looking for K type multimeter, and thermistor

Andy Dingley explained on 23/05/2008 :

As did mine from Mr Maplin and that included the K-type probe with built in LCD display meter.

Reply to
Harry Bloomfield
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Well, test leads on a multimeter are polarised too (by colour) for certain operations but are interchangeable. I don't think you'll damage a K-type sensor with reverse polarity.

Reply to
Dave Plowman (News)

I guess numbers on the LCD should indicate it in some obvious way.

Thanks... I am really sorted now, in terms of information. I know what to look for. I know the terminology. I can see ones from maplin, I can also try CRC and ebay.

I can get multimeters with temperature dispaying LCDs from maplin... that use - I think - diodes (which seem similar to thermistors and thermoprobes). And with ones that display temp, I don't need to do any conversions.

I can look at CRC and ebay for multimeters or DVMs with K-Type socket.. that just display voltage

You've all been extremely helpful.

many thanks

now-just out of curiousity..

similarly with a thermocouple surely.. Just your meter from maplin has a temperature reading.. so I guess a chip in there converts voltage to temperature. An equivalent meter with thermistor, would have a chip that converts resistance to temperature.

so howcome all focus is in thermocouples? do the multimeters with temperature readings not tend to use thermistors?

I don't see mention of K-Type Thermocouple for N73CG , UT50C They do mention Diode though. so in defence of maplin tech. Maybe they really aren't selling multimeters with K-Type thermocouples anymore.

Reply to
jameshanley39

snipped-for-privacy@yahoo.co.uk formulated the question :

A thermistor is not suitable for the higher temperatures which would cause it irreparable damage. A diode can be used to measure temperature, but its again its junction could suffer permanent damage above say 70 - 100 deg C.

K-types are good (from memory) to in excess of 700 to 1000 deg C.

Reply to
Harry Bloomfield

You won't damage it, but the temperature will read as a negative value! Remember that a thermocouple is a voltage source. You are (basically) using the meter as a voltmeter. A thermistor, on the other hand, isn't polarity sensitive because it is simply a resistor whose value changes with temperature. A thermocouple is far more difficult to read (linearly) but handles a wider temperature range.

Reply to
mick

The pins are different widths (and different metals). Physically plugging it in backwards will break it (if you force it) and it certainly won't work.

Reply to
Andy Dingley

There's a lot special about that socket, mostly what the pins are made of. Then it's unusual and polarized to stop you mis-plugging it with anything else, or backwards.

Reply to
Andy Dingley

Read the whole of the post you replied to.

Reply to
Dave Plowman (News)

There's a more fundamental difference too. Thermocouples only measure a temperature _difference_, thermistors measure an absolute temperature. A thermocouple meter used to measure around "room temperature" must also do something called "cold junction compensation", where it applies a suitable offset to compensate for the temperature that the meter end is at. This is tricky to do well. One way is to _know_ the cold junction temperature, by sticking it into melting ice or some fairly standard temperature. Another electronic way is to buy a chipset from Burr-Brown or similar that does the hard work for you.

Of course if the thermocouple is measuring a high temperature such as a kiln, then the few degrees either way for the room temperature end is neither here nor there. Room temperature is usually constant within a few tens of degrees, or else the wetware complains. This is one reason why thermocouples are still popular for high temperature work, but the low temperature stuff was largely taken over by thermistors (from 20 years ago), and these days by semiconductor sensors (less sensitive to connection-induced errors).

Reply to
Andy Dingley

If you physically plug a K type in backwards, you'll snap something. This might not be "reverse polarity" causing the damage, but you've buggered it up all the same.

Reply to
Andy Dingley

If you read my post you'd see I was referring to my two meters which use K type sensors which plug into the normal banana plug sockets. Like every other DVM I've seen which does temperature measurement. There's simply no reason to provide a special socket for the temperature function on these since the sensor can't be damaged by wrong connection. Of course it won't work properly - but the plugs on the sensor are colour coded so that's hardly a problem.

Reply to
Dave Plowman (News)

You wrote "standard banana plugs for the probe lead.", not "piece of inaccurate crap that's so badly designed as to run thermocouple signals through banana plugs". I didn't realise you even meant thermocouples here - there are plenty of packaged thermometer probes around that will give a nice simple voltage output so they work with any meter (or measure rpm, or windspeed, or pH etc.). These quite rightly use bananas.

Fluke do (or did anyway) make an adapter for fitting K types to bananas, but it's a bad idea. It's a particularly bad idea for low temperatures not much different from the temperature of the meter housing. The problem that it isn't a thermocouple any more, it's a daisy chain of several thermocouples. You're as likely to be measuring temperature differences from how warm the meter is, rather than the junction you want. If Fluke build a decent thermocouple cold junction front end and package it into a meter behind standard jacks (Why? K sockets aren't expensive) then it probably works OK for a kiln, but I wouldn't trust it below boiling water temperatures.

Assuming that such an adapter plugged into a simple voltmeter would be any more than a random number generator would certainly be a mistake. If the contact resistance goes high and the meter impedance is that of a DVM front end, you'll just be measuring contact noise, not the couple's output at all.

Reply to
Andy Dingley

One of them is a Fluke. And it certainly isn't 'inaccurate crap' A DVM with a temperature measuring function wouldn't necessarily be of the same design as a device meant solely for that purpose. Otherwise it would have sockets scattered all over the place.

BTW - what's so special about a K type plug - apart from being polarised?

I've seen expensive temperature measuring devices designed for building into machinery where the connections to the probe are simply screw terminals...

Reply to
Dave Plowman (News)

Measuring temperatures comparable to the meter housing, it will be. It might not be inaccurate in use, but only because the valid range of use is limited to only measuring much higher temperatures. Like I said, I wouldn't trust this below boiling water.

To be honest, this is the bane of thermocouples anyway. The single most useful lesson I learned from a whole degree in applied physics is that you don't trust any measurement based on comparing the differences between two very similar direct measurements. Your inaccuracies become proportionately far more important, relative to the measurement value you're after. This is why cold-junction compensation is so important and hard to design with competently, and why so many thermocouple-based measurements of lower temperatures are woefully inaccurate.

I've worked on one site where switching a $50 input board for a $10 input board (flakey cold junction design) and a "saving" was then costing about $50k over a year in wasted energy - just from bad measurements off a thermocouple. They even had spreadsheets (based off that same thermocouple) to show how much they were now "saving" in energy. Funny thing is the fuel bills didn't reflect this.

Pins are made of matching metals to the wires. This avoids creating extra Seebeck junctions other than where you're measuring, and the single cold junction you're keeping a careful eye on.

Yes, provided that the screws and receptacles are made of the right alloys (or at least plated with them). One of the attractions of a K type is that the metals used are affordable and mechanically suitable for doing this with.

Reply to
Andy Dingley

It's as accurate as needed at room temperature - measured against a mercury thermometer. I wouldn't expect it to be accurate to within a few decimal places though.

I'd suggest you try one before spouting off. ;-)

Reply to
Dave Plowman (News)

I suggest you learn how a thermocouple works before believing that a brandname badge on the front will make physics work differently for that maker's products.

Reply to
Andy Dingley

You suggested that my Fluke is useless at anything less than about 100C.

You're wrong. Just live with it.

Reply to
Dave Plowman (News)

What accuracy and/or precision

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their picture they have the same meter I have by the looks of it.
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under £20 including the probe, but don't expect accuracy of 0.01C trust it to be within a a couple of Cs. I've tried the two I have here swapping the thermocouples around gives a range of readouts between 18C & 21C. The meters themselves seem to be about 1-2C difference with no probe connected.

What are yougoing to use it for, my use was to test the temerature of an electric cookers oven as teh numbers on teh dial had all been wiped away. I checked it with another oven and setting the oven temp to 200C got meter results between 196C and 203C or there abouts which I thought was accurate enough for my use.

The rapid meter 328 comes with the theremocouple no lead(s) required. (includes socket too)

Not really, as you'd have to construct a circuit for it. The meter has a switch position 'calibrated' for the thermocouple so just read the Cs of the scale direct.

Reply to
whisky-dave

hard drive temp.. ideally would want a flat shaped piece of metal that I can tape to the hard drive.

The place to measure, apparently is on the bottom, there is this black circular area. A small bit of flat metal will do. Like this

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> >Not sure what wires to buy with the thermocouple..

I see, from rapidonline.com

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am glad I am getting more options...

They all seem to involve the multimeter with the LCD screen, looks like the way things are done..

I don't see many options in types of probe though.. Fair enough, K-Type thermocouple probe

But the contact at the end..

Ideally, I would like it to be like in that picture.

BTW, how did you measure in the oven, did you stick the probe to the outside?

How did you not burn the jacket of the wire?

with a table with C and resistance, it should be fine though, right? no need for a circui/chip to do the conversion. I assume this is the circuit you refer to.

Reply to
jameshanley39

Either bare wires, well separated, or else you use a rigid metal tube probe, packed full with mineral (electrical) insulation and the wires separated inside.

If you make your own rigid probes, the trick is to first glue the wires into a "stick" of wire + filler + wire on a piece of thin paper, all held together with waterglass. Once dry, burn the paper off with a small blowtorch. You can pack this rigid but fragile gluestick (carefully) into the tube without shorting anything out, then pack more filler round it.

Don't place a thermocouple near an induction-heated crucible, or bad stuff happens...

Reply to
Andy Dingley

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