Using precision digital multimeters for quick checks on temperature transducers _ sensors digital image representation pdf

If your processes use temperature transducers, then the ability to quickly see if they’re good or bad is important. Digital image processing second edition This article describes how to use a digital multimeter (DMM) to perform some quick and simple tests for the most common ones—thermocouples, resistance temperature detectors (RTDs), and thermistors.

Calibration and troubleshooting are two very different requirements. Digital image processing journals Calibration maintains product quality; troubleshooting affects product quantity.


Digital image file Calibration happens on a schedule; troubleshooting happens in emergencies. Digital image processing Calibration must be precise; troubleshooting must be fast. Components of digital image processing When a production line is down, speed is of the essence. Fundamentals of digital image processing by anil k jain pdf The failed component must be isolated and replaced ASAP. Digital image representation With a precision multimeter, you can perform quick, go/no-go checks on most temperature transducers, and while these tests tell nothing about transducer accuracy, they will tell you if a transducer has failed. Digital image processing castleman And sometimes that’s exactly what you need.

Thermocouples are unpowered transducers that generate a very low voltage. Anil k jain digital image processing When two dissimilar metals are in contact with each other, a potential is created across the junction—the Seebeck effect. Digital image processing in remote sensing This voltage across the junction of the two metals is proportional to the junction’s temperature.

The “Type” of thermocouple describes the metals used to make the junction, e.g., a J Type thermocouple uses iron in one wire and a copper/nickel alloy in the other. Digital image now The junction of the metals may be sheathed in various configurations or may be exposed.

The higher the temperature, the higher the voltage produced by the thermocouple. Digital image processing ebook download (It is somewhat misleading to use the terms “high” and “voltage” in this context, since the voltage across a common, J Type thermocouple is about 1.0 mV at a room temperature of 68°F and about 1.9 mV at body temperature, 99°F).

There are two steps to checking thermocouples. Digital image file types The first is to check for a short on the terminals and the second, to make sure that voltage tracks with the temperature.

The first test can be performed with any quality multimeter. Digital image wiki Put the meter in ohms or continuity mode; on a good thermocouple, you should see a low resistance reading. Digital image processing basics If you see more than a few ohms, you probably have a faulty thermocouple. Applications of digital image processing in security If the reading at room temperature is close to 110 Ω, then you have an RTD on your hands—read on.

The second test requires a meter that can measure down to tenths of millivolts (0.0001 V). Digital image processing gonzalez A meter that can measure hundredths of millivolts (0.00001 V) makes it even easier to do this check, because the added resolution shows very small temperature changes.

Connect the meter to the terminals of the thermocouple. Digital image background Grabbing the end of the thermocouple should cause the voltage to increase slightly, since you’re warming it up. Digital image size As you release the junction, the temperature (and voltage) should drop.

Multimeters with min./max. Digital image processing third edition recording and the ability to graph electrical signals (similar to an oscilloscope) are also convenient for this application. Digital image editor Min./max. Digitalglobe image recording lets you connect the meter, walk over to the tip of the thermocouple, warm it for a few seconds, and walk back to the meter to check the results. Digital image watermarking techniques Typical values for a good thermocouple are shown in Figure 1.

Figure 1. Digital image processing viva questions Using the digital multimeter’s min./max. Digital image processing pdf gonzalez record function allows you to monitor thermocouple voltage changes over time and make sure voltage increases with increasing temperature.

RTDs operate on the principle that the resistance of any conductor changes with temperature. Digital image analysis As the temperature of a conductor rises, the increased molecular vibration impedes electron flow. Digital image processing vtu notes Thus, the higher the temperature, the higher the material’s resistance.

Most RTDs are of the PT-100 variety. Digital image processing rafael c gonzalez pdf They consist of a platinum wire coil with a nominal resistance of 100 Ω at the freezing point (or, for purists, the triple point) of water. What is digital image processing Resistances other than 100 Ω at 32°F are less common, but do occur. One digital image It helps to know what the resistance of your RTD should be.

Sometimes copper or another metal is substituted for the platinum. Digital image resource For example, in some electric motors and transformers an extra set of copper windings functions as an RTD, indicating overtemperature conditions within the motor. R c gonzalez digital image processing In these special applications, and with metals other than platinum, you will probably find freezing-point resistances other than 100 Ω.

While most low-cost DMMs with a millivolt and resistance function can be used to check thermocouples or thermistors, they may not have sufficient resolution and accuracy for testing RTDs. What is digital image For checking RTDs you’ll need a meter that is capable of indicating changes of tenths of an ohm, and you’ll want a meter that measures to hundredths—the absolute value of the resistance isn’t important, but the ability to track small changes is. Applications of digital image processing in medical field Look for multimeters with resolutions down to 0.01 mV or 0.01 Ω and added features such as min./max. Digital image types recording or a graphical display. An introduction to digital image processing Since small changes in resistance reflect large changes in temperature, their additional resolution and improved accuracy give you a clearer picture of how well the RTD being tested is functioning, giving you more confidence in your results.

RTDs can have two, three, or four leads. Digitalglobe image finder In a two-wire configuration, simply connect the meter across the leads and measure the resistance. Image definition in digital image processing For a PT-100 RTD at room temperature, this should be about 110 Ω (±20%). Digital image processing in medical field If you grab the tip of the RTD, you should see the resistance increase. Digital image download Let go, and you should see the resistance gradually settle back after you release the tip.

Three-wire RTDs are commonly used when a measurement system is made up of resistance bridges. Digital image processing 3rd edition solution manual The wires that connect the tip to a measuring device have a temperature-dependent resistance of their own (as do all metals). Digital image watermarking pdf The extra wire helps the bridge balance out the effects of lead resistance. Colour image processing in digital image processing When checking a three-wire RTD with an ohmmeter, all you need to know is that two of the three wires should be shorted. Digital image processing remote sensing Usually, the shorted wires are the same color. Digital image transfer Between any of the shorted wires and the third wire, the transducer should act just like its two-wire counterpart. Digital image recovery software That is, at room temperature, the meter should read about 110 Ω for a PT-100 RTD, and resistance should increase slightly as the temperature at the tip increases.

Four-wire RTDs are less common than the other types. Digital image processing by gonzalez 3rd edition pdf If you do come across one, it should have two shorted pairs of wire. Digital image processing applications Again, the shorted wires are generally the same color. Digital image processing question paper The resistance between different colored wires should have a reasonable value at room temperature and should increase if you heat the tip.

Thermistors are made of semiconductor material and they work in a way opposite to RTDs. Digital image processing midterm exam While RTDs experience an increase in resistance with increasing temperature, thermistors tend to exhibit a lower resistance with higher temperatures. Digital image collections This is because semiconductor materials tend to conduct more electrons as the temperature goes up.

Although many types of thermistors are available, two-wire thermistors are the most common for general-purpose temperature measurement. Digital image photography Checking a thermistor involves performing a resistance measurement. Digital image processing doc Using the DMM’s resistance function, you should be able to watch the resistance of the transducer stabilize at room temperature and drop as the tip of the transducer is heated.

Thermistors generally have a large change in resistance per degree of temperature, so just about any meter can be used to quickly test a thermistor’s response. Digital image processing jayaraman pdf free download Graphical multimeters can take advantage of this property by graphically displaying the changing resistance. Digital image processing meaning Figure 2 shows a plot of resistance over time for a thermistor that was heated briefly.

Figure 2. Digital image warehouse Using a multimeter with a graphing function lets you see how the thermistor behaves when the temperature changes—this thermistor was heated briefly, causing its resistance to drop.

Temperature transducers usually fail in a big way. Digital image processing 3rd edition pdf download Rather than drifting, they usually just stop working. X ray digital image processing While there can be no substitute for regular calibration and certification, in a pinch a precision DMM can work for you as a solid troubleshooting tool.

I received my November 2003 issue of Sensors today and was shocked to read the misinformation about how thermocouples work in the article, page 33, “Using Precision Digital Multimeters for Quick Checks on Temperature Transducers”. Digital image processing exam questions It is incorrect to state that a voltage is produced across the junction of the two dissimilar metals of a thermocouple wire. Digital image processing and analysis Rather a thermocouple circuit contains two junctions, a measuring junction and a reference junction. The digital image The voltage is not produced at the measuring junction or across the measuring junction. Digital image processing video lectures mit Rather the voltage is produced along the portions of the thermocouple wires that experience temperature differences. Digital image processing in java And the voltage produced is related to the difference in temperature between the measuring junction and the reference junction.

The author then gives the example that a type J thermocouple would produce about 1 mV at a room temperature of 68F and 1.9 mV at a room temperature of 99F. Digital image processing with matlab This would be true only when the reference junction is at 32F. Digital image processing by jayaraman If we connect a thermocouple to a multimeter to see if it is working, we would need to chill the multimeter voltage input terminals (the reference junction in this instance) to 32 F for the voltages used in this example to be valid.

On page 34 the author suggests that a good thermocouple will measure only a few ohms and if it reads more it is probably faulty. Digital image license And he states that if it reads 110 ohms then it is an RTD. Digital image recovery software free download The resistance of a thermocouple is simply the series resistance of the two wires of dissimilar metals in the circuit. Digital image recovery mac This wire resistance simply depends on the wire materials, the wire cross sections, and the total length of the wires. Digital image processing techniques pdf Small thermocouple wires can be perfectly accurate and functional and yet have a resistance of more that a few ohms. Digital image processing notes I assure you that I have a perfectly functional 36 gage Type T thermocouple that is approximately 100 inches long and its resistance measures 110 ohms. Digital image art And I assure you that it is not an RTD.

It is likely that many of your readers look to Sensors magazine for correct information on various technical subjects. Image of digital multimeter Those who read this incorrect information about thermocouples are subject to become mislead and confused. Digital image processing 3rd I would agree that the author was correct with his concepts in a very general way but his specifics are way out of whack.

Mr. Digital image format crossword Gaffney, I’m OK with you addressing the issues you have with the article. Digital image viewer I agree that I should have specified a length when determining if a transducer is a resistive device. Digital image recovery android I was working with transducers of a few meters, connected to patch panels. Applications of digital image processing in agriculture Of course any long conductor, especially with a small cross-section, will have a significant resistance.

I don’t agree with your other two issues in the context of this article. Fundamentals of digital image processing The article does not claim that the voltages given in the article would actually be observed. Digital photo recovery software I used voltages from the tables, rather than measured voltages, since I could not predict precise values. Artplus digital photo recovery free They are given simply to illustrate the point that we are talking about thousandths of volts.

Your comments do uncover an assumption in the article — since the meter itself is acting like a reference junction, I am assuming that the the thermocouple measuring junction is being warmed relative to the meter. Digital image watermarking project report If ambient temperature is high, this technique falls flat. Digital image editing services So while this works ok in the lab, on a hot factory floor or out in the field, you might need to use a more aggressive heat source to see a significant voltage increase. Digital image processing 2nd edition I will add something to this effect to the article in case we use the information again.

Referring to the voltage “across the junction” seems like a reasonable simplification, especially in the context of the article and the voltage measurement being performed.

While I do believe the article could be improved, I think we have different ideas regarding the specific issues. Digital image processing interview questions I think it would be best if you addressed your issues in your own words. Digital image processing lecture notes ppt I appreciate your careful reading and technical integrity. Image digital studio I will support you in any way I can.