How to test an anti-static mat that says \$10^7 \sim 10^9 \Omega\$ surface resistance

Question

I just bought a well-made mat that says surface resistivity is 10⁷ ~ 10⁹ Ω.

The multimeters on the market test up to like 20 MΩ.

If I buy a multimeter to test it:

1. If it displays infinite, I won’t know if it’s just because it’s between 20 MΩ and 1 GΩ, or that it’s truly infinite;
2. If it displays 20 MΩ, I won’t know if it’s actually 20 MΩ, or else if its between 20 MΩ and 1 GΩ, or else it’s infinite.

(I am very new to this and have not used a multimeter before so please pardon my noob questions.)

How do I test that it actually is working as intended (resistance not so high as to fail to protect the computer parts on it from ESD, and not so low as to fail to protect the human from electric shocks)?

Additional information: It says surface resistivity 10⁷ ~ 10⁹, grounding cord resistivity 10⁵. Should I test these parts separately or connected together?

Answer 1

This can be tested with a cheap multimeter, rather accurately even. Multimeters have a rather well defined input impedance when measuring voltages, often either 1 MOhm or 10 MOhm.

So you can set up a voltage divider with the mat like below and estimate its resistance.

$$R_{mat}=R_{in}(\frac{V_{src}}{V_{meas}}-1)$$

If you can't find any info on \$R_{in}\$ of your meter, then put a 100 kOhm resistor in parallel to your meter and use that as \$R_{in}\$.

You can also use a battery or whatever as the voltage source. If the mat is fully insulating, you would read no change in the voltage if the voltage source is present or not.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Building an answer from the comments:

You want to test your ESD mat. To do this properly you need to buy an ESD mat tester. They are not cheap (GBP, EUR or USD are all 400plus) but are designed for the job. I've used them myself to prove out an anit-static floor covering during building work. They aren't cheap, but a quick search shows what options are avalible from your prefered suppliers (such as RS, Farnell etc). They do the job and are worth it when relying on it for production and you're spending tens/hunderards of thousands on building work. You may be able to rent one, but I have no idea how easy that will be in you locality.

But you've asked how to test it. I question why you need to test a mat for hobbiest use. Assuming the mat was from a reputable supplier are you're using it properly (wrist straps, proper earthing connections), I personally wouldn't bother testing it. You can often get away with no ESD protection for hobby type things, yes it harms perfromce and reduces life, but that doesn't stop many people not bothering with ESD protetion and building their gaming PC and having no issues for years. So if you're using the mat properly, I'd say testing it is not worth the cost. If you'd bought a cheap thing that claimed to be an ESD mat but you weren't sure, then test it, but it would have been cheaper to buy a proper mat to start with. If you happen to have a tester avalible, then go for it. But as it is, congratulate yourself for doing more than many and relax.

It is good to see you are thinking about it, but is it worth the price? I'd say no, trust your supplier (or change supplier to one you can trust).

Answer 3

How do I test that it actually is working as intended (resistance not so high as to fail to protect the computer parts on it from ESD, and not so low as to fail to protect the human from electric shocks)?

The computer's enclosure is a Faraday cage and works way better at protecting the parts inside than any ESD mat would. Keep the computer assembled and you won't have to worry about any of it.

If you're servicing the computer, then you don't need a mat either.

1. Have an anti-static wrist strap with a cord with a crocodile clamp at the end. You'd clamp it to the case of the computer.

2. Only place computer parts on the metal parts of the chassis while doing the servicing.

I presume that normally you're not swapping out motherboards, but maybe addding RAM or storage. For such occasional use, an antistatic mat is unnecessary, and a wrist strap will offer sensible protection to the parts from being touched by you. No other protection is necessary. A mat without a wrist strap is just about useless.

I've been messing about with my PCs for almost 4 decades and I've never used a an ESD mat, and I had no failures that could be reasonably attributed to ESD. I keep one hand or a forearm resting on the chassis, and that's enough to dissipate any charges my feet or my bum would be accumulating from rubbing the floor/chair. If I feel too lazy to pay attention to my arm posture, or if I'm doing stuff with sensitive T&M gear, then I put on a wrist strap and keep it connected to the rack or the chassis. There are some expensive components in RF test gear that are extremely ESD sensitive and can't include ESD protection elements while retaining their primary function (e.g. RF mixers).

Everyday PC stuff isn't even nearly in the same category, and every semiconductor component on the boards in a PC has individual ESD protection on its pins. The subassemblies thus have redundant ESD current sinks on most circuits. All you need then is not to have your body at a potential far above or below the chassis - and the wrist strap does that admirably.