Inhalation Tower Nebulizer Calibration and effect of Nebulization Rate on target Humidity range
The Inhalation Tower controller nebulizes at 50% efficiency during calibration in order to calculate a better average nebulization speed. The efficiency it shows after calibration is the maximum efficiency, so it is multiplied by 2 from what you would get by hand calculation.
As a general rule with all neb heads, the calibration is not perfect nor is the rate perfectly linear across all volumes and durations, making it common to see the head empty prematurely or have a little left over. To prevent the possibility of drug being left in the head at the end of the nebulization duration, the typical recommendation is to enter twice the volume into the software than what is physically being put into the head. This will run the head at twice the rate, emptying it in about half the allotted time. In almost all scenarios, ensuring that the proper volume of drug is administered is more important than the time it takes to nebulize. For example, if the researcher wants to nebulize 0.05 ml in 30 seconds using an RC system, we would recommend entering 0.1 ml in 30 seconds into the software, but only putting 0.05 ml into the head. This will ensure all of the drug is nebulized within the set time frame.
The above approach is the recommendation all applications EXCEPT the Inhalation Tower since the Volumes and Durations are typically much larger for this application and ensuring accuracy is key. Entering twice the volume for the Inhalation Tower application would likely result in unachievable nebulization speeds.
Unlike all other Respiratory applications, you MUST regulate the nebulizer to a target humidity, not time, when using the Inhalation Tower. This is because condensation will form if the inside of the tower becomes too humid, resulting in the animals swallowing the drug into their stomachs, rather than breathing it into their lungs. To combat this you must meticulously monitor the Tower humidity using a T/H sensor and sampling manifold to determine a combination of Volume and Time that will not exceed 50-60% humidity. For most researchers, the Volume is nonnegotiable, which means that they must extend the nebulization Time to reduce the speed or nebulization and therefore, the humidity within the Tower.
Using the T/H sensor to monitor the Tower humidity, defining a Volume of 10 ml and a Time of 60 minutes results in the Tower humidity shooting up to 95% within a minute of nebulization.
This would indicate that the nebulization speed is too high to maintain a reasonable internal humidity. Since drug volume (and often concentration) are both firm requirements, the user can extend the Time to 2 hours, for example, and try again. In this example, let's say this reduces the Tower humidity 75%-- it is an improvement, but still not perfect. Should further extending this Time to 2.5 hours result in the Tower humidity dropping to a consistent 50-60%; then this becomes the length of time necessary to nebulize in order to ensure excess condensation does not occur.
If an adequate nebulization time cannot be set using the Controller UI directly, then you can use the Tower Application (attached) to set a custom Volume and Duration. The Tower Application will also log the internal Tower Controller data.
The Tower controller should already have Drierite columns on the top for the following ports:
- Inflow - to allow only dry air to get to the tower, ensuring any humidity present is due entirely to the nebulization, not the environment
- Outflow - to protect the Mass Flow Sensor from humidity.
- It is critical that the Outflow Drierite is replaced when it is all pink since this is the only protection the Mass Flow Sensor has from moisture corrosion.
- The Balance port does not have a filter or Drierite on it, but the airflow through this port is comparatively small and therefore, should not be needed. Additionally, putting extra resistance here is a bad idea because it will decrease the Tower’s ability to control pressure quickly.
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