Inaccurate reading of water temperature gauge
Sometimes you want to check the accuracy of a thermometer, or you simply don't trust the temperature displayed. We'll start with one of today's biggest misconceptions when it comes to checking water temperature. It can also be used to compare temperature readings to your meter, but...this will be used as a base comparison, not an exact comparison. Here's why: Your thermometer measures surface temperature and doesn't have the magical ability to measure coolant temperature through aluminum or cast iron parts. The surface temperature is always lower than the coolant inside the engine, or the engine "core temperature". The surface area you are measuring has a job to do, and that is to dissipate heat. A typical temperature difference is between 5 and 25 degrees, between the meter in question and the temperature gun. It depends on how efficiently the component being fired with the temperature gun dissipates heat. So if your meter reads 195, and you aim and shoot and get a reading of 185, you can safely assume you're seeing a reasonable and realistic comparison. So, when is it a good idea to trust a temperature gun? If you suspect a low reading, everything changes.
Check the accuracy of the mechanical thermometer: A mechanical thermometer will have a thick black tube attached to the back of the meter and a sensing probe on the other end. This is called a capillary. It's filled with flammable ether gas, so you don't want to flame test the meter sensing probe.
The proper way to test one of these meters is in a pot of hot water. Place the sensing probe completely in the water and monitor the water temperature using a meter and a known good and accurate thermometer. Of course, extreme care should be taken when handling any hot fluids. Also, knowing that water boils at 212 degrees Fahrenheit (sea level) may give you an indication of the accuracy of the meter.
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Mechanical thermometers do not use replaceable sensors and are generally not repairable. Therefore, do not attempt to remove the capillary or sensing bulb for any reason.
In fact, most car speed or speed meter displays may not be able to accurately reach the current value immediately when accelerating or decelerating.
