This isn't so much of a blog post as a note to self documenting anything interesting I find whilst trying to repair a Senko SP-SGTP-O2 gas monitor.

Device overview

The Senko SP-SGTP (datasheet, user manual) is a personal gas monitor that is designed to measure the concentration of a specific chemical in the atmosphere and trigger an alarm if that concentration falls outside of a specified "safe" range.

The "O2" in the product code denotes that my monitor contains an oxygen sensor. I use it when exploring abandoned mines. In these confined spaces natural bacteria can consume oxygen as they break down organic matter, reducing the concentration of oxygen in the environment and replacing it with poisonous CO2. Not wanting to asphyxiate and die, I therefore use a gas monitor to make sure that the air I'm breathing contains enough oxygen to live (and therefore probably not enough CO2 to do me harm). There are other risky gases that can build up in confined spaces, particularly abandoned mines, but they are much less likely to occur in the mines that I'm exploring (or have less severe effects than asphyxiation), and multi-gas monitors are expensive, so I have made a decision not to monitor for them and to control my risk exposure in other ways.

The second "P" in the product code denotes that this is a maintainable device rather than a disposable one - i.e. you can (in theory) switch out the battery and sensor if they fail.

Senko is a South Korean company with quite a good reputation for making this type of PPE, but this particular device is definitely on the budget end of their offering.

What's inside?

At first glance this device certainly does appear to be pretty maintainable. The back of the case can be removed with 6 identical machine screws, giving easy access to the battery which just pops out for replacement.

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The circuit board is held into the front of the case with 2 shorter self tapping screws. Removing these allows the board to be removed, allowing the filter on top of the sensor to be removed and replaced.

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But what about the sensor itself? On further inspection it appears to be a fairly standard three pin electrochemical sensor, with the pins labelled on the board:

• "WE": working electrode, does the work of reducing O2 molecules by providing electrons. The number of electrons provided, and therefore the charge on this electrode, varies depending on the oxygen concentration.
• "CE": counter electrode, counters the working electrode by providing the electrons required to reduce the working electrode (essentially just closing the circuit) creating a current with the working electrode that can be measured.
• "RE": reference electrode, sits in the electrolyte but doesn't do any reducing, allowing us to have a reference potential so that the working electrode can be calibrated to provided a stable current.

Unfortunately the pins on this sensor have been soldered directly to the circuit board, so it can't actually be removed without a soldering iron.

In other respects the circuit board itself is only remarkable for being quite so unremarkable. The only ICs are a STM32L0 microcontroller which manages the LCD display and logic, a 12 pin driver for the piezo, and an op-amp which I suspect is used to provide a constant sensor current for stabilisation - everything else appears to be discrete!

Repair

My gas monitor is a couple of years old now, and when I turn it on it complains both that the battery is low and that the oxygen level in my office is about 10%. This isn't correct, as I'm not dead. As such I presume that the dying battery means that the sensor isn't being biased properly, and it's giving duff readings. That, or both the battery and sensor have died simultaneously. My multimeter gives a voltage of 3.61v from the battery, which is the reference voltage, but this isn't under load, so it may well still be the problem.

Change the battery

I bought a new Tekcell SB-AA02, installed it, and waited for a day for the sensor to stabilize. Now when I turn it on the monitor goes through the usual start up check, then shows both the "Stabilization Success" and "Stabilization Failure" indicators. It also shows a 3 digit number which must be a sensor reading as it drops when I breath on the sensor, however neither the "%VOL" nor "ppm" indicators are shown, so it's not clear what this reading actually represents...

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Debugging

Now this monitor can be configured using an IR (infrared) link, and I suspect that if I owned one of those I could adjust the settings and see what's going on. But I don't, so I'll have to try something else...