TEMPERATURE

Air temperature is measured with a common thermometer. Not surprisingly, the thermometer indicates the temperature of the exposed sensor tip, or bulb, which has reached an equilibrium with the surrounding environment. The sensor tip must not be exposed to radiant energy, such as direct sunlight or a heating system radiator, as this will increase the sensor tip temperature. Any measurement taken would not be representative of the surrounding air temperature. Be sure that your measured temperature is representative of air in the zone of importance, usually the area where animals spend most of their time. Air temperature in a central aisle, where air mixing is relatively unrestricted, is probably not indicative of air temperature at the back of the adjacent animal confinement area. A simple maximum-minimum thermometer that can be left in the area of interest is an inexpensive tool that can help determine whether wide temperature swings occur in the building over a period of time. Digital thermometers also are becoming more common. They are easier to read and offer remote sensing capabilities in hard-to-reach animal areas. Digital readouts may offer a false sense of accuracy when meters have an accuracy of ±3 percent yet the readout displays temperature to a resolution of one-tenth of a degree.

Min-Max Thermometer

HUMIDITY

Humidity is commonly measured as "Relative Humidity," which compares the "relative" percentage of moisture in the air to how much moisture the air could potentially hold at that same temperature. Air can hold more moisture as its temperature increases. The traditional way to measure humidity is a two-step process: both wet bulb and dry bulb temperatures are obtained, and then converted to relative humidity using a psychrometric chart. (Use of the psychrometric chart is covered in fact sheet Psychrometric Chart Use G-83)

The traditional instrument, called a sling psychrometer, contains two thermometers. One indicates the dry bulb temperature and the other, with a wet wick, indicates the wet bulb temperature. The sling psychrometer is swung around swiftly (900 ft/min) on a jointed handle for about three minutes to obtain the relative air movement needed to extract the wet bulb temperature. An aspirated psychrometer operates on the same principles as the sling psychrometer, except that a battery powered fan moves air over the wet wick. Cleanup of the aspirated psychrometer wick can be awkward. Air speed over the wet wick is better controlled by an aspirated psychrometer than it is by whirling a sling psychrometer. In order to take a reading on a sling psychrometer, the whirling of the psychrometer must stop, which begins to change the properties of the wet wick. Hence, the aspirated readings are usually more reliable. Accuracy of the thermometer and careful reading of results are important.
Sling Psychrometer	Aspirated Psychrometer

Dry bulb temperature is the commonly measured thermometer temperature. Wet bulb temperature is determined by moving air past a wetted fabric wick covering the sensor bulb. As water evaporates from the wet wick, temperature falls and the sensor reflects a wet bulb temperature. The best accuracy is provided by a clean bulb wick soaked with distilled water. The wick will have to be wetted periodically. With a wet wick, measured temperatures must be above freezing. Air movement can be provided by an aspirated box (with a fan) or by whirling the sensor through the air.

Relative humidity can be measured directly, rather than being determined by two temperatures and a psychrometric chart, by an instrument called a hygrometer. Newer hygrometers measure relative humidity with solid state devices and electronics. The sensor is a matrix material in which electrical properties change as water molecules diffuse into and out of the special material in response to air moisture content. Other hygrometers use materials which indicate electrical changes as water molecules adhere to their surface. Matrix material changes are interpreted and displayed by the hygrometer. Careful calibration is essential. The sensor materials may not tolerate conditions near saturation. Hygrometers offer the advantage of direct humidity measurements and are available in several cost-accuracy categories. A relatively inexpensive, thick, pen-shaped instrument provides digital dry bulb temperature and relative humidity readings. These pens can take several minutes to display a correct reading and provide humidity measurements with an unimpressive accuracy of ±5 percent. More accurate hygrometers (accuracy +/- 1 percent), with an increased price, are better. On some models, maximum and minimum temperature and humidity can be captured over a pre-determined time period.

Hygrometer

AIR SPEED

Air speed is measured with an anemometer. In livestock building applications, two types of anemometers are common, depending on the type of air flow being measured: vane anemometers and hot-wire anemometers. Both instruments are composed of two connected parts: one is the sensing probe and the second displays air speed. One key technique in using an anemometer is to take measurements while air speed and direction are minimally altered by the instruments placement. The operator should stand away from the air flow being measured.

A hot-wire anemometer has a very fine, short wire, often the thickness of a human hair, positioned horizontally between two upright supports. Another design uses a thicker, vertical wire, which incorporates a temperature-sensing thermistor. The wire is heated by electronic circuitry and air flowing over it causes the wire temperature to decrease. By detecting this temperature decrease, or by evaluating the amount of current supplied to keep the temperature of the wire from decreasing, the anemometer determines the speed of the passing air. Calibration is important for relating hot-wire temperature effects to air speed. The hot-wire portion of the instrument is fragile and great care must be taken to protect it from physical damage, which can be caused by large dust particles, airborne bedding, feathers, etc. A hot-wire anemometer is the instrument of choice for low air speed applications. Air moving less than 50 feet per minute (fpm) is considered still air. This condition exists in many animal pens and in many draft evaluations. Due to their small size, hot wire anemometers can be used in small places, such as an inlet jet of a ventilation system, or in hard to reach spaces, such as a duct.

Hot Wire Anemometer

Vane Anemometer Small Headed Vane Anemometer

The vane anemometer is a more rugged instrument that is well suited to several livestock applications. Designs vary, but most have an approximately three-inch-diameter vane propeller which is turned by moving air. Since it makes an air speed measurement based on a larger area than the hot-wire anemometer, it is better for determining air flow over the face of a fan, or a large duct or sidewall opening. It is not ruined by dust and small airborne debris since it can be carefully cleaned. It does not measure low air speeds because the mass of the vane requires a fair amount of air movement to rotate. Vane anemometers are not considered accurate below 50 to 70 fpm, even though the meter provides a readout at these low air speeds. Vane anemometers must be used in air streams which are at least as wide as the vane diameter. They will not accurately measure narrow inlet air jets which are smaller than the vane anemometer propeller. Vane anemometers with small, one-inch diameter vane heads are available for small jet flow measurement, yet they still cannot detect low air speeds. For low speed air (< 50 fpm) and most small jet measurements, a hot-wire anemometer is required. One option on vane anemometers is an averaging mode where velocity is displayed as a running average value over time. This aids in scanning a fluctuating air stream. Velocity manometers may be used in well-defined air streams of fairly high velocity. A Pitot tube is positioned so air flow directly affects the sensing tip, so streamlined air is more desirable than turbulent flow. A velocity pressure is detected, from which air speed is determined. A bouncing ball in the instrument's air tube indicates the velocity reading. Although relatively inexpensive, these flow meters provide accurate, if fluctuating, readings. Velocity Manometer

AIR FLOW VISUALIZATION

It is helpful to see where air is mixing or forming dead zones that influence animal comfort. Unusual air leaks may affect the operation of a ventilation system. Visualizing streamline patterns in livestock buildings has some limitations, but several methods have worked. Devices that generate smoke are the most common and come in gun, stick, candle, and bomb formats, with an increasing amount of smoke, respectively.

Smoke candles are rated according to their duration and volume of smoke they produce. A common beehive smoker provides an inexpensive diagnostic tool for local air flow effects. Smoke bombs have been used, but the abundant smoke quickly obscures air flow patterns and is an irritant to confined animals. Animals should not be present if harmful techniques are used, but since the presence of animals usually affects how air flow patterns develop under normal housing conditions, animal removal may provide unrealistic air flow patterns. It is best to keep the animals in place and use compatible air flow visualization methods. The above smoke devices use combustion to produce smoke, so they also generate heat. This thermal effect tends to produce rising smoke.

Smoke Candles

Smoke Sticks Smoke Gun

Smoke sticks and guns use chemical reactions to produce smoke, so they exhibit few thermal effects. Smoke sticks produce the equivalent of three cigarettes smoke and look like glass tubes filled with cotton. They produce smoke for ten minutes once the end is broken off with pliers. A smoke gun or puffer (a plastic bottle with a cap on the tip) provides small smoke puffs. This allows smoke to be produced intermittently, rather than the unstoppable stream provided by the combustion devices. A rubber bulb on the handle of a smoke gun provides smoke in puffs or continuous stream. The disadvantage is that the small amount of smoke dissipates quickly and may not photograph well. Smoke and stored sticks are irritating and corrosive. Also rubber parts of the smoke gun may deteriorate from chemical corrosion. Very small, neutrally buoyant soap bubbles, constructed with helium and compressed air, can last long enough to show airstreams within an enclosure. Bubbles are surprisingly durable in a free airstream but will not tolerate many impacts with obstructions. The apparatus used to generate bubbles is cumbersome and expensive compared to other air flow visualization devices. Children's soap bubble toys can be useful in faster-flowing airstreams but are not neutrally buoyant. The bubbles exhibit downward gravitational effects which may not represent true air flow.

A set of air speed streamers may be used to detect air speed at various locations in a building. Threads of material or ribbons, such as string or plastic tape, can be "calibrated" to a size which blow horizontally at a particular air flow of interest. These inexpensive tiny posts with attached free-to-spin streamers can be positioned in many locations as indicators of the "calibrated," desired air flow and direction. As conditions are changed in a livestock building, a quick survey of the streamers will indicate which areas are receiving desirable air flow. For example, a mechanical ventilation system inlet air speed of 700 fpm or faster is desirable. Streamers which have been "calibrated" to blow horizontally at 700 fpm are positioned at various inlet locations to observe whether inlet air speed is at least 700 fpm.

Streamers showing inadequate (left) and adequate (right) air speed.

SURFACE TEMPERATURE

In cases where large differences in temperature exist between the animal environment and surrounding surfaces such as walls, ceiling, and floor, determine the radiant temperature, or surface temperature, of those surfaces. Surface temperatures have a strong impact on animal comfort, yet often are ignored in environment analysis. A hot ceiling temperature, from the summer sun, for example, can provide a large radiant load on the enclosed animals. This load would not be detected by a regular, dry bulb air temperature measurement. Surface temperature measurements will indicate ceiling areas with poor insulation. Similarly, very cold surrounding surfaces can make animals feel chilled even though the air temperature seems adequate. Radiation is a very strong form of heat transfer, yet is purely a surface phenomena that can be characterized by an object's surface temperature. An object must "see" another surface in order to feel its radiant heat transfer effect. "Line-of-sight" is a straight, unobstructed pathway where radiant energy wavelengths can travel. Animals in enclosures will be influenced by temperatures of the surrounding walls, ceiling, and floor even though they have limited or no contact with these surfaces. Even a surface outside the barn can cause heat stress if the enclosed animals can "see" it. For example, a black asphalt pavement may heat to 200°F on a sunny day. This surface adjacent to a curtained, naturally-ventilated freestall dairy may affect cow comfort when the curtains are completely opened, since there is a clear radiant heat transfer sight line between the cow and the hot surface.

Infared Thermometer

An infrared thermometer measures surface temperature. This is a line-of-sight instrument and detects the radiant temperature of object(s) it can "see." Readings are calibrated, or zeroed, on a black disc which is at the same temperature as the air temperature in the enclosure being evaluated. Infrared thermometers look like a hand-held hair dryer with a small, circular sensing element that is aimed at a surface. It does not touch the surface, but it detects the wavelength of thermal energy emitted from that surface, which is displayed as a radiant temperature. The instruments field of view widens with increasing distance between the object of interest and the instrument. Therefore, be sure that it is not also detecting adjacent surfaces. Small objects will require having the instrument close. A large object, such as a ceiling, can be evaluated while standing several feet away at floor level. Be sure to evaluate surfaces that the animals "see" from their enclosure.

GAS LEVELS:

Ammonia, hydrogen sulfide.

A portable and relatively inexpensive way to detect gas levels is with a hand-held sampler pump. This manually operated, piston-type pump draws an accurate sample of ambient air through a detector tube. It is very important to hold the pump so the air pulled in through the detector tube comes from the location of interest; this means holding it near the floor during the sampling period for floor-level measurements. Remote sampling is possible for hard-to-reach areas.

The thin glass detector tube is specific to the type of gas that you are measuring. For example, if ammonia is a concern in veal calf housing, a detector tube filled with an ammonia-sensitive material would be attached to the pump. The contents of the tube react with the air contaminants and change color. The length or shade of the color change in the detector tube indicates the concentration of gas in the sample. Tubes come in a choice of measurable ranges so that accurate analysis is possible. For example, one manufacturer offers ammonia detection tubes in 2-500 parts per million (ppm), and 20-1000 ppm ranges. Each tube is used once to obtain a reading and then discarded.

Dozens of gas- and vapor-specific detector tubes are available, including ones for ammonia, hydrogen sulfide, carbon dioxide, and carbon monoxide. Several types of sampling pumps are available, such as a design with rubberized bulb that is squeezed for sampling. The pump and detector tubes must be compatible. As with other instruments, the pumps need to be periodically checked for leakage and calibration.

DUST

Dust is the most difficult environmental parameter to measure and the appropriate equipment is quite expensive. Dust particles need to be separated by size to determine the respirable portion. This dust goes directly to the lungs and contributes to animal and human health problems. Dust, in general, is detrimental to animals, workers, and equipment with moving parts. Air samples may be taken and submitted to a lab where a cascade impactor, or similar device, is used to determine dust levels in a range of sizes.

SUMMARY

Determining air characteristics in livestock housing environments allows us to evaluate problems and their potential causes. This is the first step in correcting any problems that are detrimental to production. A healthy and comfortable indoor environment will lead to productivity gains for livestock. By quantifying air characteristics such as temperature, humidity, air speed, and contaminant levels, we can see where we are falling short of optimal conditions. Changes in management and environmental conditions are the next step. Then air quality can again be quantified for comparison. Progress in improving the environment can be determined and animal health and comfort changes documented.

Each air quality characteristic, such as temperature, humidity, air speed, and flow pattern, can be measured in more than one way. The cost of instruments often is weighed against the accuracy of readings.

Certain instruments are appropriate only for specific applications. Best readings are obtained when the basic principles of how the instrument detects an environmental characteristic are understood. Proper technique will minimize human impact on the air being measured. This fact sheet has outlined many features of commonly used instruments. Part 1 of this fact sheet series explored the Principles of Measuring Air Quality, while Part 3 covers Evaluating Mechanical Ventilation Systems.

Periodic checks on environmental conditions, with instrument readings, are a supplement to the everyday observation of building conditions, animal behavior, and production records.