How can I detect when the air flow in a room stops and report it?

Question

I have a server room with an air conditioning unit. The unit is supposed to run 24x7, but ND tends to get 100 °F in the summer and -40 °F in the winter. This is a little harsh on the equipment and we have had failures. I have a temperature sensor, but I want a little more warning.

I am trying to cook up some solution that monitors the air flow and sends an e-mail when the air stops. I have looked at various anemometer (wind speed indicators), but they all seem to log data and not provide updates, plus they all seem to be rather large and not a good thing to have spinning in a server room. I want to hang the sensor from the ceiling in front of the air conditioner.

Answer 1

For a very simple sensor you could hang two parallel strips of thin (very flexible) foil in front of the air vent. When air is flowing they'd end up pushed together, giving you (essentially) a switch closure you could read.

You'd still need something to notice and give a yell, like the Arduino + ethernet shield mentioned elsewhere, but this is about as simple a sensor as you could get.

Answer 2

Use a hot-wire anemometer, they can be very small.

Answer 3

For something this critical, I'd seriously consider buying off the shelf.

But if you want to roll your own the simplest solution is two temperature sensors. One in the airflow and one in a "dead" air spot. With air flowing, measure the two sensors and apply the difference to one as an offset (static_offset below) so the difference == 0. This is your calibration step. Due to self heating the sensors should work as a hot wire anemometer and when the air flow stops, the temp difference will go up and that will be your trigger.

Basically you are constantly reading (T1 - T2 + static_offset). When this calculation exceeds a threshold you identify, send the alarm.

Answer 4

Perhaps a pressure meter would do the trick? Use a tube and align it so that the airflow is going into the tube. Use the pressure meter at the other end of the tube. When the airflow stops, the pressure should drop. Sparkfun sells a barometric pressure sensor that, "Under ideal conditions, this sensor can detect the pressure difference within a 9cm column of air." http://www.sparkfun.com/products/8161

For a mechanical solution, you could use a flex sensitive resistor strip (http://www.trossenrobotics.com/c/robot-force-sensor-fsr.aspx). Hang it so that the airflow will force the strip to bend. You'll have to be careful that the strip doesn't oscillate, but that would be very easy to interface to a microcontroller, or a comparator to turn on a LED or a buzzer.

Answer 5

If you want to monitor just one fan, maybe a Hall sensor on the fan for the rpm.

Answer 6

I design equipment for data centres so get where you're coming from here.

Gallamine is on the right track, you want a pressure sensor but a differential sensor like the ones you can get from RS or Farnell. Some are just switches and some have a I2C interface or voltage output that you could read. You can then look at the outlet air press compared to some place out the air flow.

I would also use temperature as I guess there could be times the unit will switch off the fan when down to low temperatures and is not required - depends on design, but normally run 24/7.

Microchip make the explorer 16 kit which you can get a Ethernet plug in module that allows you network access. They also have a full TCP/IP stack you can load onto it and this will allow you to send emails - I know cos we have done it. Would also give you a web interface so you can go look at pressure / temperature remotely to have the feel good factor its still working.

Other options include Arduino and Ethernet shield that again will give you networkable access - seen people sending tweets as alarms.

Others are right - not a quick simple project but possible. Finding an off the shelf unit in a one off will be hard and you may well have to give on your requirements.

Answer 7

How about buying a standard wind meter and hooking it up to one of your free computers. http://www.weathershop.com/wind_.htm

Or you can build one @ http://www.instructables.com/id/Digital-Anemometer-wind-meter/

Answer 8

A temp sensor in front of the outlet might be better than an airflow sensor, since as monpjc mentioned, the fan could continue to run even if the unit is not cooling. The temp there will generally be cooler than the ambient temp in the room, so you could probably find a threshold that would give an early indication that the unit is not cooling before the room temp rises noticeably. If the airflow does stop, the temp in the airstream area will quickly rise to the general room temp, so you are covered both ways.

Answer 9

Surely the core need is to monitor room temp, which you say you are already doing, so isn't trying to monitor airflow just overkill and creating unnecessary complexity? At the end of the day: room = cool enough = happy you?

When I looked into automating the monitoring of the server room in which I worked, I ended up with a USB temp sensor off ebay for very little indeed and we just plugged it into one of the PCs already in the room. There were plenty of apps on the internet to read the sensor and dump the temp to a file and I had a simple batch file to read this and send an email if the temp rose above a set value.

My only extravagance was to look at a USB IR Tx/Rx module that I planned to program to learn and send the aircon on/off commands as the room was a semi-office area and could be adequately cooled by opening some windows when the room was occupied, but once in a while the occupants would forget to turn on the aircon again when they closed up and left for the night. Ultimately, I left the job before putting this part of the plan into action.

Answer 10

There is a cheap alternative that you can put in every point you like. It's a USB Temperature Sensor from Magiant.it that sends email and/or SMS when temperature is above the thresold and could also shutdown the pc/server.