Figure 1: Passive sensor head with active component
by Deryck Webb
There are many safety precautions surrounding the operation and maintenance of an NMR spectrometer. Most precautions center around the use of cryogens when filling the magnet, such as the use of gloves and protective eye wear.
One safety device essential for any NMR facility is an O2 sensor which would monitor O2 levels of the laboratory environment, not just during cryogen fills, but 24 hours a day, 7 days a week. My recent experience with the structure and function of O2 sensors has inspired me to write this article.
Some sensors passively sample the air via diffusion while others actively pump air through the sensor. There are also sensors small and compact enough for personal use in different locations (see Figures 1-3).
All O2 sensors should trigger a visual or audible alarm when levels of O2 are below a normal level. This lower alarm level limit is sometimes adjustable, but can also be preprogrammed or hardware coded into the unit. Some detectors, when in a low limit alarm, will also activate an emergency exhaust system evacuating the area and replenishing it with fresh air. When designing a facility, this exhaust and re-supply of air must be considered in the ventilation designs for the NMR environment.
Figure 2: Actively pumping unit through sensor and exhausting
Structure of an O2 Sensor
The basic structure an O2 sensor can be seen in Figure 4 (below).
An oxygen cell can simply be considered as an enclosure (either a metal can or a plastic molding) which holds two electrodes: a flat PTFE tape coated with an active catalyst, the cathode, and a block of lead metal, the anode.
This enclosure is airtight apart from a small capillary at the top of the cell, which allows oxygen access to the working electrode. The two electrodes are connected, via current collectors, to the pins, which protrude externally and allow the sensor to be electronically connected to an instrument. The entire cell is filled with conductive electrolyte, which allows transfer of ionic species between the electrodes.
The rate at which oxygen can enter the cell dependent on the size of the hole at the top of the sensor. When oxygen reaches the working electrode, it is immediately reduced to hydroxyl ions:
Figure 3: Personal oxygen sensor
O2 + 2H2O + 4e- 4OH-
These hydroxyl ions migrate through the electrolyte to the lead anode where they are involved in the oxidation of the metal to its corresponding oxide.
2Pb + 4OH- 2PbO + 2H2O + 4e-
As the two processes above take place, a current is generated which can be measured externally by passing it through a known resistance and measuring the potential drop across it. Since the current produced is proportional to the rate at which these reactions occur, current measurement therefore allows accurate determination of the oxygen concentration.
As the electrochemical reaction results in the oxidation of the lead anode these sensors have a limited life. Once all the available lead has been oxidized they cease to function. Typically oxygen sensors have 1 - 2 year life span, however increasing the size of the anode or restricting the amount of oxygen that gets to the anode can extend this.
Figure 4: Schematic of oxygen sensor.
In our case at Nanuc we have installed the O2 sensor seen in Figure 1. When our 800 MHz magnet quenched in September 2002 it did not activate the emergency exhaust system. It was regularly maintained and the active component was recently replaced, but the sensor did not register an alarm and therefore did not activate the exhaust fans.
Many theories were put forth as to why the sensor did not function; the favourite at the time was that the sensor froze. This is not a bad theory considering the sensor head was 4 meters away from the emergency exhaust port while 700L of liquid He became 490 000L of supercool He gas. It was surmised that the electrochemical reaction, described above, which is responsible for the sensing the lack of O2 slowed down significantly. It was also conceivable that the active component of the sensor ruptured with the cold.
A few days after the quench it was decided to replace the O2 sensor with a temperature switch in the same location. The O2 sensor head was serviced and moved to a new location out of the line of fire for the exhaust port. It was tested and no problems were found.
Testing the Sensor
It was tested with N2 and He gas and reacted satisfactorily, in that the O2 level could be seen going down to about 15%, but there was no alarm. It was discovered that the low limit alarm was set to only 8.5%. With liquid He venting out of a transfer line for 3 minutes directly below the sensor the O2 level went down to only 15%.
The readings continued to drop very slowly, but it would have taken about 20 minutes to half an hour for levels to reach 8.5% if at all. The lower alarm limit was readjusted to have a lower limit of 17.5%. The exhaust fan was triggered within 2 minutes when tested with venting cryogens.
We noted there were many false alarms from the O2 sensor when it was first installed. Modifications had to be made, in the form of shielding of wires and components from the strong magnetic fields.
During this time the lower alarm limit was reduced to 8.5%. During a quench an O2 level of 8.5% might be achieved, but having an O2 sensor set to trigger an alarm and activate an exhaust system at that level will not save anyone in the room.
Assuming a person was unable to react to a quench or other O2 displacing event, they would depend on the exhaust fan to evacuate the He or N2 gas and draw in more O2. They would certainly be beyond help if the O2 level was down to 8.5%. Combined with extremely low temperature danger, O2 deprivation would have resulted in a tragic outcome. Below is a table provided by the Compressed Gas Association regarding the effects of low oxygen levels.
|15 - 19%
||Decreased ability to perform tasks. May impair coordination and induce early symptoms in persons with head, lung or circulatory problems.
|12 - 15%
||Breathing increases, especially in exertion. Pulse rate up. Impaired coordination, perception, and judgment.
|10 - 12%
||Breathing further increases in rate and depth, poor coordination and judgment, lips slightly blue.
|8 - 10%
||Mental failure, fainting, unconsciousness, ashen face, blueness of lips, nausea and vomiting.
|6 - 8%
||8 minute exposure may be fatal in 50-100% of cases; 6 minute exposure may be fatal in 25 to 50% of cases; 4-5 minutes, recovery with treatment.
|4 - 6%
||Coma in 40 seconds, followed by convulsions, breathing failure and death.
In conclusion my advice to NMR facility managers is to start and/or maintain a regular maintenance schedule for your O2 sensors and any other emergency response equipment as well. Gain an understanding of how, why, and when the equipment will function or malfunction in order to best safe guard the working spectrometer environment.
Deryck Webb is the NMR technologist at NANUC, and can be contacted at firstname.lastname@example.org.