Features built into Graphtec GL series instruments allow you convert voltage into meaningful engineering units (psi, lbs, etc.). This video demonstration shows you how.
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Context-sensitive Help
Categories: FAQs, General Data Acquisition, General Data Logger
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Have you tried lately to access the help file for a fairly complex application? There’s a lot of information in there and digging through it to find the one nugget necessary for the task at hand can be time consuming. That’s why, a very long time ago, the Wizards of Windows had a great idea. They provided a framework that developers could use to enable context-sensitive help for their applications. Instead of bringing users to the help files, context-sensitive help brings help files to the users. Specifically, you select the pull-down menu item that you want to know more about, and then press the F1 key. The help file activates and automatically indexes to the section that describes the details of the highlighted item. Beats the heck out of slugging through a long help file, right?
The problem is that almost as quickly as Microsoft supported the standard, they abandoned its use in their own applications. The result is that even if you know what context-sensitive help is, you’ve probably never used it since some of the most popular applications on the planet don’t support it.
The next time you open a DATAQ Instruments WinDaq application, save time and trouble by giving context-sensitive help a try.
Strain Gage Module Functionality Testing
Categories: FAQs, General Data Acquisition
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Strain gage modules are used in data acquisition products to provide a supply (excitation) voltage to strain gage-based transducers and to amplify the output, which is then passed on to a data acquisition instrument such as the DATAQ Instruments DI-718B.
Often, when a test engineer or maintenance technician notices that the values they’re observing aren’t quite what they were expecting, or are all together outside the norm; their data acquisition hardware and/or software become suspect number one. Naturally, they begin to poke and prod, checking an input impedance here, an excitation voltage there. And while these measurements may seem relatively straight forward, that’s usually not the case. Take for example the DI-8B38 strain gage module. Once or twice a month a customer will call in and say “Hey, I measured across the excitation pins with a voltmeter and I see 17V! The sticker on the module says 10V excitation!” It’s true, measuring across the excitation pins of an unloaded DI-8B38 module will yield approximately 17V. It’s only when you connect a transducer to the module that you see 10V across the excitation pins.
Due to the way that these signal condition modules are engineered, it’s often the case that a load or bias resistance must be applied in order to test the functionality. The OEM was in fact able to provide us with a procedure, using bias resistors, to verify proper operation of the DI-8B38 module (below).
Single-ended vs. Differential: Confusion Reigns
Categories: FAQs, General Data Acquisition, General Data Logger
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There is a persistent quandary out there concerning the difference between single-ended and differential analog input configurations. Specifically, many have the impression that analog inputs configured to be single-ended cannot measure negative-going voltages inputs. I’m not sure where or how this rumor got started, but it’s pervasive and has led many an end-user astray.
Here’s an example. The following is drawn word-for-word from a recent customer email who had purchased one of our DI-718B data loggers with several DI-8B41-09 amplifiers:
I’m a bit puzzled regarding the SE inputs. According to the datasheet the input modules convert a ±40 V to a ±5 V signal. Might it be that the DI-718B actually offsets the signal by 5V and the reads the signal as 0-10V??
It may not be obvious, but experience was screaming that this customer was under the impression that negative going signals cannot be measured by single-ended (SE) inputs. Hence, he thought that before the logger’s single-ended input could make the measurement it would need to apply a +5 V offset to the amplifier’s output to yield a signal in the range of 0-10 V, suitably denuded of any negative components.
When we replied that single-ended inputs can measure signals less than zero, and after the customer proved this to himself through experimentation, the issue was resolved.
So, what’s the difference between SE and differential input configurations? The nearby figure shows four signal configurations, two each for differential and single-ended. The signal is a common sine wave of the type you’d get from a function generator. 
Configuration A and B show that the polarity of the signal generator is connected to match the polarity of the input amplifier, “+” to “+” and “-” to “-.” Notice that the SE configuration has no problem duplicating the negative portion of the sine wave as does its differential counterpart. However, in configurations C and D where we swap the polarity of the input signal generator versus the amplifier, “+” to “-” and “-” to “+”, we have a problem. Since by definition the low side of the SE amplifier is connected to ground, swapping the polarity of the input signal essentially shorts the “+” output of the generator to ground causing the amplifier’s output to show no activity other than zero volts. However, the differential configuration of D simply swaps the polarity of the sine wave to show an inverted sine wave at its output, which is precisely consistent with the polarity of the input signal.
Single-ended input configurations suffer at least one disadvantage when compared to its differential counterpart, but the inability to handle signal levels less than zero is not one of them.
What’s All This Sink and Source Current Stuff?
Categories: FAQs, General Data Acquisition, General Data Logger
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Anyone who has connected to the digital outputs of data acquisition or data logger systems has wrestled with sink and source current specs, and they continue to be a topic of confusion for many. When does sink current matter, and what is it? Same for source current.
References to sink and source current are made relative to the current switching device inside the system. In most cases, that’s a simple transistor. Source current refers to the ability of the digital output port to supply current. Sink current refers to the ability of the port to receive current. The following figure may help.
In the above, our goal is to light a simple light-emitting diode using the switching ability of a digital output port. The top shows a current source application, and the bottom a current sink approach. Notice the direction of current in both cases: Source supplies current and sink receives current. When the port supplies (or sources) current it’s limited by the current limiting resistor R. You may or may not be able to draw enough current to light the LED depending upon its current requirements, but in any event you won’t damage the port. Let’s assume that the LED lights very dimly (or not at all) so that sourcing current is not an option. In other words, the digital port cannot provide (or source) enough current. What can you do?
One option is to provide your own power source, one that can provide adequate current, use the port to control whether or not the current flows, and thus turn the LED on and off. This configuration is shown at the bottom, of the figure. Now the digital port no longer supplies (sources) current but is configured to receive (or sink) it. To prevent damage to the port you need to be cognizant of its maximum sink current spec. Sinking more than this figure will fry transistor switch Q and render the digital port permanently inoperative.
Now let’s put some meat on this application and assume that the maximum current sink spec for a given digital output port is 10 mA. We ignore any LEDs that require more current than 10 mA and locate one that’s rated for 5 mA and has a forward voltage of 2 V. Ignoring the ON resistance of Q (usually very small), we can calculate Rsink to be (5-2)/0.005 = 600 Ohms to ensure that the port doesn’t sink more than 5 mA, well below its 10 mA maximum rating.
A final point is to note that the control signal at Q’s base for the current source versus the sink application is inverted. A logical 1 applied to Q’s base in the current source application turns the LED off. A logical 1 turns it on for the current sink application.
A Simple Humidity Verification Test
Categories: FAQs
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Since we sell many data logger products that can measure temperature and humidity customers often ask us how they can verify accurate humidity measurements. That’s a good question. Verifying temperature is easy, but humidity is an entirely different matter. Fortunately, ordinary salt possesses a valuable characteristic that we can use to our advantage. It turns out that table salt (preferably non-iodized), properly moistened and maintained in a sealed environment over time, will generate a predictable humidity of 75% over a very wide room temperature range. Here’s how it’s done:
What you’ll need:
(1) zip-lock plastic bag, large enough to comfortably fit a shallow glass container and your data logger
(1) small, shallow open glass container that easily fits in the zip-lock bag
(1) tablespoon table salt (preferably non-iodized)
(1) oz. of distilled water, or less
- Pour the salt into the glass container and add just enough distilled water to make a paste. You’re looking for a consistency similar to a sand and water mixture that you could use to sculpt a sand castle.
- Place the glass container containing the water/salt mixture and the data logger (in its actively logging state) into the zip-lock bag, ensuring that the salt mixture does not come in contact with the loggers.
- Firmly seal the bag making sure that plenty of air is trapped inside it.
- Place the sealed bag in a reasonably controlled temperature environment of about 25 °C, not in the sun or near a HVAC duct.
- Wait eight hours (don’t cheat), remove the logger, and upload the data. Humidity should have moved from ambient to 75% near the end of the eight hours while the bag was still sealed.
Run WinDaq and Begin Recording on Windows Startup
Categories: FAQs, General Data Acquisition
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A new application, currently for use with DI-718 and DI-730 Ethernet products, allows you to run WinDaq and begin recording on Windows start-up. The application, titled LaunchWinDaq, lets you “set it and forget it”.
After following through a short procedure to save your default conditions (calibration, scaling, annotation, etc.), and copying a few files in the WinDaq installation directory, you’ll simply place the LaunchWinDaq executable in the Windows startup folder. Every time you start Windows, WinDaq will run and begin recording data. Support for additional DATAQ instruments is in the works.
Click here to download the application along with instructions.
