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Category Archives: Unrelated but Interesting

Interesting articles unrelated to data acquisition or data logger products.

How To Measure Current

Categories: Data Acquisition, Data Logger, FAQs, Unrelated but Interesting Leave a comment
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…current is always measured indirectly from an external sensor that produces a voltage output that is proportional to current. Since the only difference between a voltage and current measurement is the scaling that you apply to the result, voltage measuring instruments are almost always used to measure current.

Model CL601 current data logger is a rare example of an instrument with a built-in current sensor.

Model CL601 current data logger is a rare example of an instrument with a built-in current sensor (click for details.)

 

 

We often run across applications that require general-purpose AC or DC current measurement. In many cases, customers are surprised to learn that dedicated current measurement options appear to be limited.Simply stated, this is because current is always measured indirectly from an external sensor that produces a voltage output that is proportional to current. Since the only difference between a voltage and current measurement is the scaling that you apply to the result, voltage measuring instruments are almost always used to measure current.

The only exception to the above are instruments that happen to have a current sensor built into them, which is very unusual. An example is model CL601 AC Current Data Logger from DATAQ Instruments (see accompanying picture), which features a built-in, clamp-type current probe to accommodate AC current measurements.

Some examples are in order to crystallize the much more common concept of using voltage instruments to measure current, and they are best categorized based upon the most common types of current sensors.

Current Measurement Using Shunts

 

Current shunt

A typical current shunt

A current shunt is a precision resistor of very low value that is installed in series with the conductor from which current will be measured. Since its output is scaled to produce typically 50 or 100 mV at full-scale current, a voltage measuring device is used with the result scaled into units of current.

Instrument sample rate comes into play with ac current measurements, since the shunt passes an unfettered current waveform. In cases like that, you might connect an RMS voltage amplifier to the shunt’s output to generate a DC output that is proportional to RMS current, minimizing both sample rate and the size of recorded files.

Note that the instrument used to measure current shunt voltage will often need to be isolated. 

Current Measurement Using Probes

Example of an AC/DC current probe sold by DATAQ Instruments.  Click for more information.

Example of an AC/DC current probe sold by DATAQ Instruments. Powered by a 9-V battery, the probe outputs a DC voltage proportional to applied DC and AC RMS current (Click for more information)

Current probes provide the convenience of being hung on, or wrapped around a conductor. Unlike current shunts, they do not require that you install them in series, and all provide inherent isolation. However, these conveniences come at the price of accuracy and price when compared to current shunts.

Some current probes measure AC and DC current, and others only measure AC. Further, some probes provide built-in amplifiers that produce an RMS output for AC current measurement. Those that do not yield a current waveform. So, care should be exercised when matching a probe to an instrument, especially for AC measurements. It wouldn’t make sense to use a probe without an RMS amplifier to measure 60 Hz line current using an instrument that can sample at only a 10 Hz rate.

Current Measurement Using Current Transformers

The final method commonly employed to measure current is a current transformer (CT). Some CTs can be used to measure AC and DC current, but most measure only

A typical current transformer. A conductor is passed through the hole, and an output is provided for indirect current measurement by an instrument.

A typical current transformer. A conductor is passed through the hole, and an output is provided for indirect current measurement by a voltage measuring instrument.

AC current. CTs commonly have a hole in them through which the conductor is passed, and an output for connecting to a measuring instrument. The output is actually a current output and must be shunted with a known resistance to develop a voltage. Failure to do so results in extremely high and possibly lethal voltages present on the secondary, so be sure to carefully read and follow the manufacturer’s operating instructions.

Match the Current Sensor to the Instrument

Whether you choose a current shunt, probe, transformer, or some other current sensor, the key takeaway from this article is that current measurement is always a voltage measurement by any other name. So it follows that the the same precautions you’d exercise to measure voltage apply equally to measure current:

  1. Ensure that the expected output voltage range of the current sensor is within the measurement range of the instrument.
  2. Ensure that the instrument has the sample rate flexibility to provide meaningful results if the sensor passes a true current waveform.
  3. Ensure that you understand the isolation requirements of the measurement, and use a suitably isolated instrument if required.

 Additional Reading:

Isolated True RMS Amplifiers

Isolated mV amplifiers suitable for use with shunts

Using WinDaq software to calculate true RMS

Learn the Importance of Isolation in Four Easy Lessons

 

WinDaq System Used in Bridge Monitoring

Categories: Data Acquisition, Synchronous, Unrelated but Interesting Leave a comment
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The I-35W Mississippi River Bridge in Minneapolis, MN connects Downtown East to Marcy-Holmes. Following its catastrophic failure on August 1, 2007, a decision was made to rebuild the bridge with a state-of-the-art monitoring system. The contractor chose DATAQ Instruments hardware and software for the job due to our expertise in distributed and synchronous data acquisition technology. Since the bridge’s reopening on September 18, 2008, DATAQ Instruments hardware and software have played a vital role in monitoring bridge health, a contribution that was recently featured in a Minneapolis news report, which you can find here.

Additional Reading:

Bridge Structural Monitoring System

Big Brains. Small Films. Benoît Mandelbrot, The Father of Fractals

Categories: Unrelated but Interesting Leave a comment
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IBM and http://IBMblr.Tumblr.com celebrate the life of Benoit B. Mandelbrot, IBM Fellow Emeritus and Fractal Pioneer. In this final interview shot by filmmaker Erol Morris, Mandelbrot shares his love for mathematics and how it led him to his wondrous discovery of fractals. His work lives on today in many innovations in science, design, telecommunications, medicine, renewable energy, film (special effects), gaming (computer graphics) and more.

 

How To Make 4-20 mA Current Loop Measurements

Categories: 4-20 mA Current Loop, Data Acquisition, Data Logger, FAQs, Unrelated but Interesting 1 Comment
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It seems that at least one 4-20 mA (milliamp) measurement is required by our typical customer, and the way to do it is a constant source of confusion for many. So I thought I’d zero in on the various 4-20 mA current loop configurations and elaborate on the specifics that  you need to know to make a successful measurement. The following discussion is ordered from the most to least common configuration, and I hope to cover all those that I have encountered in customer applications. If yours isn’t included, please use the comments section to fill me in.

4-20 mA Current Loop Basics

Sensors or other devices with a 4-20 mA current loop output are extremely common in industrial measurement and control applications. They are easy to deploy, have wide power supply requirements, generate a low noise output, and can be transmitted without loss over great distances. We encounter them all the time in both process control and basic measurement data logger and data acquisition applications.

The idea behind 4-20 mA current loop operation is that the sensor draws current from its power source in direct proportion to the mechanical property it measures. Take the example of a 100 psi sensor with a current loop output. With 0 psi applied, the sensor draws 4 mA from its power source. With 100 psi applied the sensor draws 20 mA. At 50 psi the sensor draws 12 mA and so on. The relationship of mechanical property measurement to current output is almost always linear, allowing the resulting current loop data to be scaled with a simple mx+b formula to reveal more useful measurements scaled into engineering units.

How you actually measure the 4-20 mA current loop signal is a function of the sensor’s architecture and the capabilities of the instrument you’ll use for the measurement.

Terminology

So that my discussion translates well across the various kinds of 4-20 mA current loop configurations, I’ve opted to standardize the terminology I use to describe each. Here’s an overview:

“E” (dc excitation)

Most configurations that follow will show a DC voltage excitation source that I denote as “E”. Many who use current loop sensors for the first time are surprised to learn that they need to supply this excitation source. Nonetheless, unless the sensor is self-powered (i.e. AC line powered) an external dc source is required. The good news is that this can sometimes be supplied by the instrument, and the range of acceptable values is usually very wide, typically 10-24 V dc.

“R” (shunt resistor)

Here’s a bit of trivia for you: No instruments measure current directly. They all do it indirectly by measuring the voltage dropped across a resistor of known value, and then they use Ohm’s Law to calculate actual current. The resistor is referred to as a “shunt”, is absolutely required to make a current measurement, and is either supplied externally to, or built into the measuring instrument. For clarity, I assume that it’s supplied externally.

“i” (current loop value ranging from 4-20 mA)

This is the 4-20 mA current signal generated by the sensor. Note that some sensors may draw 0-20 mA and even other values, but the vast majority of them use the 4-20 mA convention.

“v” (shunt voltage that’s proportional to current)

This is the voltage actually measured by the instrument. Since our industry has standardized on a shunt value of 250 Ohms, “v” will range between 1 and 5 volts for a 4-20 mA current loop signal (v=i * resistance). Note that shunt resistor value is arbitrary as long as it’s known. You also need to ensure that it doesn’t burden the loop, so lower values are better than higher. Yes, I mean lower. Remember that we’re working with current, not voltage, so the rules are inverted. Just as infinitely-high resistor loads work well for a voltage source, you can take the load all the way to zero Ohms for a current source without consequence.

 Self-powered Sensors

Self-powered 4-20 mA current loop sensorI promised to order these configurations from most to least common, and the self-powered sensor just noses out the first runner up. Self-powered sensors are those that, well, power themselves. The sensor may have an integral ac power supply, thereby negating the need for an external DC power source. Or it may not be a sensor at all. It could be an output from a PLC or other source that is internally powered.

 

2-wire Sensors (Low-side Shunt)

2-wire 4-20 mA current loop sensorOkay, this can get confusing for first-time  4-20 mA current loop users. Yes, it is possible  to both power the sensor and measure the current it draws over the same two wires. In the 2-wire examples shown here, only two wires connect the sensor to its power supply, and the sensor draws current from it in direct proportion to the mechanical property that it measures. As current changes, the voltage developed across resistor R will change, thus providing a signal that’s suitable to connect to a measuring instrument like a data logger or data acquisition system.

In most situations, care should be taken to place the resistor in the low-side of the loop as shown here, as opposed to the high-side. Doing so will allow non-isolated instruments to make the measurement. In the next section, I’ll deal with a high-side shunt placement and discuss these cautions in more detail.

 

2-wire Sensors (High-side Shunt)

2-wire 4-20 mA current loop sensorThis configuration is almost exactly like the low-side, 2-wire approach, but it places the shunt resistor in the high-side of the loop. Note that while the voltage across the resistor is proportional to the current drawn by the sensor (just like the low-side approach), there is also a common mode voltage (CMV) present on either side to ground. On one side to ground the CMV is equal to the supply voltage. On the other side to ground it’s equal to the supply voltage, less the voltage dropped by the resistor (v). The presence of the CMVs places conditions on the instrument that you use to measure v. Specially, the instrument needs to have an isolated front end so it can float to the level of the CMV and still successfully make the measurement. Try this with a non-isolated, single-ended instrument and you will short-circuit the sensor to ground. A non-isolated differential instrument will either saturate or provide erroneous results.

3-wire Sensors

3-wire 4-20 mA current loop sensorThree-wire sensors with a process current output have a separate wire for ground, signal (4-20 mA), and the power supply. This configuration is the easiest for current loop beginners to grasp, one input for power and a second for the current loop with a common ground. The primary advantage of a 3-wire sensor over its 2-wire counterpart is its ability to drive higher resistive loads. Resistors drop voltage for any given current in direct proportion to their resistance value. Holding current constant, higher resistances drop more voltage. Turning back to the 2-wire sensor and holding current constant, as the shunt resistance increases the voltage drop across the sensor also increases. You might reach a point where the voltage dropped by the shunt lowers the voltage drop across the sensor below the minimum required for it to operate properly.

We had a customer whose 2-wire current loop measurements functioned beautifully until loop current reached about 18 mA, at which point everything went  haywire. Upon close examination, we determined that the supply voltage she used was too low by at least 0.56-V. She needed 2 mA more measurement to reach full scale, which translates to 0.56 V with her 250-Ohm resistor. The solution was to use a higher voltage power supply to ensure that the voltage drop across the sensor stayed above the minimum level. She could have also used a 3-wire sensor, which ensures that the voltage applied to the sensor is independent of shunt resistor voltage drop.

Watch Your Grounds (or use an isolated instrument)

Contrary to what many believe (and have been erroneously taught in school), grounds are almost never the same in industrial settings, exactly where most 4-20 mA current loop sensors are used. Two or more grounds that are the same means that they are at the same potential. If so, a measurement between the grounds of the various field sensors and the instrument using a digital volt meter (DVM) on both its DC and AC settings will show zero volts, or very close to it. In reality, you’ll measure at least several volts, and I’ve seen as much as 75 Volts. When grounds that are not at the same potential are tied together (which you need to do to make the measurement), current flows through them, creating several possible measurement outcomes for non-isolated instruments:

  1. The measurement is noisy.
  2. The measurement is inaccurate.
  3. You irreparably damage the instrument.
  4. You saturate the instrument (it’s not damaged, but you can’t make a successful measurement, either.)

To remedy these problems requires the following:

  1. Use an isolated instrument for your 4-20 mA current loop measurements. This single decision allows you to ignore all other grounding issues in exchange for successful measurements in any situation. If you don’t have an isolated instrument, read on…
  2. Ensure that the loop power source is isolated. This means that its output ground (the one connected to the sensor) is not tied to its input ground (the one that connects to AC line power.) An isolated power source means that the output ground can be tied to another ground (like a non-isolated instrument) without consequence.
  3. In self-powered applications, ensure that the low-side of the loop is isolated from its power source.
  4. If you lack control over the power sources and determine that they are not isolated, then your only option is to power ALL devices (power supplies, self-powered sensors, the instrument, and its connected PC) from exactly the same power outlet. Don’t make the mistake of using outlets that are close to each other. If you run out of receptacles on a single outlet, then use a power strip.

Again, it’s worth repeating that all of the cautions associated with proper grounding disappear if an isolated instrument is used to make the measurement.

Sensors with 4-20 mA outputs are encountered in all disciplines and in many configurations. Contact us with any questions that arise in your unique situation.

Additional Reading:

4-20 mA Current Loop Products

4-20 mA Current Loop Measurement Resolution Calculations Made Easy

4-20mA Current Loop Data Acquisition

DI-8B32-01 4-20 mA Current Loop Amplifier

DI-8B42-01 2-wire Current Loop Amplifier with Power Supply

The Graphene Supercapacitor

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Batteries can hold a lot of energy, but they take a long time to charge. Capacitors can charge in seconds, but do not hold a lot of energy. What if you could have a device that promises to deliver the energy capacity of a battery with the charge time of a capacitor. THAT could change everything. Enter the Graphene Supercapacitor.

Departing Space Station Commander Provides Tour of Orbital Laboratory

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In her final days as Commander of the International Space Station, Sunita Williams of NASA recorded an extensive tour of the orbital laboratory and downlinked the video on Nov. 18, just hours before she, cosmonaut Yuri Malenchenko and Flight Engineer Aki Hoshide of the Japan Aerospace Exploration Agency departed in their Soyuz TMA-05M spacecraft for a landing on the steppe of Kazakhstan. The tour includes scenes of each of the station’s modules and research facilities with a running narrative by Williams of the work that has taken place and which is ongoing aboard the orbital outpost.

 

 

The Nighttime Earth From Space Like You’ve Never Seen It Before

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The United States

SAN FRANCISCO — The Earth at night looks more beautiful than it ever has before in these incredible new images from NASA’s Suomi NPP satellite.

More images at this link: (click)

 

Cheetahs On The Edge

Categories: Unrelated but Interesting 1 Comment
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Cheetahs are the fastest runners on the planet. Combining the resources of National Geographic and the Cincinnati Zoo, and drawing on the skills of an incredible crew, we documented these amazing cats in a way that’s never been done before.

Using a Phantom camera filming at 1200 frames per second while zooming beside a sprinting cheetah, the team captured every nuance of the cat’s movement as it reached top speeds of 60+ miles per hour.

The extraordinary footage that follows is a compilation of multiple runs by five cheetahs during three days of filming.

(link: http://vimeo.com/53914149)

 

See how it was filmed:

(link: http://vimeo.com/53129783)

 

Why is the Sky Dark at Night?

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A question that bothered me a lot when I was younger, and it bothered me even more when I found out about Olber’s Paradox. Here’s my take.

Get astronomy twitter posts here:

http://twitter.com/DeepAstronomy

Music used:

Revised Youth – Broken Kites:
http://www.archive.org/details/brokenkites_the_envious_dead

Touch the Sky – Iambic^2:
http://www.archive.org/details/laridae031

Google’s data centers: an inside look

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Very few people have stepped inside Google’s data centers, and for good reason: our first priority is the privacy and security of your data, and we go to great lengths to protect it, keeping our sites under close guard. While we’ve shared many of our designs and best practices, and we’ve been publishing our efficiency data since 2008, only a small set of employees have access to the server floor itself.
Today, for the first time, you can see inside our data centers and pay them a virtual visit. On Where the Internet lives, our new site featuring beautiful photographs by Connie Zhou, you’ll get a never-before-seen look at the technology, the people and the places that keep Google running.
 In addition, you can now explore our Lenoir, NC data center at your own pace in Street View. Walk in the front door, head up the stairs, turn right at the ping-pong table and head down the hall to the data center floor. Or take a stroll around the exterior of the facility to see our energy-efficient cooling infrastructure. You can also watch a video tour to learn more about what you’re viewing in Street View and see some of our equipment in action.