Temperature Measurement

Posted by Jeff on Oct 26, 2009 in Biomedical Engineering |

Why is temperature measurement important?

When you are calibrating equipment, you will probably want to account for temperatures associated with the system.  Here is some information about temperature measurement that might help you better understand.

Types of temperature measurements

The majority of physical processes are affected, to one degree or another, by temperature.  Whether these changes are exploited as a technique for temperature transduction or used to identify temperature induced artifacts, the measurement of temperature is of fundamental importance.  Some physical effects used as the basis for temperature transduction are listed below:

  1. Thermal expansion
  2. Thermocapacitive effect
  3. Thermochemical effect
  4. Thermoelectric effect
  5. Thermoresistive effect
  6. Pyroelectric effect
  7. Radiation effect

Thermal expansion is the basis for function of mercury or alcohol thermometers while thermocouples employ the thermoelectric effect.  Like the resistance temperature detector (RTD), the thermistor is also a temperature sensistive resistor.  Thermistor is an acronym for thermally sensitive resistors.  While the thermocouple is the most versatile temperature transducer and the Platinum RTD is the most table, the word that best describes the thermistor is sensitive.

Major sensor categories

Of the three major categories of sensors, the thermistor exhibits by far the largest parameter change with temperature.   Thermistors are generally composed of semiconductor materials.  Although positive temperature coefficient units are available, most thermistors have a negative temperature coefficient (TC); that is, their resistance decreases with increasing temperature.  The negative T.C. can be as large as several percent per degree Celsius, allowing the thermistor circuit to detect minute changes in temperature, which could not be observed with an RTD or thermocouple circuit.  The price we pay for this increased sensitivity is loss of linearity.  The thermistor is an extremely non-linear device that is highly dependent upon process parameters.  Consequently, manufacturers have not standardized thermistor curves to the extent that RTD and thermocouple curves have been standardized.

The Steinhart-Hart Equation

An individual thermistor curve can be very closely approximated through the use of the Steinhart-Hart Equation:

\frac{1}{T_{i}} = A + BlnR_{i} + C(lnR_{i})^{3}

Here,

T_{i} = temperature in degrees kelvin for i = 1,…,n (number of data points),

R_{i} = resistance of the thermistor (ohms) for i = 1,…,n, and

A, B, C = Curve-fitting coefficients.

Temperature transduction technique factors

The choice of temperature transduction technique depends on a variety of factors, including temperature range, cost, size, weight, power consumption, sensitivity, aging effects, etc.  Another prime consideration that affects a choice is the range over which measurements are to be made and the precision of the readings.  Other considerations that may not be so obvious include selection of a thermal mass to be compatible with the bandwidth requirements of the anticipated temperature variations.

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