Experiment 1 - Measurement

Introduction

Temperature Measurements - Thermometer

In any temperature measurement we need to approach the measuring instrument at proper eye position, at eye level. With a mercury thermometer, when we are up close to the mercury level at eye level, we will observe that the surface of the mercury forms a "lens-shaped" that bulges upward. This is called the meniscus. Unlike the meniscus that we observe with a liquid like water, the mercury forms a positive meniscus. In taking the measurement, we always read the top of the positive meniscus. Below is an example of a temperature reading. The diagram shows an expanded view of the portion of the thermometer near the surface of the mercury.

Mercury Thermometer

Let's analyze this measurement:

  1. Upon inspection of the thermometer, we observe that the finest division is a 1 oC division.
  2. We can always make an estimate of a reading to within the finest division. Therefore, to the best of my judgement, I conclude that this particular thermometer is able to provide a reading accuracy to +/- 0.2 oC. (Note: A plus or minus 0.2 oC reading accuracy means that a temperature measurement using this thermometer will contain uncertainty in the first decimal place.)
  3. The top of the positive meniscus almost lines up with the 30 oC mark.
  4. The temperature reading is determined to be 29.8 oC. This temperature reading has 3 significant figures. (Note: the '8' in this measurement is an uncertain figure. This measurement is accurate to 29.8 +/- 0.2 oC. It means that using this thermometer, the accuracy of the temperature is between the range 29.6 oC to 30.0 oC.)

 Alcohol Thermometer

The Alcohol thermometer is an alternative to the mercury thermometer. The range of usefulness of the thermometer is set by the boiling point of the liquid used. In the case of the ethanol-filled thermometer the upper limit for measurement in 78°C. Thus, alcohol thermometers cannot be used to measure high temperatures. The ethanol version is the most widely used due to the low cost and relatively low hazard posed by the liquid in case of breakage. When reading the alcohol thermometer, make sure the eye position is level with the surface of the alcohol level. You will not observe a meniscus, as the alcohol level will be flat.  Determine where the alcohol level lines up with the nearest mark on the scale and follow the above procedure to report the temperature to within the finest division.

 


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