IOT cooling contamination II
Feb 12, 2009 2:54 PM, By Russell Brown
Figure 4a
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Figure 4b
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Proper maintenance
Proper maintenance will keep a transmitter’s cooling system clean, and this, in turn, will lead to a longer life for your IOTs. The first thing that should be done is to have the coolant tested at least every year. The coolant manufacturers will usually test their own glycol, for a fee, and report whether it needs to be changed out. You can also use an outside lab to test your coolant; just be sure they are qualified to do this type of testing. Dow Chemical will do an analysts of your glycol, but their reports tend to be limited in scope, more of a pass/no pass, whereas an outside laboratory can give you a much more detailed report. In fact, a good way to compare the results would be to have an analysis done as soon as the transmitter is installed and use that report for comparison.
Figure 5
Some of these detailed analyses list levels of inhibitors, but without a reference, it’s hard to tell if it’s within limits. This is where consulting with the tube manufacturer comes in handy. (See Figures 4a and 4b.)
The main criteria for any glycol mix are the pH level, conductivity, oxidation and the level of corrosion inhibitors. The presence of glycolate is an indicator that the glycol is breaking down.
A simple test that any engineer can perform is a pH test using an inexpensive pH meter. After drawing a clean sample, you can read the pH of your coolant. It should be within the range of eight to 10.5. (See Figure 5.) Another check you can perform is to make sure the level in the reservoir tank is high enough that the returning coolant is below the level of the tank. If the returning coolant is above the tank level, then the coolant will be aerated, which contributes to the corrosion process.
Figure 6
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Once you have determined that the coolant is getting close to the limits for pH and/or the levels of inhibitors, it’s time to change out the coolant. In the past, you could purchase additives to bring these levels back within range, but many believe that the cost of changing the coolant is far smaller than the risk of a damaged IOT. It is believed that the life span of glycol in a typical transmitter is about five years, but no manufacturer recommends that you wait that long. As long as the coolant is regularly tested, it will be apparent when it’s time to change it. (See Figures 6 and 7.)
At the minimum, the coolant should be drained and refilled, but it is always better to do one flush with distilled water and then refill the system if there were no indications of corrosion. But as mentioned above, if the analysis shows signs of corrosion, then a full cleaning should be performed before refilling the system.
Figure 7
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Acknowledgments
Vijay Patel, Mark Strohecker and Neil O’Sullivan of e2v contributed to this tutorial.
Next time
The next “Transition to Digital” tutorial will cover multidepressed collector IOT cooling.
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