating on them.

Also I have found the MFD can read high if the car has been sitting in the sun, but settles down quickly once I get moving.
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#4 (permalink) Old 06-28-2006, 06:55 PM
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It's possible that both are accurate. We know that our atmosphere is made up of different layers of air which can be of different temperatures. Some of those layers are pretty thin. So, depending on where the bank temp sensors are located, they could be correct as well as the car's sensors.

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#5 (permalink) Old 06-28-2006, 07:41 PM
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In general, I think that most thermometers that we (normal people) run into fall far short of being able to be called accurate. I just had my heating and A/C systems replaced at home and had various thermometers around the house to see how even they were working. I couldn't get any 2 thermometers to agree with each other, even if they were right next to each other. I think all you can reasonably expect from any given thermometer is that it always reads the same when the external temperature is the same. I guess that is more precision than accuracy.
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#6 (permalink) Old 06-28-2006, 08:27 PM
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Thermometer accuracy has a lot to do with where the probe is placed and under what conditions. A bank thermometer may not necessarily be placed in a position where it is receving a correct ambient air temperature. A thermometer or probe placed in an enclosure may prevent an accurate reading by shielding the probe from significant changes in temperature or by absorbing heat and then continuing to radiate that heat in the presence of the probe as the outside temperature drops.

I think that Toyota has done a good job of shielding the on board temp sensor from temperature readings that may be generated by solar heating, however that same protection of the sensor prevents it from quickly reporting major temperature changes, like driving out of a parking garage basement to a hot day. You go from a 70 degree environment to a 90 degree environment in a matter of seconds and the probe doesn't sense the temp change for many minutes.

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#7 (permalink) Old 06-28-2006, 10:52 PM
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Quote:
Originally Posted by jeromep
Thermometer accuracy has a lot to do with where the probe is placed and under what conditions. A bank thermometer may not necessarily be placed in a position where it is receving a correct ambient air temperature. A thermometer or probe placed in an enclosure may prevent an accurate reading by shielding the probe from significant changes in temperature or by absorbing heat and then continuing to radiate that heat in the presence of the probe as the outside temperature drops.

I think that Toyota has done a good job of shielding the on board temp sensor from temperature readings that may be generated by solar heating, however that same protection of the sensor prevents it from quickly reporting major temperature changes, like driving out of a parking garage basement to a hot day. You go from a 70 degree environment to a 90 degree environment in a matter of seconds and the probe doesn't sense the temp change for many minutes.
The outside temperature reading in my Canadian 2005 Prius is almost always 1 degree C higher than the outside ambient temperature. I've seen this winter and summer.
If there is a car ahead of you, that car will mix the air quite a bit, so there shouldn't be any question of layers of air with different temperatures.
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#8 (permalink) Old 07-01-2006, 11:46 AM
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Default Air Temperature Measurement 101

I frequently measure air temperature in experiments. When you do this, you perform an "energy balance" on the sensor, be it a thermocouple, thermistor, or RTD. The sensor temperature will be affected by heat transfer to/from it via convection with the air, radiation with the environment, and conduction down the sensor lead wires. To get the sensor temperature as close as possible to the air temperature, you design the sensor installation to maximize convection with the air and minimize the other two heat transfer terms. So a good air temperature probe will be aspirated with air and surrounded by radiation shields (to block both sunlight and infrared radiation). Also, the lead wires leaving the sensor will be run for a certain distance through a region of uniform temperature to reduce heat conduction along the leads.

I haven't examined the sensor in the Prius, but it will be affected by the surrounding metal being heated by the engine and by sunlight. If they've located it properly, it should be shaded from sunlight and not "see" much hot metal and it should be well exposed to the flow of outside air due to motion of the car and/or the radiator fan (obviously it would need to be located upstream of the radiator). The sensor will have some thermal mass, so it should take a few minutes to settle down to the outside air temperature after it has been exposed to the flow of outside air. The best reading should be after you've been driving at high speed for a while. If the sensor itself is decent, it should give you a more precise reading than a bank thermometer that is sitting in still air.
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#9 (permalink) Old 07-01-2006, 01:11 PM
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I've seen bank thermometers read 150 degrees, so I never believe them. Sensor placement is usually poor and the readings unreliable.

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