July 15 & 16 – Power tower fixes and IRGA Calibration

Joined: 23 Mar 2004 Posts: 74 Location: Barrow, AK

Yesterday
As it was my first day back from Atqasuk, I spent much of the morning getting caught up on Journal entries and around 10 am met Jenny and Kirstin at the BASC warehouse. They had gone to the airport to pick up an engineer from ABS Alaska, the company that designed the tower that is supposed to supply power to the eddy covariance tower at BEO.

The tower is an alternative energy source with both solar and wind electrical generation capabilities. The solar panels and wind turbine are designed to charge up four 6V batteries that will supply power to the eddy covariance equipment and data loggers. However, the power generator has not worked for the entire summer. This is the reason that we need to go to BEO everyday and run the generator to recharge the two 12V lead acid car batteries at the site.

The power generation tower at BEO

Earlier this week, while Kirstin and I were in Atqasuk, Kim and Jenny found the tower completely dead several days in a row. The batteries had both run down to less than 6V. They did several re-wiring tests and hauled batteries back and forth from the tower a couple of times to try recharging them back at the lab. Finally, the folks down in San Diego decided to arrange for Jim Norman from ABS Alaska to come up for the day to see what he could learn.

So Jenny, Kirstin, Jim Norman, Matt Irinaga, the BASC Logistics Coordinator, and I headed out to BEO. We brought plenty of trail mix and the field equipment for take soil moisture and thaw depth while Matt and Jim worked on the tower.

While we did our daily business - refuel and start the generator, download the data - and our weekly business - measure thaw depth and soil moisture along the three transects - Matt and Jim puzzled away.

Matt and Jim work on the tower

Finally, three main problems were determined: the blades on the wind generator were backwards, so it was not operating efficiently, the system batteries had been run down so low that the tower could not produce enough power to recharge them (due to the high internal resistance of the batteries at low voltage) and the main unit that controls whether the generated power is going to the batteries or the discharge unit was not working properly and needed to be replaced. With these fixes made, we left the larger generator running to charge up the batteries at the power tower and planned to come back later that evening to see how things were progressing with the battery recharge.

Kirstin and I spent the afternoon running back and forth from CMDL to calibrate its IRGA and reinstall it , while Jenny took UniSpec measurement there.

Jenny, Kirstin and I did hike back to BEO after dinner that night to refuel the generator and see what the battery voltage was reading. It was about 24.5V, so we set up the generator to run overnight.

Today
We went to BEO first thing this morning to check on the tower battery recharge and to get the BEO IRGA for calibration. The generator had quit the night before without running out of gas, because we had left the gas cap turned to off, but the batteries were holding good at 25.7V. We topped off the generator gas tank with some of the gasoline that we had packed out and started it back up.

Then we gassed up and started the generator that was hooked to the eddy covariance tower, downloaded the data and took down its IRGA for calibration.

Kim and Kirstin removing the IRGA for an earlier calibration

The IRGA at each tower is calibrated every week and this week, I got to “run the show” for the calibration of the BEO IRGA.

Calibration
Over the course of time the measurements of CO2 and H2O that the IRGA takes begin to drift and the instrument needs to be reset. When we calibrate the instrument we let it read a known gas and then tell the calibration program what it should be reading. The program resets the instrument to that value. We reset values for the low end of the scale (called the zero) for CO2 and for H2O and then we reset the values for the high end (called span) for both gases as well.

The IRGA and its “brain” on the calibration bench

To begin the calibration we hook up the IRGA “brain” to the computer and put a sealed cylinder into the path of the IRGA instrument so that we can control the gas that it reads.

Inserting the IRGA closed path cylinder

The IRGA with the closed path cylinder installed

The first step is to set the zero for CO2 by running a small flow rate of air through a CO2 scrubber, which is just a plastic tube filled with soda lime, a substance that absorbs CO2. When the values on computer screen show us that the IRGA is reading close to zero and the readings have stabilized then we tell the program to reset the current value to read zero.

Soda lime scrubber attached to IRGA

The IRGA calibration program screen

Next we set the zero for H2O by running the flow of air through a desiccant to dry it and then through the IRGA. Again, we wait for the values on the program to get lower and stabilize and then tell the program to reset the reading to zero for H2O.

Desiccant container for airflow

The next two steps involve setting the span of the instrument. To do this we put in a flow of gas that we know the concentration of CO2 and H2O, but this time the gases have relatively large concentrations so the instrument can properly read the high-end values. To set the CO2 span, we put a flow of standardized air into the closed path. This is basically a compressed gas cylinder that is full of air, but it is air with a precisely measured CO2 level, called a standard.

The CO2 standard with its pressure regulator

When the program shows that the IRGA is reading a stable number, we tell it what the standard SHOULD be (usually around 600 ppm for the gas cylinders that we have) and tell it to reset the reading.

Lastly, we do the same thing for H2O, but to feed a known H2O concentration, we use a machine called a dew-point generator. This machine can create a flow air that has a precise dew point temperature.

The dew point generator

Dew point temperature is the temperature to which the air would have to be cooled for water in it to begin to condense. It is a measure of the concentration of water in the air and is similar to relative humidity, except that relative humidity is a comparison of the amount of water in the air to the maximum that it could hold.

We set the dew point that we want to be about 4 C below the existing temperature so that no water will condense inside the instrument (the closer the dew point to the current temperature, the closer to saturation the air is and thus the higher its humidity). Again, we let the instrument reading get close to the standard value and wait for it to stabilize and then tell the program to reset the value to what we know it should be.

When we finished, we reinstalled the IRGA back at the tower. We checked the power generation tower batteries again and they were over 27V and the diverter was getting warm, so we turned the generator off. We will see if the batteries hold their charge overnight with just the solar and wind power generation. If they do, then we will try reconnecting that tower to the eddy covariance tower on Sunday. Then we could actually take one day off a week!

Two trips to BEO per day for two days in a row, including hauling heavy equipment and we were BEAT!