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Temperature Measurement, Part 3

As promised in my last blog, I will start immediately with my discussion of semiconductor sensors. The basic sensor is normally just a diode, and you could stick with just that if it suits your purposes. There are several documents on implementing the diode technique like this one from TI: “Diode-Based Temperature Measurement“; or this from Linear Tech: “Accurate Temperature Sensing with an External P-N Junction.”

A calorimeter is a device that measures the caloric value of a substance. In this machine, coal is ground to a powder and placed in a ceramic crucible. It is then placed in a sealed container (with a water jacket) in the presence of oxygen. The coal dust is ignited, and the resulting combustion warms the water. This calorimeter measures the temperature rise of the water using a string of diodes. The control was realized with an Intel iSBC80/10B single-board computer (based around the i8080) and an external 12-bit ADC.

A calorimeter is a device that measures the caloric value of a substance. In this machine, coal is ground to a powder and placed in a ceramic crucible. It is then placed in a sealed container (with a water jacket) in the presence of oxygen. The coal dust is ignited, and the resulting combustion warms the water. This calorimeter measures the temperature rise of the water using a string of diodes. The control was realized with an Intel iSBC80/10B single-board computer (based around the i8080) and an external 12-bit ADC.

Aside from the simple diode, there are more than a few semiconductor sensors that produce an analog voltage in proportion to the temperature. The venerable LM34 and LM35 are still going strong, with many newer additions like the Linear Tech LTC2997 , which allows external semiconductor sensors; or the AD590 from Analog Devices. Each of these will require some form of ADC, but the semiconductor temperature sensors have evolved to include additional functionality unavailable on the other sensors.

Now that we have dealt with all the sensors that provide an analog output, I would like to talk about the conversion and software side. If your micro doesn’t have a high-resolution ADC, you may want to consider an external device like the TI ADS1246/7/8 or its LMP90100 series. They provide excellent support with the WebBench software, which you can access here. An Analog Devices CN017 Circuit Note also gives information on a precision multichannel thermocouple solution.

I have been rather glib about the resolution of these ADCs. The fact of the matter is that considerable effort may be needed to reduce the levels of noise before the accuracy of the design becomes acceptable.

It is no secret that when it comes to micros I favor the Cypress PSoC device, and when I was about two-thirds of the way through this series of blogs I was invited to attend a seminar on the use of the PSoC3 and PSoC5 as applied to temperature sensing. Cypress has taken a comprehensive approach to the subject. The PSoC has blocks of IP called “components,” which are configured through different register setups on the device. The IP hides this setup, and you simply get to tailor it to your application.

There are components for the thermocouple, RTDs (in 2-, 3-, or 4-wire configuration), thermistors, and diodes. The delta-sigma ADC can be configured for 20-bit operation, and you (or the IP) can add amplifiers. It can handle an enormous number of analog inputs and so is ideal for multiple temperature sensors. This ability to allocate and reallocate the I/O pins has an interesting application. Apparently RTDs are used extensively in the windmill industry, and, because of the vibration, the wires (4-wire devices are used) fatigue and break.

It is possible for the PSoC (through the IP) to detect the wire break and maintain operation by dynamically altering the connections (albeit with a downgrade in performance) from a 4- to a 3-wire and then to a 2-wire system. Another option in the IP for the thermistor is to choose whether to use formulae (included in the IP) for the temperature calculations or to revert to lookup tables. I will use this as a segue from the PSoC back to the general discussion.

Setup screen for the RTD and thermocouple components with some of the configuration options.

Setup screen for the RTD and thermocouple components with some of the configuration options.

The thermistor and the thermocouple especially require linearization, and you have two options. You can look at their formulae relating the measured property (resistance or voltage) or you can use lookup tables, which is the age-old software conundrum: execution time or code size. You can opt for the lookup table and reduce its size by piece-wise linearization, but complete thermocouple tables, especially if you are catering for different types, can chew up a lot of memory.

All of this and yet there is still more to discuss. In part 4, I will finally get to look at “intelligent” sensors with digital outputs. Seventh inning stretch now — stand up, yawn, and get some popcorn.

17 comments on “Temperature Measurement, Part 3

  1. Davidled
    April 21, 2014

    These are good information for LM and LTC sensor. If I get a chance to select sensor among these, the sensor which has less interface circuit including A/D, would be selected to be simply.

  2. etnapowers
    April 22, 2014

    “The fact of the matter is that considerable effort may be needed to reduce the levels of noise before the accuracy of the design becomes acceptable.”

     

    The noise levels are often comparable to the output voltage of the thermocouple, so an appropriate amplification with a good loop gain for a accurate noise rejection is required. 

  3. antedeluvian
    April 22, 2014

    A blog by our own Dennis Feucht covered CJC for very accurate measurments

    Thermocouple Nodules, Cold Junctions & Integration Opportunities

  4. antedeluvian
    April 22, 2014

    There was a recent article in EDN on using the LMP90100 series AFE using thermocouples and more

    What makes industrial sensors go awry?

  5. Netcrawl
    April 22, 2014

    @antedeluvian thanks for a great links, Both Thermocouples and RTDs have advantages and limitations, the key is to select the right solution for the specific applications, Thermocouples can maintain high accuracy if operated under controlled conditions. Thermocouples are good enough for higher accuracy applications. However without any monitoring or attention being paid to each measurement point, an RTDs could be a great choice or alternative.  

  6. geek
    April 25, 2014

    “Both Thermocouples and RTDs have advantages and limitations, the key is to select the right solution for the specific applications”

    @Netcrawl: What are the applications that Thermocouples are better useful for and when do RTDs come in handy? Does the seggregation occur over the type of industry in the picture?

  7. Netcrawl
    April 26, 2014

    @tzubair, each type of temperature sensor has a set of conditions for which that is best suited and work well, for example for RTDs, RTDs has a wide temperature range( -200 to 850 C), it can be used in almost all but the highest temperature industrial processes.

    If accuracy, linearity and stability are your application's primary concerns then you need to go with RTDs, RTDs are far more accurate and more repeatable, RTDs can produced more repeatable and stable readings.

    Thermocouples are designed to be more durable, a major selling point for thermocouples is their temperature range- up to 2700 Fahrenheit. 

  8. geek
    April 29, 2014

    “Thermocouples are designed to be more durable, a major selling point for thermocouples is their temperature range- up to 2700 Fahrenheit. “

    @Netcrawl: Thank you for elaborating about the thermocouples and RTDs. Given the high temperature range for the thermoucouples on the higher side, I think they fit well with most metal industries where measuring temperature at extreme high levels is a norm.

  9. samicksha
    April 29, 2014

    I read technical documentation and found that it can do continuous background sensor diagnostics in that case does that mean it can detect short circuit condition as well.

  10. amrutah
    April 29, 2014

    @Samicksha:  This document is regarding any specific device to temperature sensor?  The tempsense and short-circuit are two independent events, meaning at high or low temperature someone can short a pin or line.  But are you talking about an application where temperature is monitored for short-circuit detection and protection thereof?

  11. Victor Lorenzo
    April 29, 2014

    @Netcrawl, one more parameter to take into account is the form factor. RTDs tend to be larger than thermocouples which is logical from their operating principles.

    RTDs are very 'popular', in fact almost like 'industry standards', for fluids temperature control systems due to their mechanical construction and encapsulation characteristics.

  12. Victor Lorenzo
    April 29, 2014

    @samicksha points us to something that in my oppinion could be very interesting. In many applications it is crucial to detect any sensor or sensor cable failure. Imagine an oil closed loop temperature controlling system, in the eventuality of a sensor failure the whole system could get uncontrolled and even catch fire if this situation is not properly handled.

    Some old ECG devices injected a high frequency common mode signal (filtered out later before processing) for detecting sensor connection errors.

  13. SunitaT
    April 29, 2014

    It very important to be keen when it comes to selecting temperature sensor, I think all sensors are good but there are some differences in the way they operate and conditions of their operation. When you compare RTDs and Thermocouples you will find that they operate differently depending on the temperature condition. RTDs can work well to measure temperatures not above 800 C without a lot of attention paid to it while thermocouples will be accurate and can measure high temperatures that RTDs can't reach but under controlled conditions.

  14. geek
    April 30, 2014

    “I read technical documentation and found that it can do continuous background sensor diagnostics in that case does that mean it can detect short circuit condition as well.”

    @samicksha: If the short circuit condition produces enough heat that raises the temperature beyond the threshold set for the sensor to activate, then I believe it should be capable of detecting them as well.

  15. eafpres
    April 30, 2014

    Hi Aubrey–great stuff as usual!

    I'm reminded of a long time ago I raced go-karts; these had small displacement high RPM 2-stroke engines from which we squeezed maybe 15 hp from a 100cc engine. To get the most from these things, we used two types of temperature measurements, both based on thermocouples.  One was cylinder head temperature, which typically terminated the thermocouple in a crimped sleeve with a lug on it that fit under the spark plug, effectively replacing the spark plug gasket with the sensor.  These were plugged directly into an analog gauge–a small enclosure with an analog meter on the face of it.  The typical way of using these was to wear them on a belt, so that when you sat in the seat, you could look down and glance at the gauge.  Typically we put a piece of electrical tape on the face to indicated the target temperature.

  16. eafpres
    April 30, 2014

    Hi Aubrey–I meant to also mention the other way we measured temperature, which was to weld a thermocouple well into an exhaust header, otherwise the same setup.  The advantage of exhaust gas temperature was the dynamic response was much better.  But when you are zooming around at 60 m/h you don't have a lot of time to watch the gauge, so it turned out the self-integrating response from the cylinder head temperature was good–it didn't jump around so you could look every so often and make adjustments.

  17. eafpres
    April 30, 2014

    When I was in the early part of my career I worked in NIST (then the National Bureau of Standards).  Once I had a project to measure the heat capacity of a fairly nasty and unstable solution (liquid).  For that we used a DSC–differential scanning calorimeter.  This required to put a sample into tiny aluminum pans which crimped together to form a sealed sample, then put that into the device.  The measurement depended on very precise temperature measurements.  I want to say the machine used RTDs but I'm not sure I'm remembering correctly.

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