Understanding the leak rates and duty cycles of dehydrators
The overall purpose of the dehydrator is to eliminate moisture in the waveguide of the transmitter. Moisture will affect the reflected energy and increase the Standing Wave Ratio (SWR) of the system.
Dehydrators deal with moisture in waveguides differently than systems pressurized with inert gasses.
In pressurized systems, the system is sealed, and moisture is kept out by the same seal that keeps the gas in. If the pressurized system develops a leak the inert gas leaks out and moisture can then accumulate in the system. In most cases, these pressurized systems then need to be re-pressurized to find and repair the leak, then be evacuated with a vacuum pump, and refilled with the inert gas. This can become a long and expensive process.
In comparison, a dehydrator supplied waveguide is constantly having the air in the waveguide replaced with desiccated air and its pressure is varying from the low set point to the target set point. This delta P is the operating pressure for the system.
The operating pressure (∆P) is determined by the wave guide’s manufacturer’s recommended max pressure and the lowest pressure the customer is comfortable with as a minimum to the system. The max pressure is generally dictated by the feed horn window material.
In the NETCOM dehydrator it is possible to set the pressure parameters with four set points, these are:
- Low pressure alarm – this is the level at which the unit will present an alarm. This needs to be lower than the low limit pressure
- Low Limit Pressure – this is the level that will cause the dehydrator to start the compressor to pressurize the system. This must be at least .1 PSI lower than the High Limit Target Pressure
- High Limit Target Pressure – this is the level that the compressor will be turned off at.
- High pressure alarm – this is the level at which the unit will present an alarm. This needs to be higher than the High Limit Target Pressure
The difference between the Low Limit Pressure and the High Limit Target Pressure is the customer selected operating pressure (∆P).
Perfectly sealed systems are not only difficult to manufacture but impossible to maintain over time. For this reason, ETI strives for perfect seals but accepts very small leaks in the system as normal. Our maximum allowable leak rate on a new system is .04 psi per minute on a system pressurized at 7.5 PSI. With a dehydrator’s outlet completely blocked off this would translate to a leak downtime of 2.5 hours or more if there is a ∆P of 6 PSI between the low limit pressure and the high limit target pressure.
It should be noted that the leak downtime is dependent on the ∆P. For example, the same leak rate of .04 PSI will take 2.5 minutes when the ∆P is .1 PSI.
For this reason, it is important to understand the relationship between the ∆P, leak rate & duty cycle. To do this also requires an understanding of how the duty cycle is calculated and what it means.
The duty cycle is calculated by averaging the time of two compression cycles using the time the compressor is on divided by total time (time compressor on and time off between cycles). The on time is going to be affected by several variables, but the two most important variables are the volume of the waveguide and the flow rate of the compressor.
There is no hard and fast rule as to what duty cycle you should have on your dehydrator; it is entirely up to the customer to determine what is best for their application. In doing so it should be considered that once set, a change in the duty cycle indicates a change in the system/ waveguide. Setting the duty cycle alarm to approximately two times the selected duty cycle will allow the system to give the customer an alarm indicating a problem with the system.
Setting the duty cycle is accomplished by changing the waveguide bleed (normally mounted on or near the feed horn) to allow a constant managed leak of the system.
NETCOM compressors have a flow rate of 10 liters per min so for small systems (1l or less) you are looking at only a few seconds of compressor time for a complete fill and fractions of a second for satisfying the ∆P requirements for operating pressure.
For these reasons, it is possible to have a NETCOM pressurizing a small system with a very small ∆P that will run the compressor every 10 to 15 seconds for a period of a fraction of a second. At first, this may appear to be an internal leak on the NETCOM but looking at the duty cycle and alarms you will be able to determine that the NETCOM is simply working normally at a 5% duty cycle.
It would be advisable to reduce the duty cycle to as low as 1% for small systems to increase the time between compression cycles. This will also allow the casual observer to feel the system is operating normally without internal leaks or issues.
Another way to accomplish this is, of course, to increase the ∆P on the system slightly if the system will allow it. Again, the limiting factor is the feed horn window max pressure limitations.
As an example:
If a system is initially set to a 1% duty cycle and the duty cycle alarm is set for 2% and after several months of operation the duty cycle alarm is indicated the operator has several options. He can start looking for the leak, find and fix it immediately or determine that the leak is small enough to not warrant immediate action because there has been no effect on the SWR of the system. The dehydrator has compensated for the leak with increased duty cycle. At this point, the operator may bump the duty cycle alarm up to 3%, report the issue and put it on the agenda for future maintenance on a remote site.
Comparing the above example to an inert gas pressurized system the leak would leave the system open to atmosphere and SWR would be affected necessitating an immediate repair.
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