A 4-wire connection is often used to minimize measurement errors — 2-wires to carry the excitation, 2-wires to measure the voltage across the sensor. Very precise systems also reverse the polarity of the excitation and average the two readings to remove and inductive, capacitive and thermocouple effects. Some also pulse the excitation to minimize self-heating of the sensor due to the excitation power. A digital thermistor thermometer provides an excitation voltage or current to the thermistor.
Because the thermistor change in resistance is large compared to the resistance of the leads, a thermistor usually has a 2-wire connection. The excitation is converted to a voltage signal by the thermistor and the thermometer then converts the measured voltage to temperature. Now we have the thermometer conditioning the sensor and making an electrical measurement of its output. This signal, voltage or resistance, is the analog signal. In the thermometer this signal goes to an A to D converter Analog to Digital.
The A to D converts the analog signal into a series of pulses digital signal. Now the signal is in the digital world. The accuracy of the digital thermometer mostly lies in both the analog and the digital worlds. The precision of the excitation and the errors making the measurement plus any errors due to the change in ambient temperature in the analog world, and in the digital world, the error in the A to D converter and the precision of the curve fit equations used to match the exact curve of the sensor.
You now have an overview of all the signal processing that goes on inside a digital thermometer. Understanding how the sensor and the thermometer interact should help you apply the sensors properly and make the best possible measurements. Digital thermometers rely on the principle that the electrical resistance of metal changes with temperature. Digital thermometers are different to traditional thermometers such as liquid filled and bi-metal thermometers in that they require a sensor.
Typically the sensor used in digital thermometers will be one of the following four:. All of these sensors work by producing a resistance, current or voltage in reaction to a change in temperature. These changes are referred to as analogue output signals. Although all of these digital thermometer sensors provide the same end result in a digital display of the temperature the way they work this out varies.
In basic terms the analogue outputs of these sensors are different. If so, remove the thermometer and use a different thermometer to take an oral, armpit, or forehead reading. No-touch forehead thermometers usually have a removable cap that keeps it clean during storage. Make sure you take the cap off before use.
Digital thermometers are a popular option, which can be used orally, rectally, or under the armpit. There are some no-touch forehead thermometers that are also popular, especially in social settings. Digital thermometers are the most accurate way to take body temperature. There are many types, including oral, rectal, and forehead, plus many that….
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