Video: Measuring Temperatures

In this lesson, we will learn how to correctly use thermometers to measure the temperature of liquids and gases.

15:36

Video Transcript

In this video, we’re talking about measuring temperatures. The image we see on screen shows one way that cooks have used to approximate the temperature of a hot dish. The length of time a person can leave their hand a certain distance above a cooking pot indicates roughly the temperature of the pot. This is one way to measure temperatures when we don’t have a more precise instrument, namely a thermometer, at hand.

When we do have a thermometer though, that can give us a much more precise result. Generally speaking, thermometers can be divided into two categories. The thermometers we often see on the outside of a house to measure the outside air temperature or the one a doctor may use to measure our body temperature are known as analogue thermometers. These are also sometimes called mercury thermometers because mercury is the liquid that can be inside them. Bit by bit though, analogue methods for measuring temperature are being replaced by digital thermometers. These thermometers not only make a measurement of temperature, but they also interpret and report the result. This is in contrast to an analogue thermometer where the temperature is indeed measured, but it needs someone to interpret that measurement.

To better understand how to interpret a measurement made by an analogue thermometer, let’s consider how they work and how to read their results. When we measure a temperature with an analogue thermometer, the first thing to keep in mind is that the part of the thermometer sensitive to temperature, typically one end of it, needs to be surrounded by whatever we’re measuring the temperature of. So let’s say that we have an analogue thermometer, and the temperature-sensitive part of that thermometer is at the lower end, this tip here. If we wanted to make a measurement of the temperature of a liquid inside a container, then we wouldn’t get an accurate reading by positioning our thermometer like this. Even though the thermometer is over the liquid, it’s not being surrounded by it. In this case, our thermometer would really be measuring the temperature of the air just above the liquid rather than the liquid itself.

The measurement we really want, the temperature of the liquid in the container, would require that we position the thermometer something like this, with the temperature-sensitive end of it submerged in this liquid. A second helpful step to follow in making an accurate measurement using an analogue thermometer is to give the thermometer time to adjust once it’s in contact with the new material. For example, say that this thermometer here was measuring the ambient air temperature. And that temperature was read out at this position on the thermometer. Then say that we filled a cup with ice, and we put the end of the thermometer in the cup. We could expect this temperature to be different from the ambient air temperature the thermometer was just measuring.

If this temperature was lower than the ambient air temperature, then the level of the liquid in the thermometer would start to drop. While it does, while the level of the liquid in the thermometer is changing, we wouldn’t want to make our measurement of the temperature at that time. Instead, we would want to wait until the level of the liquid had stopped changing in the thermometer before making our reading. Before we make our reading on an analogue or a digital thermometer, it’s important that we’ve confirmed that the temperature is a stable value, that it’s not still climbing or still dropping.

The next step applies specifically to analogue thermometers. Say that the temperature in our thermometer had stabilized to this value right there. If we wanted to read what that temperature was, typically, this involves bringing our eye up close to the side of the thermometer. But the question is, where do we put our eye? Do we put it here or, say, here below the level of the liquid in the thermometer? Or do we put our eye on level with the liquid? The best way to get an accurate reading from an analogue thermometer is to put our eye on level with the liquid inside. Sometimes, this is a bit difficult. In some cases, it will be easier to have our eye at some other position relative to the thermometer liquid. Especially, if our thermometer isn’t aligned vertically like it is here. For the sake of accuracy though, getting our line of sight so it’s perpendicular to the thermometer and making sure our thermometer is positioned vertically, wherever possible, is important.

Now, if we were to zoom in very close on the level of the liquid in the thermometer, we might see something like this, that the top of the liquid actually isn’t flat but has a curve to it. This curved surface, this shape, is called the meniscus. And depending on the liquid we have in our thermometer, whether mercury or ethanol or something else, this meniscus will either curve upward, like we see it is here, or downward. The important thing, from a measurement perspective, is that when we measure the temperature, we want to measure the level of the flat part of the meniscus at its middle. Depending on the way the meniscus curves, this is either the top or the bottom of this meniscus. But in either case, we always measure the midpoint, the flat part, and where that lines up with the side of the thermometer.

Sometimes, the thermometer we’re looking at is so narrow that the meniscus is invisible. If so, there’s no need to be concerned about it. But if we can see the meniscus, then measuring from its midpoint is the proper way. And one final pointer for measuring temperature using an analogue thermometer is that if the temperature we want to measure is off our thermometer scale, we’ll want to find another thermometer whose scale does include this temperature. That is, say we have a thermometer set up with a certain scale, as all thermometers have. And say further that as we start to make a temperature measurement, the liquid in the thermometer goes all the way up to the very top of the scale. Indeed, it goes as far up as the thermometer will physically allow the liquid to rise.

In this case, it’s likely that our thermometer scale is unable to handle the temperature that we want to measure. That is, it doesn’t go high enough to measure it accurately. In that case, rather than saying that the measured temperature is the highest value this particular thermometer can measure, or even simply guessing at what the temperature maybe. It’s best to get a thermometer with a different scale that is not exceeded by this particular temperature. That way, we can be confident in the temperature reading we make.

Now, we’ve said that this is a process for measuring temperatures using an analogue thermometer. And indeed, some of these steps are specific to analogue thermometers and are not required when making a measurement using a digital thermometer. But other steps apply to both types of thermometers. And we’ll want to keep them in mind regardless of whether we’re using an analogue or a digital thermometer. Keeping all this in mind, let’s now get a bit of practice with these ideas through an example exercise.

The diagram shows three different angles at which the temperature shown on a thermometer could be read. From which angle is it best to read the thermometer to get an accurate reading?

All right, so we see in this diagram these three potential angles A, B, and C for positioning one’s eye to read off the temperature on this analogue thermometer. From angle A, our eye is looking down on the level of the liquid in the thermometer. From angle B, our eye is on level with that liquid. And from angle C, our eye is below that liquid level. So we’re looking up at it. Our question asks, from which of these three angles is it best to read the thermometer in order to get an accurate reading?

To see which angle of observation will give us the most accurate temperature reading, let’s take a very close up, zoomed-in view on the level of the liquid in our thermometer. That view looks something like this, where the level of liquid in our thermometer is on line with a smaller hash mark. And that’s one hash mark below 20. And if we look carefully at the top of our thermometer, we see that this thermometer measures temperature in degrees Celsius. Therefore, this 20 that we see here indicates a temperature of 20 degrees Celsius. Now, we don’t know the particular liquid that this thermometer is filled with. It might be mercury; that’s one common type. Or it might be ethanol, a type of alcohol. Or it could be something else.

But the important thing to notice is that whatever the liquid is, it doesn’t run up to the edge of our thermometer. That is, there’s this gap that we’ve just drawn in between where the liquid actually rises and falls in a column through the center of the thermometer and the place on the outside of that thermometer where the temperatures are marked in. This distance, this gap, is very important because it can affect the accuracy of our reading.

For example, consider what would happen if our eye was at the position marked out as A. In that case, our line of sight drawn in on our zoomed-in diagram might look very much like this. That is, our eye is over here looking down on the liquid in our thermometer. And this would mean that our line of sight, when it looks at the level of the liquid in the thermometer, passes through the marking identified as 20 degrees Celsius. So if our eye was up here, if it was around position A, then we might say the thermometer is reading a value of 20 degrees Celsius. But from our up-close view, we can see that that’s not accurate.

Now, let’s consider putting our eye at the lower position at angle C. In this case, again using our zoomed-in view, our line of sight might look very much like this. That is, our eye is positioned here, looking up at the level of the liquid. And from this position, we see the liquid level crossing here on our thermometer. Now, if we look at the big scale view of our thermometer, we see that each one of these smaller tick marks indicates a change in temperature of one degree Celsius. Which means that this tick mark here represents 18 degrees Celsius. It’s two tick marks below 20.

So if our eye was in position A, we might record the temperature as 20 degrees Celsius. And if it was in position C, we might record 18 degrees Celsius. But neither of these we can see is accurate. The better reading would be on level with the liquid in our thermometer. That is, it’s best for our eye to be here on that level. So that the distance gap between the liquid in the center of our thermometer and the thermometer markings on the outside doesn’t lead to a distorted or inaccurate reading. So we’ll say it’s from angle B that this thermometer should be read in order to get an accurate reading.

Let’s look now at a second example exercise.

Sofia wants to measure the temperature of some water that has been heated to near its boiling point. She uses a thermometer that has been on a table in the room for an hour. She places the bulb of the thermometer in the water and immediately reads the temperature on it, as shown in the diagram. She determines that the temperature of the water is 19 degrees Celsius.

Okay, before going on to our question, let’s consider this diagram. In it, we see this thermometer with its bulb, the temperature-sensitive part, submerged in some hot water. We’re told that this particular thermometer has been sitting on a table in the room for an hour before this measurement was made. And we’re also told the measurement took place immediately after the thermometer was put into the water. Now, we can see that at this moment, where the temperature was measured, the position of the red liquid inside the thermometer is at this level. It’s tricky to see, but we’re told that this level is 19 degrees Celsius. One other piece of information we’ll want to keep track of is the fact that this water, whose temperature is being measured, has been heated to near its boiling point. Now, keeping all this in mind, let’s go on to our question.

Which of the following statements explains why this answer is incorrect? A) She should hold the bulb of the thermometer just above the surface of the water. This is where the temperature is highest. B) She should submerge the whole thermometer in the water. And C, she did not wait for the reading to stabilize. The temperature of the water is actually far higher than 19 degrees Celsius.

All right, so let’s recall that Sophia recorded the temperature of this water to be 19 degrees Celsius, that that was the reading on the thermometer at this instant. We’re told that this answer is incorrect, and we want to explain why that is. Answer choices A, B, and C give us options for doing that. Let’s consider these options one by one.

Option A says that Sophia should hold the bulb of the thermometer just above the surface of the water. In other words, option A is saying that the thermometer should have been positioned something like this, where the bulb, the part that’s sensitive to temperature, is not actually in the water being measured. Rather, it’s just above that surface. The trouble with this explanation is that the bulb of the thermometer, in a correct measurement, is placed where it’s surrounded by the substance or material whose temperature we want to measure.

If we wanted to measure the temperature of the air right above the surface of the water, then indeed this would be a good position for our thermometer. But what we really want to measure is the temperature of the water. Set up this way, our thermometer would not measure that temperature. So we’ll cross option A off our list of possible explanations. This doesn’t mean, by the way, that we’re saying that 19 degrees Celsius is a correct temperature reading for the water. But we’re only saying that option A is not a correct explanation for why this number might be incorrect. Let’s move on to option B.

This option says that Sophia should submerge the whole thermometer in the water. Well, if she could find a thermometer small enough to do that, then that would be possible. And in that case, the temperature-sensitive part of the thermometer, it’s bulb, would indeed be submerged in the material whose temperature we want to measure. But in order to make an accurate temperature reading, it’s not necessary for the entire thermometer to be submerged in this material, in this case, the water. It’s only the bulb, the temperature-sensitive part of the thermometer, that must be. Option B is implicitly saying that the reason 19 degrees Celsius is not a correct temperature reading for the water is because the thermometer was not entirely submerged in the water. As we’ve seen though, that’s not necessary for an accurate reading. Therefore, we’ll cross option B off our list too.

Our last choice, option C, says Sofia did not wait for the reading to stabilize. The temperature of the water is actually far higher than 19 degrees Celsius. If we remember, this thermometer that Sophia used was at room temperature and had been for about an hour. We were told that immediately after the thermometer bulb was submerged in the water, Sophia took the reading on the thermometer. And that reading she recorded as 19 degrees Celsius. We have good reason to believe, though, that 19 degrees Celsius is not the temperature of the water. That’s because we’re told in the problem statement that the water is heated to near its boiling point. And we can recall that the boiling point of water on the Celsius scale is 100 degrees Celsius. If this water is near boiling, and we’re told it is, then we would expect a temperature reading near 100.

So the fact that the water temperature is actually much higher than 19 degrees Celsius. And the fact that we’re told Sofia made the temperature reading immediately after the thermometer was put into the water points us to the conclusion that she did not wait for the temperature reading to stabilize. And that indeed the real temperature of the water is actually much higher than 19 degrees Celsius. This, then, is our answer as to why it is that 19 degrees Celsius is an incorrect reading of the water temperature. It’s incorrect because Sofia did not wait for the reading to stabilize. And the temperature of the water is actually much higher than 19 degrees Celsius.

Let’s take a moment now to summarize what we’ve learned about measuring temperature. In this lesson, we saw that temperature can be measured using an analogue or a digital thermometer. We also learned steps in making correct temperature readings. Some of these steps apply just to analogue thermometers, and some apply to both analogue and digital. First off, the thermometer bulb, the part of the thermometer sensitive to temperature, must be surrounded by the object of interest. By object of interest, we mean the object whose temperature we want to measure. Next, we want to be sure to let our thermometer reading stabilize. That is, we want to record the temperature when that reading is neither going up nor going down but is stable.

In the case of analogue thermometers, we want to make sure to read the temperature on level with the liquid in the thermometer. And then, if a meniscus, a curved surface at the top of the liquid in the thermometer, is visible, we want to make our measurement from the middle of that meniscus. And lastly, we want to be sure to use the thermometer whose scale contains the temperature of interest. That is, the temperature we’re trying to measure isn’t above or below that scale. This is a summary of measuring temperatures.

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