How to use the Dantec hotwire anemometer
The hotwire anemometers we use are extremely fragile. An accidental bump or drop of a probe can cause the wire to separate from one of the leads. If this happens, it will take weeks for Dantec to receive, repair, and return that probe. Be very careful.
How the Hotwire Anemometer Works
The hotwire anemometer is a device we use to gather one-dimensional velocity data from fluid flows. Our probes consist of one wire strung between two leads. Electricity runs from one lead to the other through the wire, generating an amount of heat dependent on the resistive properties of the probe. As fluid flows over the wire, this heat is dissipated, and the temperature difference is measured via the circuit contained within a device called a MiniCTA. It looks like this:
This is translated to a voltage output, which can further be translated into a velocity by fitting a power law to constant-velocity calibration data.
There are two different types of hotwire anemometers used in this lab: the 55P16 and the 55R11. Though they use fundamentally the same technology and are attached to the same probe base and sheath, they have very different use cases.
The 55P16 Probe
This hotwire is designed to measure low to moderate velocities in gaseous flows only. It looks like this:
The probe includes a hotwire sensor 5um in diameter with a 0.5um coating, and is attached to a BNC connector with a cable. Because of its sensitivity to lower flow speeds, this probe should not be used in flows exceeding 20 m/s.
The 55R11 Probe
This hotwire is designed to measure low-velocity liquid flows and gaseous flows at higher velocities than those able to be measured by the 55P16. It looks like this:
The wire in this probe is 70um thick, so it can accurately measure relatively high-velocity gaseous flows without breaking. It has a coating 2um thick that allows it to measure flows of conducting liquids. The 55R11 is less fragile than the 55P16, but it still requires a high degree of care in handling as it can break in a similar fashion.
Installation and Calibration
If you have trouble with any of these steps or find some part of them unclear, look at the manuals included on the StreamWare CD for clarification.
First, make sure every component is disconnected from every other component and from the power supply. Connecting all the hardware parts in the correct order and method is essential. If you connect the MiniCTA to the power supply before connecting it to the probe, you could blow the hotwire. Think if it like a lightbulb: you want to screw in the bulb before flipping the switch to turn it on, not the other way around.
Open the packaging for the probe. You should find a small slip of paper that looks like this:
Do not lose this slip of paper. It will be necessary for calibrating the hardware.
The probe will have a metal sheath that covers and protects the hotwire itself. Keep this sheath over the hotwire during any period of time in which you are not actively collecting data. Securing it with a small piece of tape can be helpful. This sheath will retain its position along the probe as long as you move it less than an inch or so toward its cord. After this point, it will be loose and will fall away from the probe. For this reason, always hold the probe by the metal rod directly connected to its tip, not by the protective sheath, like this:
The cord connected to the probe should be 4 meters long and have a BNC connector at its end. Make sure the MiniCTA is not connected to a power outlet, then attach its BNC connector to that of the hotwire. From here, connect the MiniCTA to the DAQ, then plug the MiniCTA’s power cord into a power outlet. See below for reference:
After this intitial setup, you shouldn’t need to mess with the hardware configuration again until you need to replace the probe.
The hotwire comes with a disk of software and reference material. It looks like this:
Put the StreamWare CD into the computer. Follow the instructions of the installation software in order to install the program for the first time (do not do this if you already have it installed). You should now see an icon on your desktop that will shortcut you to the StreamWare Basic v.600 program.
Open this shortcut. Make sure that the wireless dongle is inserted into one of the USB slots on the computer, otherwise the program will not start. The wireless dongle for the Lung Flow Sensor project looks like this:
When you open the program, you should see this page:
Select “New Database” in the top left of the screen, then name your file and project. The System Set-up wizard should begin automatically. Select the image of the probe that matches the one you are trying to use. The Lung Flow Sensor project uses a probe with one wire between the leads (55P16) and therefore requires the selection circled in red below:
Next select the Type, Model, and Description that correspond to your hotwire probe. For example, select the following for the 55P16 probe:
Do the same for the probe support. For example, select the following for the 55P16 wire:
Do the same for the cable attached to the probe. For example, select the following for the 55P16 wire:
You will then see a diagram of the hardware setup according to the selections you have made. Hit the Save button. The wizard for the hardware calibration should start up immediately.
Remember that slip of paper from the Equipment Setup section? The one that came in the packaging of the probe? Find it and have it ready. The slip should have five numbers handwritten on it, like this:
The hardware setup dialog box should already be up on your computer. It looks like this:
Enter the value for R20 that is written on your slip of paper and make sure you have “Fixed Overheat” selected. From the drop-down menu, select your model of MiniCTA (i.e. 54T42). In order to calculate the overheat ratio, you’ll need the overheat calculation spreadsheet that came on the software disc. There will be two spreadsheets on the disc; pick the one that includes the designation number for your MiniCTA. The spreadsheet for the 54T42 MiniCTA should look like this:
Enter the Sensor resistance, Leads resistance, and Sensor TCR values from the slip of paper into the spreadsheet at the designated spots. The spreadsheet will auto-calculate your overheat ratio in the lower section. Return to the Hardware Setup screen and enter this ratio into the appropriate box. Select “Update Probe Settings” on the left side of the window. You can double-check that you entered everything correctly by comparing the Total Resistance and Decade Resistance values on the Hardware Setup screen to those calculated in the spreadsheet.
The three red boxes at the bottom of the window should now have updated to match the intended hardware orientation for your specific probe. Each red box contains four switches, each of which corresponds to a real switch inside the MiniCTA. Use a screwdriver to open the back of the MiniCTA, then turn it around so that the switches and their red boxes are toward the top. The word “OPEN” should be upside-down, like this:
Now the switches are ordered 4-3-2-1 in each box as you read from left to right, just like in the Hardware Setup window.
As noted below the images of the switches, the dot on the top or bottom of a switch in the software image indicates that that side of the switch should be “down”, i.e. flush with the red part of the switch box. Change the switches inside the MiniCTA to match those on the screen, then double-check that you don’t have them all reversed. Once you are confident in your switch-matching, hit OK and reattach the cover of the MiniCTA.
Velocity Calibration: How to get airspeed from voltage
Once you finish the hardware setup, you should be prompted to begin velocity calibration by the StreamWare Basic program. The window will look like this:
If you would like to calibrate the hotwire using StreamWare, select which kind of calibration you prefer, then follow the instructions from the manual and the software to complete the process. I personally had trouble with the software, so I will need to update this section later to include the steps for this calibration option.
If you are going to be using LabVIEW or some software other than StreamWare Basic, you may choose to exit the prompt window and conduct your own calibration. This involves generating fluid flow at a known velocity, measuring that velocity with the hotwire probe, and plotting the output voltage vs. the known fluid speed. You’ll then need to fit this plot with a power law curve, determining the values of constants A, B, and n for the following equation:
Velocity = A + B*Voltage^n
For the Lung Flow Sensor project, there is a pre-made set of waveforms to be used with the Flow Volume Simulator in order to calibrate the 55P16 probe on the computer at the experimental setup. These waveforms induce the constant piston speed necessary to generate in-tube airspeeds of 1 m/s to 15 m/s. You will have 15 data points with which to create a power law fit equation. The same logic can be used to generate known-velocity fluid flows for other projects, though that process will be tailored to each application.
How to Use the Hotwire Anemometer
The hotwire probe outputs an analog voltage signal that can be read through a DAQ. Via DAQExpress, LabVIEW, or some other software, you will need to create a means of measuring that voltage output over some length of time and saving it. This has already been done for the Lung Flow Sensor parameter sweep. If you want a more specialized or altogether different application than with the Flow Volume Simulator, you’ll have to make one of your own. Once you have that voltage data, convert it to velocity data using the power law fit you found in the previous section.
For the Lung Flow Sensor project, there is a specific part designed to mount the 55P16 probe inside of the test apparatus. It looks like this:
The tolerancing on this mount is not close enough for the probe to stay in place solely due to friction. For this reason, you need to use a small piece of tape or other material to secure the whole probe at a certain height inside the mount, attaching the hotwire sheath to the part itself. It should look like this:
Then extend the hotwire down into the center of the tube and secure it with another piece of tape. It should look like this:
At this point, the sheath should be attached to the mount, the probe should be attached to the sheath, and the hotwire should be securely positioned at your desired height without slipping. You can now run the Flow Volume Simulator and record air velocity measurements with the hotwire anemometer.