DAIRY SPECIAL: Smoke bombs aid understanding of building airflow

The test should also be repeated at different times of the year when housing all-year-round in order to gain an understanding of how the building performs in different weather conditions.

Be cautious when using smoke bombs in sheds with older animals as they can easily spook at the smoke.

Mr Hughes says: “Smoke bombs do not give you any readings as such. They are very crude, but it is a good way of visualising air movement throughout the whole building.

“Typically, the plume of smoke will rise and slowly drift off in the direction of the airflow, but if smoke moves in a horizontal direction towards a door or outlet, you know the wind speed there is reasonably high, so it might indicate there is a draught at calf level which is not ideal. Anything more than 0.5 metres/second is considered to be a draught at calf level.”

Mr Hughes says in some very poorly ventilated buildings, smoke will hang around for several minutes and seem to disperse within the air, not really leaving the building, or it sometimes collects in one area of a shed. Often he says areas of a single shed can be more prone to disease.

He says: “Measuring the length of time taken for smoke to disappear can be useful in calculating air changes. The guidelines state there should be 10 air changes per hour over winter and this is increased to 60 air changes per hour in summer.”

A smoke bomb will give you an indication of airflow, but they can be used in conjunction with measurements of air speed and temperature to provide a more comprehensive picture of the environment.

Depending on the result of the test, Mr Hughes says there are several changes which can be made to buildings to improve the airflow and overall environment for cattle.

“It depends on the age of the animals, but older animals usually produce a good stack effect. If the outlets are not adequate, it is important to address.

According to AHDB, the approximate requirement for ridge outlet area is 0.04sq.m per animal up to 100kg, then 0.1sq.m/head for fast growing and adult stock. However, AHDB says these figures need to be modified depending on stocking densities and roof pitch.

AHDB recommends inlet areas need to be at least twice the calculated outlet areas, and ideally four times larger.

Mr Hughes says: “A group of young animals may find it difficult to produce enough heat to create the stack effect in a larger building, so by essentially creating a shed within a shed, they are more likely to produce enough heat to move the air effectively.

“You can also install fans or tube ventilation to create positive pressure. It increases airflow in the environment as well as pressure. The air then has to go somewhere, so it is effectively squeezed out of the sides of the building.

“However, you need good guidance on how and where to install them, ensuring you are not causing a draught, but sufficient to aid air movement over animals.”

If the smoke bomb reveals signs of draught at calf level, Mr Hughes advises creating micro-environments to protect them using bales of straw and other obstacles for them to shelter behind.