Air conditioning for tog testing to BS5335
Tim Jackson 30 Nov 2015
British Standard BS5335 stipulates the following conditions for the tog testing environment.
1.Air temperature above the quilt must be controlled to 20ºC ±0.5ºC.
2.Air temperature must not fluctuate by more than ±0.1ºC for the duration of the test (approx 5 hours).
3.Air speed must be under 0.3m/sec.
4.Humidity must be controlled to 65%±5%
5.The quilts must be pre-conditioned at 20ºC ±2ºC, humidity as above.
In a small laboratory a 'standard' air conditioner will not achieve the first 3.
Absolute air temperature.
The measurement point for air temperature is between 75 and 100mm above the quilt. In a free-convection arrangement this point is around 1ºC above ambient, and varies with tog value. It is not possible to meet this requirement with a fixed ambient temperature across the whole test range. The solutions to this are either to use a hooded apparatus with local temperature control of the air above the test bed, or to set the ambient temperature to around 19ºC and adjust to suit the quilt under test. This gives difficulties if other apparatus in the room, e.g. another tog tester, requires a different temperature.
Relative air temperature.
Any temperature gradient during the test will give erroneous tog values as it will result in an extraneous heat flow into or out of the quilt, which will be added to the heat flow being provided by the hotplate.
Measurements are based on a 30-minute average of temperature. 'Standard' air conditioners work by turning the compressor on and off at two temperature thresholds typically one degree apart. This leads to sawtooth pattern of temperature with a typical cycle time of about 10 minutes. While the averaging process smooths this somewhat, there is always a partial cycle in any sample period, leading to at least a 0.2ºC fluctuation from period to period. There are again two classes of solution: plenum chamber or re-heater.
Temperature regulation by plenum chamber
If the outlet of the air conditioner is fed into a chamber where it can mix extensively before being passes to the test area, this will tend to smooth out the fluctuations. This arrangement also slows the air from the conditioner to an acceptable level to meet requirement 3.
The usual way of doing this is to use the ceiling void as a plenum chamber, with a perforated ceiling providing a low speed airflow over the test rig, although other baffling arrangements have been used successfully.
Using this arrangement it is helpful if the air conditioner can be programmed in “PWM mode” (Pulse Width Modulated) with a cycle time of one or two minutes. In this mode the compressor is switched on at regular intervals, and remains on for a period (duty cycle) proportional to the temperature overshoot. This will give more cycles to each sample period, and so reduce the difference between periods. If this is not used, then it is important that the air conditioner's temperature sensor (usually on the intake) is arranged to measure the temperature in the plenum, not that in the test area. This means either specifying a remote sensor, or arranging the conditioner to draw most of its air from the plenum, and a sufficient fraction from the test area to ensure adequate exchange.
With this method there will inevitably be some temperature rise between the plenum and the test area, so the air conditioner will have to be set lower to compensate.
Temperature regulation by reheater
The outlet of the air conditioner is fed through a small heater with an electronic temperature controller. This smooths out the fluctuations by heating the air during the cold part of the cycle. I find it best to use a “proportional forward control” arrangement rather than the more usual negative feedback, while less accurate it has a much faster response to sudden changes. So the temperature sensor measures the temperature before the heater and outputs a power proportional to the temperature undershoot, the gain being calibrated to correct the error.
Air Speed
A 'standard' air conditioner produces a draught of cold air during compressor operation. As well as causing cyclic fluctuations, this can cause local cold spots above the quilt, distorting temperature measurements. It is necessary to provide some baffling, eg by a plenum chamber arrangement as described above, or by a hooded apparatus. Even where a hooded apparatus is used, in a small laboratory care should be taken to ensure that draughts do not blow under the hood.
Draughts do not only come from the air conditioner but also from poor sealing and from convection cells set up by temperature differences in the room. An uninsulated external wall or window on a cold day can create a vortex strong enough even to push cold air under the hood of a hooded tester. One should aim for no more than a 1ºC difference between the air temperature and the wall surface temperature at the same height. This requires a U-value of less than 0.7 W/m².K in temperate climates. Any outside windows should be shuttered.
Humidity
It is not thought that humidity in the test area has a significant effect on results, however the humidity of the pre-conditioning environment is important, especially for natural fill products.
Humidity can be either described in terms of relative humidity or absolute humidity. Relative humidity is the moisture contest relative to saturation, normally expressed as a percentage (%RH). The absorption of moisture into hygroscopic materials tends to relate to relative humidity, and most humidity meters work on this principle. Absolute humidity is the mass of water per unit mass of air and may be expressed either as a simple ratio (usually given in parts per million), as the temperature at which that air would be saturated (dew point) or as the vapour pressure of water (Pa H2O). Instruments which measure this are called psychrometers, the classic instrument being the wet-bulb and dry-bulb thermometers. This works on the principle that a thermally isolated film of water in moving air will be cooled by 'wind chill' to a temperature such that the rate of heat loss by evaporation balances the convective warming. From this measurement the absolute humidity can be calculated.
Controlling humidity is considered a 'black art' because relative humidity varies rapidly with temperature, so a device controlling relative humidity in an environment controlled by a switched-compressor air conditioner will be constantly called upon to add and subtract water as the temperature oscillates. Furthermore the relative humidity in different parts of the room will differ due to variations in air temperature. However the absolute humidity is quite constant, unlike heat water vapour does not readily pass through walls. The main source of water is the operator's breath, and the main loss is condensation in the chiller if this is drained away and not re-evaporated. Condensation loss in the chiller can be avoided if the cooling element remains above the dew point, about 14°C under these conditions.
It takes much longer to change humidity than temperature. To control it, it is best to produce the absolute humidity that gives the desired RH for the intended mean temperature, rather than to try to follow actual temperature. So for example if the nominal room temperature is 20°C and the absolute humidity is controlled to a level that gives 65%RH at that temperature then the RH will remain between 60% and 70% over a range of temperatures from 19°C to 21°C without the addition or removal of water.
Hooded tog tester
Many of the air conditioning problems can be resolved by using a hood over the test bed to provide a local region of conditioned air. A fan produces a slow controlled flow over the test piece. Typically the hood would be supplied with chilled air or water from some sort of refrigeration unit. Ideally the this should have a variable speed inverter driven compressor. If it is of the switched-compressor type then a controlled re-heater is needed to stabilise the air temperature. In this arrangement the room does not need to be closely temperature controlled, although it does still need to be humidity controlled and free from major draughts. Bear in mind that the chiller may partially dehumidify the air, and need condensate drainage.
Alternatively a Peltier effect cooler can be used in the hood to provide controlled cooling (or reheating) of the airflow. This method is simpler but has the disadvantage that it is much less efficient then a compressor chiller and produces a significant rooom heating.
Free convection
The un-hooded tester has a simple open frame supporting the air sensors and is essentially convection cooled. The convection plume generated above the quilt has a significant effect on the result, especially at low tog, and so if any changes are made in the room which may affect the convection cell, then the rig should be recalibrated.