Pitot tube and differential pressure sensor recommendations?

I’m interested in measuring the air flow through a solar air heater.

One option I’ve seen is a “hot wire anemometer” such as “Wind Sensor Rev P” from Modern Device[1]. Another would be a pitot tube of some description, though most I’ve seen commercially available are quite large, intended for measuring airflow in HVAC installations. My installation will have tubes in the order of 10cm diameter, not square air-conditioning duct a metre on a side!

Does anyone have recommendations on where I could source a manufactured, surplus, etc pitot tube (or Prandtl tube, where both tubes are combined into one object) suitable for measuring airspeed inside a 10cm diameter cylindrical channel, or 10cm square box channel?

[1] http://moderndevice.com/product/wind-sensor-rev-p/

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My guess is that speed would be too low for pitot tube. There are
mass-flow-sensors (not speed) to consider. Get back to me if more info is
wanted - this is written in haste! A sonic anemometer could be considered


I was looking at sonic anemometers too. Is it really as simple as directing an ultrasonic transducer diagonally across the airflow?

late thoughts - what air-speeds are expected - and, would the data be used in a controller (is stability/reliability crucial ?) it may be that a vane/propeller anemometer would do?

My intention is to build a solar air heater connected to a drying box. I want to find out whether air temperature or air flow is more important for drying things out faster. The stuff I’ll be drying is silica gel, so I need to get the air to 40°C or so.

So first step is building the heater and proving that I can heat the air to 40–60°C at all. After that is learning to adjust the air flow (higher air flow will lead to cooler air). My thinking with the heater is that I’ll be able to “dial up” a temperature and have the micro controller adjust fan speeds to adjust the airflow in order to get the exhaust air to that temperature. Thus airspeed will be in the range of 0 to a few cubic metres per minute. Then I get to play with bigger fans, etc to build bigger, better blowers for the air heater. I’m not aiming to build a supersonic wind tunnel or anything.

Have you found any theoretical - or empirical - analysis of the physics of
drying silica gel? My quick look found data only for the final equilibrium
state of the gel, nothing on rates. It should be similar to evaporation
from wet surfaces such as inside leaves, as is well covered in plant

This paper is what I’m wanting to replicate to start with: Pramurang & Exell, “The regeneration of silica gel desiccant by air from a solar heater with a compound parabolic concentrator.”

The fundamentals of reconditioning silica gel are that the relative humidity of the air must be quite low. Thus a high flow rate at a “low” temperature of 40°C means you are blowing a lot of not-quite-dry air over the silica gel. As the air temperature increases, the relative humidity decreases so you can suck more water out of the gel with the same mass of air (and thus a lower flow rate).


be aware it is not the relative humidity that drives the water flux from
the get, it is the difference in vapour pressure between the water and the
air. Heating the air itself has no direct effect on the flux. Just heat
the gel. Heating the gel increases the internal vapour pressure, and thus
the rate of drying. The air flow would ideally be turbulent in order to
reduce the thickness of the boundary layer through which the vapour must
diffuse, before it gets to the ambient air. So, a rough surface helps (no
prob with solid gel), and high air speed *at the surface. *


I just came across your thread and believe I have a very similar project I am working on. It’s an indirect, active solar dehydrator. I have a 32 sq.ft collector with ~ 120 CFM in line fan that blows the hot air through a filter then into a horizontal flow dehydrator. I am trying to do performance testing on the unit to see how efficient it is. I have several humidity/temp sensors throughout the system as well as a light sensor to try to approximate performance against available solar irradiance. Also have PWM control on fan (not using yet) to try to keep a constant dryer temp. I am data logging all the sensor outputs to an SD card using an arduino. My biggest challenge is trying to find a method for realtime measuring of flow. I have several areas where I could measure flow but I am trying to measure it now at inlet end of the dehydrator. Flow should be around 2 ft/s under full fan at this point. I am using a “Wind Sensor Rev P” but I am not getting values that are even remotely close to correct… Have you been able to come up with something that works?

I haven’t even gotten as far as building the box for it :(

As far as the sensor goes, it needs to be calibrated to a reference. I don’t have any kind of reference to calibrate it to, as per the Modern Devices blog entry: http://moderndevice.com/uncategorized/rev-p-wind-sensor-data/