This article is transferred from the old MEDTEQ website written in 2012. Since that time a significant amount of experience testing humidifiers has been obtained and it is planned to update the material below when time allows.
———————————————————————————————————
Design and test of humidifiers are difficult, because the exact temperature and humidity provided to the patient is difficult to know and control. Accurate measurement of temperature high humidity flow of air is problematic, and the difficulties of measuring humidity in any environment are widely documented. It is not surprising that many manufacturers simply rely on open loop control based on parameters back at the chamber, where the humidify is 100%, allowing a degree of certainty. But in that case, the actual humidity output to the patient is heavily influenced by flow rates, length of tubing and room conditions.
Beyond performance, the risks of burns can also be difficult to address without also triggering false alarms under normal conditions. The test methods in ISO 8185 also create some issues affecting accuracy, in particular the tests in Annex EE have a major problem, meaning that most manufacturers ignore the test and simply use an external temperature / humidity gauge to confirm the output despite the difficulty.
This article helps to provide some background and theory for those interested in performing ISO 8185 tests.
The basics (theory)
The main function of a humidifier is to heat a container of water which vaporises the water, adding it to the flow of gas or air and hence creating a heated humidified output. We normally refer to humidity as "relative humidity", such as 50% RH. However, clinically it is the volume of water per unit volume of air that is significant, which is referred to as absolute humidity and has the units of mg/L. A room with 23°C 50% RH has about 10mg/L of water, as vapour in the air. As outlined in ISO 8185, humidifiers should be able to provide at least 10mg/L in 37°C air for non-invasive applications, and 33mg/L for invasive (bypassed airway) applications.
The figure of 10mg/L is easy and safe to achieve under a wide range of conditions (it is usually well exceeded by most humidifiers). However to provide 33mg/L the risks increase: it is starting to get close to safe limit of 43°C, saturated air (60mg/L). Although the energy is only half the safe limit, manufacturers usually have to exceed 33mg/L in most common conditions, to ensure the output is still above 33mg/L in extreme conditions, such as low room temperatures, high flow rates, and taking into account the errors in measurement and control. Adding in abnormal conditions such as brief interruptions of flow, changes in in flow rates and fault conditions in the control system, it can be tricky to find the right balance.
To get some concepts of the relation between heat and humidity, we can use some physical parameters of air and water: the “heat of vaporization” (energy required to evaporate water into air) is about 2.26J/mg. The “specific heat” or energy required to increase the temperature of air is about 1.2J/L/°C. Putting these together, we can predict the energy (power) required for a humidifier to operate at a certain flow rates, input and output temperatures.
For example: at 60L/min (1L/s), heating 25°C dry air to 40°C saturated air:
Absolute humidity: 51.3mg/L (for 40°C)
Power for vaporization: 2.26J/mg x 51.3 mg/L x 1 L/s = 116W
Power for air heating: 1.2J/L/°C x (40 – 25) x 1L/s = 18W
Total power: 134W
The calculations show that power required is directly proportional to the flow rate. What is also interesting in the example is that most (86%) of the power will be used in vaporization of water, not heating of the air. A change of 1°C in the inlet gas temperature has less than 1% effect on the power, a negligible effect in most cases, particularly if feedback control is provided.
However, the dryness of the gas inlet air does have a large effect. Normal room air of 23°C and 50% relative humidity already has 10mg/L of moisture. In such a case, the humidifier only needs to add 23mg/L to achieve a target of 33mg/L, significantly reducing the required heater power to achieve that target. If feedback is employed, this might not make much effect in the long term, but at high flow rates or during warm up the power might be at the limits of the system. Thus, tests with dry air is the worst case and important.
All of the above just gives us some idea of the power needed. Unfortunately, the temperature and humidity of the delivered gas to the patient, in other words, the output end of any tubing, can be quite different. An unheated tube will have cooling and condensation along the tube, reducing both the temperature and absolute humidity. For example, measurements at a flow rate of 10L/min, with a chamber output of 40°C and 51mg/L has a delivered gas of only 34°C and 38mg/L at the end of a 150cm tube.
Where does the water go? In a closed system, it might be expected that the water content leaving the chamber should be the same as what comes out the end of the tube. What happens is that water condenses on the inside of the tube, and thus is not delivered to the patient. When enough water builds up, it collects and runs either back into the chamber or to the patient depending on the angle of the tube. To prevent the patient getting this water, the tubing is usually positioned such that the water drains back into the chamber. Thus, the tube in a sense absorbs water from the flow of humidified air or gas.
If the tube is not heated, the humidity at the output is usually 100%, but the temperature is much lower than that at the chamber output. Also there will be a lot of water in the tubing. The temperature at the end of the tube is difficult to predict, but follows a trend with flow rate: at high flow rates the cooling per unit volume of gas is less (because each litre of air spends less time in the tube), leading to higher temperature and humidity output at the patient. However, as mentioned above, at the highest rates the system may reach the limits of heater power, resulting in a fall in output. Thus, with unheated tubing the absolute humidity output usually follows an inverse bathtub type of curve.
Tube heating is often applied, mainly to prevent excessive build up of water. In this case, the humidity at the patient side can be less than 100%, and the temperature higher than the chamber outlet. The other benefit of tube heating is that overall less heater power is required: because the amount of fluid absorbed in the tube is less, the humidity at the chamber output can be less.
Some tricks in testing
One of the problems in the standard is that dry gas gets used up quickly. Medical grade air is expensive, making the tests very expensive. One trick to lower the cost is to use nitrogen: this is much cheaper and has been found to provide the same results in tests when compared to medical grade air (technical websites indicate also the same). Even then, a 7000L gas bottle can be used up quickly when running tests at high flow rates: just running a 30 minute stabilization period at 60L/min can use 1800L, let alone the actual test.
Another trick was to use room air for stabilization, followed the real test with the dry gas. This works fairly well in mid winter, when the air is dry, but less so in spring or summer. Even with winter air, there is a noticeable change in humidifier performance when switching over to dry gas as can be expected, requiring again a period of stabilization.
To get around this, the air can be dried using a combination of cooling (passing the air through iced water) and then silica gel. The first stage drops the absolute humidity down to around 5mg/L, and the second stage to 1-3mg/L (at 23°C), and amount which is unlikely to influence the results of most tests. At higher flow rates the drying process can be less effective, but this can be overcome by expanding the system. This dried air is perfect for stabilization, and mixed to ensure the residual amount is <1mg/L even at high flow rates.