I am the lucky owner of a Fluke 289 multimeter: it is a great digital multimeter, albeit pretty large… Still, I believe this is one of the the best portable DMMs you can find in 2012, with great precision, large display, excellent logging capabilities, very rugged and so on – I am not here to do a review of the device, it has already been done with great talent.

Unfortunately, this multimeter is plagued by one small issue, which can become a very big issue when using it for recording data over long periods of time: its infrared communication port is too sensitive to ambient light!

In practice, this means that if the IR port is exposed to sunlight or any sort of strong light, it will end up crashing the DMM after a few minutes.

Update: Fluke released in 2013 a firmware update to the 289, which fixes this issue, according to the changelog!

In general, what happens is that the screen suddenly switches off after 5 to 10 minutes, and will sometimes display a black line for a few seconds before switching off. When you turn the DMM back on, it will have lost its whole logging session, which is a very big problem! The issue is well known and you can find numerous YouTube videos or forum posts from people who are describing exactly this.

To correct this issue, I designed a small port cover, which I printed on Shapeways: the model is available here.

This cover, pictured below, fits perfectly but there is one problem with it: the material used by Shapeways (Nylon) lets infrared light through! This means that out of the box, the cap does not do its job.

I managed to correct this using Sugru and coating the cap with it. Now it does protect the IR port from sunlight, though I did have one crash while logging with strong sun, once. Note that it is not a good idea at all to leave your DMM in the sun!

Still: if someone reads this post and has an idea on how to make this cap using an IR-proof material at a sensible price, I would be very interested!

Here is the download link for the CAD file:  Fluke Protection Cap

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