Just got an IR tester, is off scale normal.

[JohnW2]

(or, of course, get it calibrated!)

realistically for an instrument like this "calibration" probablly consists of little more than sticking a few resistors on it and checking the results are in-tolerance. It's not exactly a precision instrument in the first place, nor does it need to be.

Indeed. I have to say that I usually have a quick check of the IR and low-resistance ('continuity', if you must!) measurements of my ('uncalibrated') MFT whenever I use it, using my little 'resistor box' which always lives with it.

The tester is only specced down to 3M (not 2M as I said in my original post).

That's quite a high lower limit for many purposes. My Fluke MFT measures 'IR' right down to 10kΩ (which it displays as 0.01MΩ).

When I tried with a 1M resistor it gave a reading arround 1.8,beeped at me (it's supposed to beep below and showed a corresponding "buzzer" inducation on the display. A short circuit did much the same. Slightly more worrying was the 2M2 case, the beep and it's indicator on the display were still there and the decimal point were still there but a large number of digits on the display turned off leaving a display fairly similar to the over-range case (which shows a "1" on it's own with the decimal point and all the other digits turned off), which seems like a potentially confusing software bug, especially if you were using the meter in a noisy environment. ... A 10M resistor and a 3M3 resistor read fine.

I agree that what happened with 2.2MΩ could be a bit confusing but, other than that, what you report sounds consistent with a machine that has a 'lower limit' of 3MΩ.

Kind Regards, John

 

[plugwash]

That's quite a high lower limit for many purposes.

Agreed, clearly one of the places where they have cheaped-out.

I agree that what happened with 2.2MΩ could be a bit confusing

and confusion of the sort that could lead to passing stuff that should have failed or at least raised concern

In a quiet lab envionment where the meter is being used by someone who knows what the beep means the risk is minimal but in a noisy industrial environment or if the meter is used by someone who doesn't know what the beep means I could see that being misintepreted as an "over range" reading.

but, other than that, what you report sounds consistent with a machine that has a 'lower limit' of 3MΩ.

The trouble is while the 3MΩ lower limit is given in the manual it's not printed on the meter itself and may not be obvious to the user.

IMO if an IR meter is going to have a "lower limit" as high as 3M it should clearly and unambigously indicate the under-range condition. It should not respond to a short circuit by giving a reading that per BS7671 represents a concern but not an outright fail.

Overall while this meter will serve my purposes i'd struggle to recommend it to anyone else because of the hazards it could present if used by someone who wasn't aware of it's behaviour and limitations.

I have sent my thoughts to MCM (tenma appears to be a brand of theirs, www.tenma.com redirects to http://www.mcmelectronics.com/manufacturer/TENMA/01001018 and the address listed for tenma on the meters box matches MCMs address)

 

[plugwash]

it's supposed to beep below

That should have said it's supposed to beep below 4.3 megohm

 

[JohnW2]

That's quite a high lower limit for many purposes.

Agreed, clearly one of the places where they have cheaped-out.

Indeed. I can but presume that, unlike things like my MFT, it does not reduce the test voltage in the face of low resistances - but merely use the full wack voltage and see what current results, and 'chickens out' if that current is unacceptably high - which is obviously (a little) simpler/cheaper. My Fluke (and I imagine most machines which measure IR) is essentially a constant-current 1mA source, the selected 'test voltage' merely being the highest that the voltage is allowed to go. Hence, if confronted with a 10kΩ resistance, it reduces the test voltage to just 10V (and displays what test voltage it used, as well as the resistance), even if the meter is set to a '1000V test'. If the resistance is 100kΩ, then it allows the voltage to go up to 100v, etc.

IMO if an IR meter is going to have a "lower limit" as high as 3M it should clearly and unambigously indicate the under-range condition.

I agree totally - but frankly think that a meter with such a high 'lower limit' would probably have very limited usefulness, anyway (even if it unambiguously reported 'under-range' resistance) - and all for the cost saving of not using a constant-current source for the tests.

Kind Regards, John

 

[ban-all-sheds]

Yeah, if installation testing is your goal then your cheapest option seems to be a multifunction tester, something like
http://www.tester.co.uk/seaward-powertest-1557-installation-tester?gclid=CNC4-NufrcYCFUTKtAodCUsCug at 358.80 (inc vat and delivery)

Or eBay.