|UG04: Measuring Temperature For Under $35 In Hardware Using LArVa|
MEASURING TEMPERATURE FOR UNDER $35 IN HARDWARE USING LARVA
One of the most universal needs in the lab is to measure temperature. This could be the temperature of a tightly controlled experiment, the temperature of a portion of a structure under thermal feedback or simply the air temperature in the room.
Most data acquisition hardware costs hundreds of dollars or more and offers much more bandwidth and resolution than a typical temperature measurement requires. This application note shows how to use a resistive temperature device (costing less than $3), an Arduino Uno (costing less than $30) and the LArVa driver (free) to measure temperature over a wide variety of ranges.
LArVa is a free driver that turns an under $30 Arduino microcontroller into an ultra-low cost data acquisition system. While spending hundreds, thousands or more dollars on data acquisition system is necessary in some cases, inexpensive data acquisition can do a lot for very little cost… and with more and more amazing and inexpensive sensors and actuators available every day, there are vast possibilities.
WHAT YOU NEED
WIRING UP THE RTD
The TD5A data sheet recommends using a sense current of less than 100uA so the sense current will not heat the RTD and affect the temperature measurements. The TD5A resistance changes from 3,128 ohms at +150?C to 1,584 ohms at -40?C. For maximum flexibility, we will allow for using this full range, but for applications that don’t require such a wide temperature range, we could trade less temperature range for more resolution and accuracy.
One lead of the RTD is connected to ground. The other side of the LED is connected to the resistor and to A0. The opposite side of the resistor is connected to 5V power.
MEASURING THE RTD VOLTAGE
To measure the voltage, open the LArVa simple Graph application. For instructions on the free download and installation, see User Guide UG01 –LarVa Install Guide for Windows at http://angstromdesigns.com/larva/user-guides
We are now gathering data from our RTD, but we need better resolution and we would like the output to be in degrees, rather than in counts.
IMPROVING THE RESOLUTION WITH A VOLTAGE REFERENCE
Right now our analog input channel A0 is spreading it’s 12 bits across 0-5V range. Because our measured voltage will only go between 0.31V (for +150?C) and 0.16V (for -40?C) we do lose resolution by having this large range available. We can limit the range to use all 12 bits across a 0-1.1V range by going to the ‘Setup’ Tab in Simple Graph and changing the value of ADC Voltage Reference to 1.1V.
This is an improvement, but we’d like to do even better.
IMPROVING THE RESOLUTION WITH AVERAGING
The LArVa firmware performs fast sampling of the analog inputs by default. We can increase this sampling to get better resolution. We can also decrease the delay between samples for faster data acquisition. Finally, we can bring the data reads back to the computer as floating point numbers, which have higher resolution than integers. All of these changes will improve our measurement.
We now have much better resolution for our RTD:
We now have ample resolution for our measurement. But the units are not in degrees.
CALIBRATING THE RTD OUTPUT
Calibration is a critical part of using any sensor. Our system has many possibilities for variation in it, such as variations in the resistor value, RTD performance or power supply voltages. Calibration will address these possible variations.
APPLYING CALIBRATION AND SCALING BY NUMERICAL APPROXIMATION
To convert from A0 in counts to temperature we need 3 equations.
Solving these equations numerically in a spreadsheet gives a relatively linear relationship between A0 counts and RTD temperature:
Iteratively changing the R0 value until the calibration of 191.2 counts corresponds to 20.1?C gives a final value for R0 of 1859.9 ohms. For a two- or three-point calibration we would fine-tune the values for the linear and quadratic terms for the RTD equation. However, our one-point calibration gives a linear approximation of temperature versus counts of T = 1.361*A0 – 243.22:
It is important to keep in mind that the linear approximation is a source of error, as can be seen in the graph above. This may not be appropriate for some measurements, see below for scaling that does not use a linear approximation.
Returning to Simple Graph we can plug in these values into the ‘Scaling’ tab. Type 1.361 into ‘Analog Data Scaling’ and type -242.62 into ‘Analog Data Offset.’ We can refine the offset value until the calibration is exact for room temperature to remove the errors in the linear approximation, but know that this just shifts our linearization errors to different temperature values. We can also rename our Y axis to be temperature in degrees C and rename channel A0 in the ‘Graph’ tab.
We can now repeat our test of touching the RTD with our fingers to see a high resolution graph with proper units on the axies:
APPLYING CALIBRATION AND SCALING BY MODIFYING SOURCE FILES
Simple Graph has values for analog data scaling and offset in the ‘Scaling’ tab. As we’ve seen above because the temperature is not linearly related to the analog input reading we will not be able to get accurate scaling over the full range using these settings. In order to get accurate scaling over a large range, custom scaling will be needed by modifying the source files that are included with the free, LArVa Simple Graph application. Note that a three-point calibration spanning the expected temperature range would also be best for sensitive measurements.
Open LArVa Simple Graph.vi using Labview 2010 or higher. For download and installation, see User Guide UG01 –LarVa Install Guide for Windows at AngstromDesigns.com.
We want to include all of the calculations that were performed in the calibration spreadsheet in Labview, so we will need to replace the scaling and offset controls in the ‘Scaling’ tab with new controls and add the rest of our calibration controls:
On the block diagram we see that the scaling values are applied in the data display thread (the bottom of the three threads) in the Append_Data subVI. Below, we have bundled up all of the controls we will need and passed them to this subVI:
Inside the Append_Data subVI we perform the calibration functions, just as in the section above but without the linear approximation:
We return to the modified Simple Graph application and repeat your measurement. Don’t forget to set the front panel values, such as Y autoscaling, points to average, ADC return, loop delay and ADC Voltage Reference as above.
That’s it. We now have calibrated measurement data in the proper units in real time. Note that final adjustment of the offset values is not necessary when we do this exact scaling.
SAVING THE DATA
To save the data, go to the ‘Save’ tab and click the button. The data will be in text format, so using the .txt file extension would be best.
MOVING FORWARD TO OTHER SENSORS
This application note shows how to gather small signals using very inexpensive hardware. Many sensors have signal outputs and calibration issues that are very similar to RTDs, so the lessons used here can be applied to many different sensors. This will enable you to gather signals at a fraction of the cost of more traditional data application systems.
LEARN MORE AND STAY IN CONTACT
For more applications, user guides, help, questions and too see what else is going on with LArVa, go to www.AngstromDesigns.com.