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Temperature sensors are broadly referred to as thermometers. There are actually several classes that utilize
different technology to produce the temperature reading.
Bimetal Indicating Material Glass Bulb Vapor/Gas Filled Galileo
RTD Thermistor Thermocouple Infrared
BIMETAL thermometer. Two different metals are bonded together with one end attached to an indicating
needle which aligns with a circular scale on the face of the instrument. Since the metals expand at different rates, movement occurs depending on the temperature
fluctuation and the needle moves. They are quite durable, fairly accurate, and are generally available for under $50. They are not so good for rapidly changing
temperatures since some lag time is needed for the bimetal to stabilize. They do cover a fairly wide range of temperature - from about -40 to +1000 degrees F. Most
of us are familiar with room thermometers.  In these cases, the indicating dial is in the same location as the temperature of interest. What to do when the temperature of interest is in a high temperature or hostile
environment? Simple, some models have stems up to 36” long to reach into those nasty areas.
INDICATING MATERIAL
. A variety of temperature crayons and pellets are available that melt at specific temperatures. These do not really measure temperature directly, but do indicate the maximum temperature
that a material was exposed to. Using several different ranges one can determine maximum temperature
within a fairly broad range. They are available in 12 degree ranges from 113 degrees F to about 525
degrees, then 50 degree increments up to 2500 degrees F. These cost about $10 each. Indicating labels
are similar in function with up to about 10 indicating “windows” on each label. Each window corresponding to
a different maximum temperature. The labels are purchased for specific ranges, in increments of about 40 degrees from 100 degrees to 550 degrees F. They cost about $1 each in packs of 10.
GLASS BULB thermometer.  This was one of the earliest forms. A bit of mercury (or organic liquid) is
encapsulated in glass bulb at the base of a glass graduated tube with a very small interior diameter.
Of course the tube is totally sealed. As the temperature increases, the volume of the mercury/liquid
increases and it has only one place to go - up the inside of the tube. While they are more fragile than
the bimetal variety, they are generally less expensive, survive well in aggressive chemicals. Standard
mercury filled units generally cost less than $20 and are available in a variety of temperature ranges from -40 to +750 degrees F. NIST traceable models cost in the $200 range.
VAPOR/GAS FILLED thermometer. These operate on a similar principle to the glass bulb type except in this
case it is the gas expands, rather than a liquid. These have a significant advantage in that the bulb can be
quite remote from the indicator. They are connected by a capillary tube (generally armored to protect it) up to
30 feet long. This lets the bulb to be in some nasty location while the indicator is well protected from the
unfriendly environment. The vapor filled units are generally less than $150 while the more accurate gas filled
models are around $350. Of course, the costs vary based on the materials of construction, the size of the indicating dial, and the length of the capillary tube.
 GALILEO thermometer: These interesting models operate based on principles of specific gravity. In a
transparent liquid filled column, sealed glass floats are fitted with weights. The weights vary depending on
the temperature to be indicated. As the temperature of the fluid increases the relationship of relative
“weight” between the fluid and the floats changes. The higher the temperature, the more buoyant each
float becomes. These tend to be used in decorative settings around the home or office.
ELECTRONIC DIGITAL RESISTANCE (RTD and Thermistor). These are based on the change in
resistance of a conductor when the temperature of the wire changes. The RTD (Resistance Temperature Detector) units have a  typical operating ranges between -250 and +1500 F. with typical accuracies of better
than +/- 0.5 degree F.  The Thermistor units have an extremely quick response time and are generally less expensive than the RTD models. A down
side is their limited temperature range of from -40 to +300 degrees F. and accuracies in the +/- 1 degree range. Both the RTD and Thermistor units are quite durable compared to glass
thermometers. The electronic displays provide easy to read information and a variety of options such as high or low hold provide users with more options than the older vintage technology
. Costs for hand held RTDs are in the $150 range while the Thermistor counterpart is in the $75 range.
THERMOCOUPLE: These operate based on the temperature change that occurs at the junction of two
dissimilar wires. When the temperature changes a small current is generated by the junction. This current is
then compared to a reference junction (calibrated standard or ice water bath) and converted to a temperature
by electric or electronic means. So the system includes the thermocouple itself, connecting wiring and some
method (Generally a digital meter) to display the temperature reading. Types J, K, T , E, R/S, B and C consist
of different types of wires and are for different temperature ranges from -200 to +5000 degrees F. Once past
1200 degrees thermocouples are about the only option available for sensing temperature.  A protective sheath of stainless or higher temperature tolerant material is generally used
to protect the thermocouple wires from a variety of harsh environments. Another significant advantage of the thermocouple is that the indicating
instrument can be a very long distance from the thermocouple environment. Once the system is calibrated and the current from the thermocouple is
captured, a variety of electrical options are available for getting the information to a display unit. A simple
portable industrial thermocouple kit for with all the goodies costs around $300 to allow measurements of up to 2000 degrees F. Of course costs vary tremendously depending on the application.
INFRARED THERMOMETER / PYROMETER: We’ve all experienced the look of a red hot cooking burner
on the kitchen stove. Experienced blacksmiths and foundry workers can tell temperature reasonably well
based on the color of the metal. So, it’s reasonable to expect that the temperature of a material, effects the
color. As it turns out, the infrared light spectrum works very well for this and is the basis for the infrared thermometer or pyrometer.  These units do not require a contact with the material and are available as hand
held units for portability or as remote sensors. They can sense temperatures from -20 to +2000 degrees F with a response time of under 1 second. The range to the target can
be up to 60 feet for some of the long range units. The cost range is from under $100 for short range, less accurate models, to over $1000 for the longer range, higher accuracy
units. So what’s the catch? Well, accuracy can be a problem. Particularly at long
ranges, exactly what is being “seen” by the infrared sensor? Additionally, there’s this
little term called emmissivitiy. It turns out that when a bright shiny surface and a dull black surface are at the same temperature, the emmissivitiy is quite different. And the
emmissivitiy effects the infrared reading. Provided you are checking the same thing all the time, the values will be relatively correct but accuracy should be established by
measuring the temperature of the object with other NIST traceable means and adjusting the emmissivity of the infrared thermometer to produce the known temperature value.
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