how do digital thermometers work?

Discussion in 'Non Technical' started by JETzx, Mar 16, 2003.

  1. JETzx


    i got this little digital thermometer with an external sensor which is wires running into this small 100x40mm plastic labeled "sensor". I've seen other sensors and half of them use metal probes the other half also plastic. My question is what would i find inside this plastic? just the two wires but plastic is used for protection against water? would i be able to remove this plastic/will it affect the accuracy?Thanx
  2. CHILI

    CHILI Indestructable Target

    Without seeing it , the "plastic" you refer>

    to is probably a Thermistor. This is an alternate to the metal Thermocouple you have seen. Removing the plastic will ensure that your Thermometer will never work again.Cheers
    :-0 :p
  3. chewy

    chewy Active Member

    three possible ways >>

    The most common sensor is a thermoresistor (or thermistor). This device changes its resistance with changes in temperature. A computer or other circuit measures the resistance and converts it to a temperatureRemote sensing of an object's temperature can be done using its infrared radiation. This technique is a very good way to detect the temperature of a person's eardrum. Thermocouples are the most popular temperature sensors. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve.In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients. Standard tables show the voltage produced by thermocouples at any given temperature. Unfortunately it is not possible to simply connect up a voltmeter to the thermocouple to measure this voltage, because the connection of the voltmeter leads will make a second, undesired thermocouple junction. To make accurate measurements, this must be compensated for by using a technique known as cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to a thermocouple does not make several additional thermocouple junctions (leads connecting to the thermocouple, leads to the meter, inside the meter etc), the law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, however, thermocouples junctions are made by welding the two metals together (usually by capacitive discharge) as this ensures that the performance is not limited by the melting point of solder.All standard thermocouple tables allow for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term 'cold' junction compensation). Maintaining a ice bath is not practical for most measurement applications, so instead the actual temperature at the point of connection of the thermocouple wires to the measuring instrument is recorded. Typically cold junction temperature is sensed by a precision thermistor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. For less critical applications, the CJC is performed by a semiconductor temperature sensor. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error in the measurement of cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip. As well as dealing with CJC, the measuring instrument must also allow for the fact that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5th to 9th order depending on thermocouple type). Analogue methods of linearisation are used in low cost themocouple meters.
  4. Bollie7

    Bollie7 New Member

    Good answer Chewy

    Off the top of your head no doubt.
    Good to see when someone takes the time to answer someone elses question in such detail.
    Peter Marr
  5. JETzx


    Thanx for the info guys (n/m)


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