How to measure frost fan rotation.

I would like to take a quick moment to recap my previous two posts;

Firstly, there is a method of setting noise limits for frost fans that preserve the ability for a person to sleep. It is based on guidelines from the World Health Organisation, creates an environment where each frost fan operator has an unambiguous noise level to meet and takes into account cumulative effects of multiple frost fans.

Secondly, once an acceptable noise level is set for a frost fan (either via the above process or via an experts test results) compliance with this level can be verified by measuring the speed of rotation of the fan without needing to measure the level of noise.

So… If the speed of rotation of the fan is accepted as the measure of compliance with conditions of consent, then in order to confirm compliance all that is required is a suitable means of carrying out the measurement of the rotation.

I can think of at least four methods that could be used with depending on the level of access to the fans and the will to carry out trials to provide suitable accuracy. 

(1) Tachometer

A tachometer is used for the study of rotating, reciprocating, oscillating or vibrating machine parts. There are various examples available, and in the case of measuring frost fans an optical variant would be required.

It’s principle of operation involves shining a light on a rotating object. As the object rotates light will either be reflected back or not depending on what part of the machine the light is shone at. 

To measure a frost fan a light would be shone on an area that the blade passes through so that when it does pass, the tachometer starts a time measurement. When the next blade passes in front of the beam of light the time that is taken between the reflections is used to calculate the speed of rotation.

Adjustments would need to be made depending on whether a two or four bladed machine was being measured. And for measurement from a respectable distance some form of lens may be required to allow the light source and measuring device to ‘reach’ out to the fan.

In theory, a modest laser could provide a focussed source of light and a garden variety tachometer coupled to a low power telescope could provide the ability to measure rotation at a considerable distance (hundreds of metres).

If access to the fan was available, commercial devices exist that should be able to make the measurement without modification.

(2) Stroboscope

A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects. 

A stroboscope works by using a lamp capable of emitting brief and rapid flashes of light. The frequency of the flash is adjusted so that it is equal to, or a unit fraction below or above the object's cyclic speed, at which point the object is seen to be either stationary or moving backward or forward, depending on the flash frequency.

The frequency can then be directly converted to the rotational speed of the fan.

Given direct access to a frost fan, this method would be the simplest to achieve quick, accurate results. Machines to do this task are available commercially.

If measurement via this method was desired at a significant distance (hundreds of metres), some form of optical coupling would again be required for the light source (low power laser) or a suitable light source would need to me modified to provide a focussed, modulated beam.

(3) Timing the rotation

By timing the rotation of a frost fan about its vertical axis and knowing the gear ratios for the coupling between the main mast and the blade, a value for rotation of the blade can be determined. This will depend on the availability of data on the frost fans being measured.

It does have the benefit of being able to be carried out simply using a video camera that can perform sufficiently well in the low light conditions that exist when frost fans are operating. Again, illumination by a sufficiently focussed light source will aid measurement.

(4) Adaptive frame rate.

Film and video cameras capture pictures at specific rates. i.e A certain number of frames per second.

By using a camera that can adapt the frame rate at which it captures the images, a video can be produced of a frost fan that operates in the same way as a stroboscope, except without the need to provide a flashing light.

If the frame rate of the camera is adjusted to the rate of rotation of the fan, it will appear to be ‘standing still’. Above or below the speed of rotation the fan will appear to be turning forwards or backwards slowly. A steady form of illumination from a focussed light would aid the capturing of the image especially in low light and since a relatively fast shutter speed would be required.

Research would be required to establish the practicality of carrying out measurements using this technique. In theory it would work well from a distance with a focussed light source if a suitable camera was available.

There is no doubt that there will be other methods that would be suitable. Including the most simple of all which is to have the data recorded by the fan itself and transmitted as part of the information that is captured at many frost fan installations.

Given the lack of any measurements being carried out by the Marlborough District and no doubt other Councils, the idea of measuring rotation appears logical and there are avenues for carrying it out that are practical and can be bought off the shelf.

Why are they not being pursued?