Solid State Temperature Sensor Primer

What is a temperature probe with a solid state sensor?

Let's start with some background information.

If you're looking for a temperature probe, you may have heard of some of the more common sensor types.  RTD's and thermocouples are very common, and for very low cost applications, thermistors are also common.  Solid state temperature sensors are a rarity in sensor probes, so they deserve some explanation.

But first some simple definitions:

  • temperature probe - A temperature probe consists of a protection tube, a sensor element, and optionally a connection head.  The sensor element is placed inside of the protection tube and a connection head is attached to the other end of the tube.
  • sensor element - This is the device that actually measures the temperature. 
  • protection tube - The protection tube protects the sensor element from the outside world and proves an anchor the can be mounted to something or held by hand.
  • connection head - the connection head is where the wires that are attached to the sensor element are attached to the outside world.  Many hand held temperature probes simply have a cable attached directly to the protection tube and do not have a connection head.  Connection heads are common in industrial settings where the sensor wiring is run through conduit.  The connection head is anchored to the protection tube.

Before explaining how solid state temperatures work, a brief introduction to the other common sensor elements is in order.

  • Thermistor - These are essentially resistors who's resistance changes with temperature.  A typically value of resistance at room temperature is 10K ohms.  This value is significant because it makes them easy to interface to.  The device that measures them is essentially just an ohm meter.  A few ohms of additional resistance in the connection doesn't significantly change the accuracy of the measurement.   Unfortunately, their initial accuracy is poor and they tend to have non-linearities which limit their useful measurement range.  This is because the accuracy and linearity is a function of the material composition which is difficult to control.  They are typically used for OEM applications where the designer can carefully take into account their inherent limitations.  This Wikipedia article about thermistors is quite extensive.
  • RTD - Resistive Thermal Device.  These typically have a thin platinum wire whose resistance changes with temperature.  Unlike thermistors, the resistance and linearity are inherent properties of the platinum material.  This means that they can be manufactured very easily and the non linearity can be corrected during measurement.  A typical resistance value is 100 ohms.  This is a problem, however, as a few ohms of extra resistance can make the measurement completely invalid. Worse yet, is that the observer may not even know that the measurement is invalid.  RTDs come in different grades which all have very impressive looking specifications.  Unfortunately, unless you're in a laboratory, or are very diligent about your metrology, you will never be able to achieve this accuracy in the real world.    (More on this later)
  • Thermocouples - Thermocouples work on the Seebeck principal where a junction between two dissimilar metals creates a voltage which is proportional to temperature.   Many metals exhibit this effect, but in order for the effect to be great enough to measure, specialized material need to be employed.  Iron and constantan are two typical materials.  Unfortunately, they eventually need to be connected to some sort of volt meter to measure them.  This measuring device will typically have copper wires and connections.  When you connect the iron lead from the thermocouple up to the copper wire of the measuring device, the same Seebeck principle works against you and changes the voltage.  This extra voltage has to be taken into account and the process is called "cold-junction compensation".   Often, solid state temperature sensors are used in the compensate circuit, limiting the overall accuracy of the thermocouple to be no better than the solid state sensor.  The other problem is that the thermocouple voltage is low and one needs to measure microvolts (that's 0.000001V).  These measurement limitations take extra effort to overcome and limit the use of thermocouple for many applications.  Wikipedia also has a good article on thermocouples. 

So what is a solid state temperature sensor and how do they work?

A solid state temperature sensor uses the PN junction of a diode to measure temperature.  In a similar manner to a thermocouple, a voltage is developed across this junction and is measured.  This voltage changes predictably according to the temperature of the junction.   This signal is referred to as a band gap voltage, because the mechanism that forms the voltage is a quantum effect called an energy gap or band gap that occurs between the N and P type semiconducting materials.  Because this effect is so predicable, it can be used in a feedback loop to create a voltage constant with temperature instead of one that varies with temperature.  This arrangement is called a bandgap voltage reference.  The bandgap effect is very predicable because it relies on inherent material properties.  The effect also depends on what is called the doping density of the N and P type materials.  This source of variation is well understood in the semiconductor industry and the industry has developed reliable and sophisticated methods to calibrate out this effect as well as other sources of error.  Essentially it is the time that it takes to calibrate the sensors at the semiconductor fabrication plant that determines the cost and accuracy of the finished sensor.

One of the major advantages of the solid state sensors is that the required signal conditioning electronics can be built along side of the actual sensor.  This eliminates what is often the most expensive cost of the installation which is the signal conditioning module.  A solid state sensor can accurately drive an Analog to Digital Converter (ADC) directly.  Thermocouples and RTDs both require specialized electronics to maintain an accurate measurement.  

Although solid state sensors are not commonly packaged as a completed probe, they are very common.  For example, the computer that you are using to view this page probably contains multiple sensors, the most notable one inside the CPU itself to prevent overheating.  They are used in the medical industry, electronics industry, for science and research, etc.

Why use a solid state temperature probe?

There are many reasons why a solid state temperature probe is the best choice.  Two important reasons are accuracy and ease of use.  When installed in a factory, wire nuts can be used to make connections to the sensors and to make splices in cable runs.  Let say you've just pulled a new cable 50 feet trough complex conduit from the control box to the tank where the sensor is mounted.  But you're two feet too short!  The obvious solution is to splice some more cable and finish the job.  If you're using a solid state sensor, you're done.  If you're using a thermocouple, it means either using an expensive and specialized junction block that you probably don't have, or it means pulling new thermocouple wire.  With an RTD, you might be tempted to just use wire nuts to splice the cable and finish the job.  But the added measurement error introduced by the resistance of wire nuts can be both significant and unknown.  You either have to pull a new piece of wire, or solder the splice.

Solid state sensors are also easy to connect to a measurement device.  This is especially true if you're using a computer with a generic data acquisition device.  With a solid state sensor, you connect the wires and you're done.  With any other temperature sensor you will probably need a signal conditioning module.  One signal conditioning module can easily cost $200.  Furthermore, the fine print on the specifications of the signal conditioning module often show that the module itself dominates the measurement error. 

When is an RTD or thermocouple better?

A solid state temperature probe can only be used over a relatively narrow temperature range (-40C to 125C).  Solid state temperature probes made by other manufactures besides Brew Data can also have further limitation on temperature range or cable length.  RTDs and especially thermocouples come in many different sizes and shapes and may be have a more appropriate physical shape.  Also, if extreme accuracy is required, RTDs can be more accurate, but be aware that it will also require lab grade measurement devices and very careful wiring to achieve a very high level of accuracy.

Check out Brew Data's selection of temperature probes that use solid state sensors.