Help purchasing a multimeter? - Page 1
Forum section: electronics / general

author: p0tato123
author_rank: Newbie
timestamp: July 15, 2015, 08:10:58 pm
content: Hi, I'm new here and I'm sorry if this is the wrong place to post this.I am a student researcher and looking for a multimeter to monitor the output of a thermocouple. The multimeter needs to have .2 mV sensitivity or better (.02 mV would be ideal), USB connectivity, and a low price tag (no more than $200). Do you have any recommendations? Thanks a lot for your help.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 15, 2015, 08:54:37 pm
content: Quote from: p0tato123 on July 15, 2015, 08:10:58 pmIs this all it will be used for? Or will it also be used for higher energy circuits. Does it have to be USB or will RS232 (serial port) or RS232 with USB adapter work? What kind of accuracy do you need? You mentioned resolution but the other side of the coin is accuracy.

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2015, 09:09:54 pm
content: Note that a thermocouple needs a "cold junction" reference to function properly. Modern "cold junctions" are actually thermal blocks with a thermistor to measure the block temperature and add an appropriate voltage. Also, the "cold junction" will have two outputs that are copper metal in order to get a voltage to the meter without inappropriate thermal EMF errors. Some voltmeters have a cold junction built in for a common thermocouple type (probably J or K) and a standard thermocouple connector.You may be better off looking for a purpose-built thermocouple meter, rather than using a general-purpose DMM.Quick example: $285 new. 120/220 VAC power, linearized analog output from a temperature-specialist company:http://www.omega.com/googlebase/product.html?pn=DP302-KC2Perhaps you can find one used and drive a standard meter from the linearized analog output.

author: Lightages
author_rank: Unknown
timestamp: July 15, 2015, 10:49:23 pm
content: If you are using a K type thermocouple, and you range of temperature is not in the cryogenic temperatures, then this might be useful:http://www.ebay.com/itm/Digitek-DT-4000ZC-TekPower-TP4000ZC-Data-Logging-Multimeter-with-Temperature-/200912705694You would need an adapter to use your TC connector. Don't expect great accruacy nor resolution.Or there is the Brymen BM829s with the PC cable option. But that gets you to closer to $250.There are many multimeters with TC measurement functions, but unless you need the multimeter part, a dedicated USB TC device might be better. Unfortunately I do not have any recent experience to be able to recommend one of the many available. It would also be useful to know what temperature range you need to measure, what environment as in liquid or gas or solid and if it is corrosive or not, and what accuracy. All of these play a role in determining which TC type to use and whether a multimeter is suitable.

author: p0tato123
author_rank: Newbie
timestamp: July 16, 2015, 06:40:55 pm
content: Hello everyone,Thank you very much for your replies, I will try to work through them 1 by 1.QuoteIt will be used for a thermocouple and it may also be used for measuring the voltage in an inductance coil. For this second measurement, the needed sensitivity is lessened, but probably will not reach a high value of volts. With respect to the port, I am a high school student and I'm attempting to do all the coding myself with a custom Python script and either a Raspberry Pi or Arduino microprocessor. This software aspect is already turning into an incredible pain in the ass because of the difficulty associated with turning on/off a USB port. I don't think Arduino/Raspberry Pi support RS232, and I don't have a computer with that port at all, so USB is definitely better. I could probably find an adapter but I'm not sure I want to go down that path. For accuracy, the thermocouple produces a signal of 40 uV/K. So, if there are 4 magnitudes of sensitivity then the measurements are accurate to 4 degrees Kelvin, if there are 5 magnitudes of sensitivity then it's accurate to .4 degrees Kelvin. 4 degrees Kelvin is a bit much for what I am doing, so at 4 magnitudes of sensitivity then it would have to be fairly accurate. At 5 magnitudes, accuracy shouldn't be much of a concern provided it's actually sensitive to 5 magnitudes.QuoteThis does sound like a good idea. This particular unit will not be suitable because the temperature range we are examining is 320-370 degrees Celsius. Thank you for this suggestion though, I will definitely look into it.QuoteI don't yet know the type of thermocouple, and the temperature range is 320 to 370 degrees Celsius. I'll take a look at that link, thank you. The material is solid, unlikely to be corrosive. Accuracy preferable to .4 degrees Kelvin.

author: TimFox
author_rank: Super Contributor
timestamp: July 16, 2015, 08:44:04 pm
content: Quote from: p0tato123 on July 16, 2015, 06:40:55 pmYou really don't want to run the low-voltage TC output into the same full-scale range as a high-voltge signal. Also, remember to do something about cold-junction compensation for the TC. Otherwise, you become sensitive to the temperature of the voltmeter.

author: p0tato123
author_rank: Newbie
timestamp: August 07, 2015, 06:55:11 pm
content: Thank you everyone for your input, I am still looking for a solution. I bought a thermocouple reader for the thermocouple but am still looking for a regular multimeter to gauge the voltage on a pickup coil. All I'm looking for now is 10mV precision and some way to output the data in real time to a text file. Hopefully under $40 as well. Does anyone know of such a device?

author: lapm
author_rank: Frequent Contributor
timestamp: August 08, 2015, 08:25:09 am
content: Quote from: p0tato123 on August 07, 2015, 06:55:11 pmArduino? Not super fast but i believe its built in adc has around 1 mV resolution.. Might be enough, depending on maximum input voltage and possibly needed voltage division.. Also might not be accurate enough... But might be worth looking.Text file saving you may need to code yourself i think...

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 11, 2015, 04:34:44 pm
content: ALDI had multimeters recently, I bought a few, they come with thermocouples but i cannot recall what they cost me.

author: Mephitus
author_rank: Regular Contributor
timestamp: August 11, 2015, 06:44:50 pm
content: Based on your requirements, I think that a Brymen 500 series (or its rebranded kin) might fit the bill. I have a Greenlee brand DM-820. Accuracy at 60.00 mV ± (0.12% + 0.02 mV). It is a rebranded Brymen 527.Manual with specs:http://greenlee.ebizcdn.com/9b91d245c953f54a7f10aba72a4d0022I was able to snag it for $30. Here is a current posting at $40:http://www.ebay.com/itm/311418779878(Lifetime warranty on them too)As something else that would help, if you go on youtube and search for"eevblog multimeter shootout", it will return about a dozen very thorough reviews of meters within your budget range. If you have the time and fortitude to be patient, deals for under 50% MSRP can definately be found. Dont be afraid to buy used too. All of the reputable brands build them pretty rugged and ready to stand up to years of abuse.A third option may be to consider some of the older "bench" DMM's. For example the Fluke 8000 series can readily be found for $50 or less. The specs far exceed what you cited and I have attached an example below. Then buying a dedicated thermocouple meter.