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I have to work out the air mass flow coming out each vent on a server and have proposed to use a hot wire anemometer to measure the air velocity at each vent by placing it slightly inside one outlets of the vent (it is a grid pattern), then multiply that by the vented area and air density to find mass flow.

My concern is that I am instructing technicians to do it on site, and I am worried that the measurements may vary significantly depending on where on the vent they are taken from i.e. if they take it from the corner or side of the vent it may be different from the centre.

This is my first time trying to measure something like this and was wondering how others in the industry do it. Is there a better way of doing this?

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  • $\begingroup$ What will you use for the discharge coefficient? Also the velocity is not constant across the vent... $\endgroup$ – Solar Mike Nov 26 '17 at 5:46
  • $\begingroup$ A relatively simple and quite accurate and foolproof approach is to measure time to inflate a plastic bag of known volume. With all the cracks unaccounted for, uneven flow (typical rotary fan will provide much more flow near the edges where airspeed of the blades is much higher), surface rounding errors etc, measuring a single window of a grid will create awful errors. Measure the entire flow, don't try to multiply vents. $\endgroup$ – SF. Nov 28 '17 at 15:28
  • $\begingroup$ Mike yes you are correct. I have come up with a possible way to measure it, have asked for feedback on another post. SF that is an interesting idea with the plastic bag. Do you know how I would carry it out (fit it originally with the server running)? $\endgroup$ – david_10001 Nov 29 '17 at 6:28
  • $\begingroup$ @david_10001: You'd need a release mechanism to allow precisely timed start of filling. It could be e.g. a rubber band net stretched over a frame matching the server profile, affixed by one end, held by hand on the other end. (the frame would be holding the edges of the bag too). Place the frame against the "venting" side of the server, quickly (not to overheat the server as you momentarily block the airflow) release the rubber bands (allowing the bag tucked beneath to expand) while starting a stop watch. $\endgroup$ – SF. Nov 29 '17 at 10:09
  • $\begingroup$ Keep in mind that the velocity across the vent could vary significantly. The velocity at the center will probably be the highest, falling off toward the edges. $\endgroup$ – geekly Mar 30 '18 at 18:06
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The Hot-wire anemometer useful for accurate turbulence measurements. Before using it, one has to calibrate the hot-wire with a fine Pitot tube for the range of velocity interested in. In which case you might require local laboratory/workplace temperature compensation.

So, unless you need a turbulence-intensity/spatial or temporal correlations, you can go for a well designed pitot tube.

Also the measurements have to be carried out in the entire outlet region with reasonable spatial resolution.

Then you can calculate the mass flow rate using the below formula (assuming data taken in a 2D-plane )

$\dot m = \bar{\rho(T)}\int\int u(x,y) dx dy = \bar{\rho(T)} \sum_{i=1}^{n_x}\sum_{j=1}^{n_y} u(i,j) \Delta x_{i,j} \Delta y_{i,j}$

where, $\bar{\rho(T)} $ is density of air compensated for temperature.

Hope this helps you.

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  • $\begingroup$ Thank you for that. I have sorted out a low velocity pitot tube meter, doing some tests tomorrow. However I think a simple density x velocity x area calculation will suffice for this purpose $\endgroup$ – david_10001 Nov 27 '17 at 11:15
  • $\begingroup$ Why not make a calibrated orifice that you could then use on site and reduce many of the errors? $\endgroup$ – Solar Mike Nov 27 '17 at 16:04
  • $\begingroup$ good that you had uniform velocity at the exit. $\endgroup$ – mustang Nov 28 '17 at 4:11
  • $\begingroup$ Sorry Mike what do you mean by a calibrated orifice? $\endgroup$ – david_10001 Nov 29 '17 at 1:10
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Remote site measurements can be problematic, especially if the technicians have not already done a similar measurement. It sounds like this is a measurement that has not been done previously.

Assuming that the measurement will be sufficient for the purpose, here are some tips for remote-site instructions:

  1. draw a technical sketch, showing where you want the sensor to be placed
  2. Submit a procedure text, instructing tools and items to use (power supply, voltage measurement, etc) and cabling parameters (type of cable, length limitation, etc)
  3. do the measurement yourself at your location, to see if more info is needed
  4. Take and use a photograph to show how to validate the location of the placed sensor (sensor edge to vent edge)
  5. make a phone call to check if everything is understood as intended, and to convince the technician that this setup is important for your project, etc.

Extra note for the experiment: this seems to be a rather rough method to measure the mass flow. And I am assuming that the measurement will not result in critical decisions (like design change, server replacement, etc). And I guess the output temperature of the vent air would be an important parameter, too.

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  • $\begingroup$ Yes we are gathering the temperatures as well, however what I am concerned about is that the air flow measurements will be rough. I would like to get an accurate idea of the airflow, which is the reason for my question. How would we go about doing this/what is the industry standard for measuring mass flow inside of a server? $\endgroup$ – david_10001 Nov 26 '17 at 21:23
  • $\begingroup$ I guess I misunderstood the main goal of the question. I thought it was more about how to ask and instruct a remote person about measuring the flow for you. Not how to measure it in a better, more reliable way. Anyways, I’ll keep the Answer, it might be useful for someone else. :) $\endgroup$ – Gürkan Çetin Nov 27 '17 at 17:26
  • $\begingroup$ It's all good. I should have asked the question a bit better. I would upvote but I am not able to. I will post another question but will try be a bit clearer in my wording. $\endgroup$ – david_10001 Nov 29 '17 at 1:11
  • $\begingroup$ No problems. :) $\endgroup$ – Gürkan Çetin Nov 29 '17 at 6:17
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If the size of your vents is consistent, you could construct a section of duct that hooks up to a vane anemometer. As long as your setup doesn't restrict flow, you can get fairly accurate, repeatable measurements. The duct needs to capture all of the flow, but not restrict it. This might not be possible in your setup, but probably worth looking at.

Typical measurements are in flow rate, not just velocity.

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