Guide Testing brake fluid for moisture percentage

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2016.5 CX-5 GT AWD titanium/black 2016 Miata Club ST MT white
I decided to order one of those brake fluid testing pens from Amazon.

https://www.amazon.com/dp/ (commissions earned)

Don't get me wrong, I flush the brake systems on all of my vehicles at regular intervals and even if the pen tells me that the fluid is ok, I'll still change it out if it's time. I just wanted to know for my general knowledge what the pen tells me about the status of the fluid.

After I ordered the $13 pen, I got to thinking on this a bit and did a bit of research on the subject. If a pen can test the fluid via electronics why can't a person use a DMM (digital multimeter) to test the fluid as well? It turns out that it can be done.

With the DMM set to read DC voltage (set to auto range or the lowest possible range) insert the positive probe into the brake fluid and touch the negative probe to the unpainted metal housing of the master cylinder (some say to touch the negative battery post instead). Allow the reading to settle. It should read less than 0.3 vdc, that's 300 millivolts DC. The reading of 0.3 vdc corresponds to 3% moisture in the brake fluid and the fluid should be changed. Anything at or above 3% needs to be swapped out.

I'm sure that some of you are wondering where the voltage comes from? It's from a chemical reaction.

"Most vehicle manufacturers use a combination of different metals to make the braking system, aluminum in the pistons and housings, and steel in the brake lines and cylinders on the wheels. When moisture mixes with the brake fluid, a galvanic reaction (corrosion) occurs. The higher the moisture content in the brake fluid, the greater the corrosion and the stronger the galvanic reaction. A voltmeter measures the galvanic reaction voltage in the braking system. The magnitude of the measured voltage depends on the moisture levels in the liquid. The higher the percentage of moisture, the higher the voltage."

I've never checked my brake fluid this way. Once my pen gets here, I'm going to try both methods and see what's what.
 
Yep let us know. Many of us have on hand the “free, well almost” multi tester from harbor freight. It seems to work fine but I don’t know if it’s accurate on something this exact.
 
Conrad,
I am a EE major. I got a multimeter, though it is an analog one.
I will give it a try. Interesting, your post is.
 
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Yep let us know. Many of us have on hand the “free, well almost” multi tester from harbor freight. It seems to work fine but I don’t know if it’s accurate on something this exact.

Conrad,
I am a EE major. I got a multimeter, though it is an analog one.
I will give it a try. Interesting, your post is.

Retired EE here myself. I have a number of multimeters, all Fluke and none of them are of the inexpensive varieties. Fluke 77, 179, and the bench model 45.

The pen tester is supposed to be here today. I'll give it a try over the weekend and compare the readings with the Fluke 77 and just for s**** and giggles the 179.
 
Retired EE here myself. I have a number of multimeters, all Fluke and none of them are of the inexpensive varieties. Fluke 77, 179, and the bench model 45.

The pen tester is supposed to be here today. I'll give it a try over the weekend and compare the readings with the Fluke 77 and just for s**** and giggles the 179.
Just remember brake fluid is “corrosive” at least to the paint. Play it with care and let us know. ;)
 
I only trust my factory trained and certified technicians when it comes to inspecting and installing OEM fluids to keep my vehicle running optimally as how Mazda engineered it.
 
I only trust my factory trained and certified technicians when it comes to inspecting and installing OEM fluids to keep my vehicle running optimally as how Mazda engineered it.

Well, okaaaay then. Good luck with that.
 
My brake fluid pen came yesterday. It's too cold in the garage to be messing around out there. I did test a cap full of new brake fluid and a cup of water though, it's nice and warm in my workshop downstairs.

The brake fluid tested with no water detected (one LED illuminated on the test pen) and the water lit up all the LEDs. So at least that works.

It's supposed to be in the 60's next week! I'll do some testing with the DMM and see what that tells me.

I believe that it's going to be time to uncover my MX and take a top-down ride next week!
 
I tested mine last year with a volt meter and it was at .1. I started to buy a $10 test pen but then found reference to dmm test.
 
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