Thursday, July 22, 2010

HP RPN online

For those like me who don't have an Reverse Polish Notation (RPN) calculator but would like to try one out or use one without actually getting a unit, here's one by Hewlett Packard.

I've always been a Casio devotee. And currently my scientific calculator of choice is the FX115MS. Love it. Has every function I need. If it were programmable it would be perfection itself.

Tuesday, July 20, 2010

Where's the spec?

I've been going over the Fluke 80 Series V user's manual but I can't find the specs for the DCmV impedance when high input impedance mode is set. Input resistance is normally 10MOhm but when the "Hz %" button is held down when the meter is turned on its goes into high impedance mode in the DCmV range.

The Fluke 87 (original series) manual actually states how much it is: "The input impedance of the function (400 mV range) is changed from 10 megohms to greater than 4000 megohms." But that's for the 87 not the 87V.

Googling turned up only one source for this datum. Fortunately, it's pretty reliable since it comes from one of Fluke's own engineers. Chuck Newcombe tells us that,
If you want to measure a high impedance dc source, and the anticipated voltage is less than 600 mV, the Fluke 87V has a power-up feature that can save the day. Just hold the 'Hz %' button while you turn the meter on to remove the 10 megohm divider from the circuit. Now, the dmm's loading effect is greatly reduced, with an input resistance of 1000 Megohms or more. 
So there. For the 87V high input impedance mode value is at least 1000 Megs.

Why this isn't in the specs section of the manual boggles the mind.

Sunday, July 11, 2010

Half a Fluke

Always dreamed of having a bench multimeter. Sure can't afford a brand new Fluke. So I settled for a couple decade old 8842A. Got it off ebay. Was excited to start using it when it arrived. But just minutes into checking the various functions my heart sank when I discovered the AC functions weren't working. "Error 30" kept popping up whenever the "VAC" and "mA ~" buttons were pressed. That could only mean that the AC circuit board isn't installed! That's despite the fact the supplier had told me personally that all the functions--including AC--were working fine. But that wasn't the worst of problems. Turned out the power switch is faulty. After turning the unit off for the first I had trouble getting it switched back on. It's a pushbutton type that mechanically latches. Apparently the mechanism is already dodgy. Had to press it several times for it to stay in the down (on) position. After turning it off for a second or third time the switch just died. That was when my heart stopped. I had bought a lemon. I now had a $300 paperweight adorning my workbench.

Lesson learned. Don't buy expensive antique second hand equipment from half a world away unless you're prepared to be disappointed and unless you can afford to have it sent back in case it's not as described in the ad and/or is defective.

Fortunately that isn't the end of the story. The following day I performed CPR on the Fluke. I kept pressing the switch--gently!--while turning the unit over and on its sides. I was banking on the off and off chance that gravity might help get the switch mechanism to engage. I don't know if the turning helped at all but after a minute or so of pressing the switch actually engaged. Well, that was it--I was not ever, ever going to push that button again. No way I'm going to risk being unable to switch this meter on again.

To power the unit up and down I'm now using an extension cord that has a similar latching pushbutton switch. I've already turned the meter on and off at least a dozen times over the past few days and the unit seems to not mind. I'm just crossing my fingers it stays that way. Will eventually have to look for a service center that can replace this switch. My concern, however, is loss of calibration when they take the unit apart, and what component they might wreck!

As a precaution I've stuck a neon yellow Post-It over the 8842A's green power switch to prevent me from accidentally pressing it! Given how absent-minded I am that's bound to happen without this warning sticker.

This is a calibrated unit (well at least that's what the seller tells me) and initial tests show that it is in fact pretty accurate. I used another Fluke and did a side-by-side test of the DCV, DCmA, and ohms ranges and readings from both meter are within 0.01% of each other. The other Fluke has much less resolution so the two meters are hardly on equal footing.