Baby Thermometers

Babies have very limited methods of communicating what they want or what is wrong with them. Crying can mean any number of different things. At this point, I am fairly good at figuring out when my daughter is tired or hungry or upset about strangers or just bored. However, sometimes she just cries and gets very upset for no clear reason and it is difficult to figure out what is wrong.

A couple of weeks ago, my daughter was very fussy and needier than usual and just refused to nap. After a while, I decided that there was definitely something wrong and that she seemed sick. One of the main tools for determining if our child is sick is our thermometer. Being a scientist (or a geek, if you prefer), I am of course interested in how different thermometers work and how they measure my daughter’s temperature.

While it seems that there are a wide variety of baby thermometers – ear thermometers, rectal thermometers, under arm thermometers and forehead thermometers – the most common types fall into just two categories of operation. The rectal and under arm thermometers use contact between the skin and a metal component on the thermometer to read temperature while the ear and forehead thermometers use the radiation emitted by your baby’s body to calculate temperature. These thermometers use very different techniques to measure temperature than the bulb thermometer many of us grew up with.

Contact Sensors

The metal contact sensors are most commonly electrical sensors. There is a thermistor in the metal detector portion of the thermometer. This is a resistor that changes its resistance depending on temperature. To understand how this works, we need to think about some simple electronics.

Consider a very simple electronic circuit like the one below:

A battery provides a voltage – 9 Volts in this picture. A constant current travels around the circuit because there is just a single path through the loop. An ammeter is a device that is used to measure the current – this is a measurement of how much charge passes through that point in a given amount of time.

Ohm’s Law tells us that voltage, V, is related to current, I, and resistance, R, as follows:

So what happens when my resistor is sensitive to temperature? When you put the resistor in contact with your body, it heats up and its resistance decreases. The battery voltage stays constant, so as the resistance decreases, the current through the system must increase. This makes sense, right? If there is less resistance to the flow of charge, more charges will pass the ammeter in a given amount of time.

By measuring this current, and knowing the voltage of the battery, you can easily calculate the resistance of your temperature sensitive resistor using the equation above. The resistance vs. temperature is something that is very well known by the manufacturer, so once you know the resistance, you know the temperature of your baby who is in contact with the thermometer.

The circuit in your thermometer has a few more components, but this basically how it works. And it does all the calculating for you and just displays a temperature.

Infrared Sensors

If you bring your child to the doctor’s office, they will likely take his or her temperature using an infrared sensor. This is what my nurse uses. She takes a device with a flat, round end and lightly moves it across my daughter’s forehead and reads out the temperature. This is very fast and is great for a kid (like mine) who hates to be touched by strangers.

How does this work? If you remember, a few weeks ago, we talked about Blackbody Radiation. Our bodies emit light, but it is in the infrared where we cannot see it with our eyes. The peak wavelength emitted from our bodies is about 9 microns (Check out the PhET website to see for yourself). The amount of light and the peak wavelength of the light both depend on the temperature of our bodies.

The thermometer uses a thermopile to measure the radiation coming from our bodies (How Stuff Works). Heat from our bodies heats two pieces of metal that make up the thermopile. A voltage is created between the pieces of metal that depends on the heat of our bodies. (Wikipedia)

How do we measure this voltage and turn it into a temperature? We could use the same circuit that we used above. This time, have a constant resistor and use the thermopile as our voltage source. We can measure the current to determine the voltage created by the heat from our bodies and determine the temperature from that voltage.

The infrared sensors are very fast, which is definitely a plus with a screaming, squirmy baby. Both types of thermometers work best when the temperature is taken inside the body – a rectal thermometer or ear thermometer. However, we were discouraged by the hospital from using a rectal thermometer on a newborn because they said it is to easy to cause them harm because they are so tiny. And ear thermometers can be a little tricky because they need to be aimed at the baby’s eardrum to get an accurate reading.

We have one of the forehead thermometers at home so I definitely wanted to make sure I understand how they worked. It turns out my daughter did not have a fever when she was sick a few weeks ago. I did take her to the doctor, though, and she did have an ear infection. The poor kid was sick for a while since the first set of antibiotics did not work on her, but is feeling better. And is now fussy for some other completely unknown reason…

I hope my daughter continues to be healthy and not have any fevers, but I do think it is interesting to know how current thermometers make use of technology to help me figure out what is going on with my little girl.

And, of course, I am looking forward to the time when she can tell me what is wrong and there is a lot less guesswork in trying to figure out if she is sick.