Guest Posted September 13, 2011 Share Posted September 13, 2011 Hi guys, I suspect one of my fridges is not working as it should and is cycling for longer than it should the book says it has a 7amp draw (but this is a theoretical max) therefore is there any way that I can measure the current drawn over a 24h period? I know that there are some very expensive multimeter's that can measure this but is there anything I can make or more cost-effective. Jason. Quote Link to comment Share on other sites More sharing options...
CwazyWabbit Posted September 13, 2011 Share Posted September 13, 2011 Does it also have 240v input? If so you could try using one of those plug in jobbies that tells you how much electric you are using to help you be greener. If there is something 'wrong' with the fridge it will use more power from whichever source it's powered from. It won't give you a current reading as such but you should be able to work it out as it will tell you how many kwh you have used. Just a thought, I'll be prepared to be shot down now Quote Link to comment Share on other sites More sharing options...
Sirocco Posted September 13, 2011 Share Posted September 13, 2011 take a voltage reading of a well charged battery, run the fridge for 24hours, take another reading then with a bit of math convert it to amps? maybe? There will be many variable factors though. The start up draw from the fridge is usually quite high before it settles down. might be worth getting it up and running to a sensible temp, then take your readings. I would do it over several days/night to get a good average. What Fridge(s) are you using? G Quote Link to comment Share on other sites More sharing options...
zardos Posted September 13, 2011 Share Posted September 13, 2011 There are probably lots of ways, depends on how complicated and expensive you want. The cheapest I thought of is normal multimeter and a webcam, just record a video and watch it back at multiple seed to see the cycle time change points. Or use one of the plug in 240v things CwazyWabbit said but just run the fridge directly off mains battery charger. Quote Link to comment Share on other sites More sharing options...
CwazyWabbit Posted September 13, 2011 Share Posted September 13, 2011 (edited) Relatively cheap idea but would take development time. Use an arduino device to measure voltage dropped across a shunt/resistor with it's analogue input and to record this using a pc, a few calculations would turn this into a current measurement and you could use your favorite spreadsheet program to draw pretty graphs etc. Depends how much time you want to put in.... or if you could find someone that has already done it..... Have a look at http://www.ladyada.net/make/logshield/lighttemp.html for some ideas Edited September 13, 2011 by CwazyWabbit Quote Link to comment Share on other sites More sharing options...
UdderlyOffroad Posted September 13, 2011 Share Posted September 13, 2011 Some multimeters have a logging function to output to a PC...and not necessirly £300 Flukes either. Try this one from fleabay - just be aware you'll need a PC with an old-skool serial port. Or this one has a USB interface and software included. Quote Link to comment Share on other sites More sharing options...
Ed Poore Posted September 13, 2011 Share Posted September 13, 2011 take a voltage reading of a well charged battery, run the fridge for 24hours, take another reading then with a bit of math convert it to amps? maybe? There will be many variable factors though. Yep - like missing the required measurements for doing the "maths". You'll need at minimum another variable (such as resistance) in order to calculate the current draw (not going to be easy on a device such as a fridge since it'll be a mainly inductive load). A simple method which can be used to measure currents (wouldn't help in the logging side of things) and particularly useful for stupidly high currents like those drawn by winches and starter motors is to accurately measure the resistance of the wire between the battery and the device (only needs to be done once). Then you monitor the potential difference (aka voltage) dropped across the wire. You then have voltage and resistance and can work out current. Of course if you're wire changes temperature then the resistance will also change - depends on how accurate you want the measurements to be. Quote Link to comment Share on other sites More sharing options...
Guest Posted September 13, 2011 Share Posted September 13, 2011 Cheers guys, Does it also have 240v input? If so you could try using one of those plug in jobbies that tells you how much electric you are using to help you be greener. If there is something 'wrong' with the fridge it will use more power from whichever source it's powered from. It won't give you a current reading as such but you should be able to work it out as it will tell you how many kwh you have used. Just a thought, I'll be prepared to be shot down now Thanks I did consider this I have a 12v converter but was not sure if it would smooth out the load and I would not get a true reading. take a voltage reading of a well charged battery, run the fridge for 24hours, take another reading then with a bit of math convert it to amps? maybe? There will be many variable factors though. The start up draw from the fridge is usually quite high before it settles down. might be worth getting it up and running to a sensible temp, then take your readings. I would do it over several days/night to get a good average. What Fridge(s) are you using? G Thanks G, this is what started me thinking as my car 110amp battery discharged from full load to 10.4v in just under a day which I know given the 7amp rating it should do but that is maximum discharge so I wanted to measure the output and see what's going on especially as the other fridge in the trailer ran all weekend without an issue I know it has 2x110a batteries but given that I was using the microwave lights etc then I would say I had 1 1/2 batteries for the fridge. The fridge in the car is a Waeco CF50 and the fridge in the trailer is a National Luna 75l single zone, I have to say that the National Luna seems a lot better insulated the lid being at least twice as thick, I did notice that on our last trip that a pillow left on top of the Waeco was cold. I therefore wanted to measure the current used and see if insulating the fridge further would make any difference. There are probably lots of ways, depends on how complicated and expensive you want. The cheapest I thought of is normal multimeter and a webcam, just record a video and watch it back at multiple seed to see the cycle time change points. Or use one of the plug in 240v things CwazyWabbit said but just run the fridge directly off mains battery charger. I again did think of that but I didn't fancy watching 24 hours of time laps images. Some multimeters have a logging function to output to a PC...and not necessirly £300 Flukes either. Try this one from fleabay - just be aware you'll need a PC with an old-skool serial port. Or this one has a USB interface and software included. I think that's the chap I have a couple of old lap tops with serial ports but may move to the USB Version as I could use it for other things. Anymore ideas? Quote Link to comment Share on other sites More sharing options...
MrKev Posted September 13, 2011 Share Posted September 13, 2011 you can get fairly cheap (£20 ish) USB sticks which are self (batery) powered temperature loggers - CPC do them as an example, Stick the logger (running,of course) inside the fridge (also needs to be running) and when you look at the results, you should see the temperature recorded versus time. A typical domestic fridge will have quite an obvious sawtooth shaped pattern as the thermostat cuts in and out, and you should be able to see the temperature being kept under control. You should then be able to diagnose the faults, e.g if it's not getting cold enough, so the compressors is running all the time, or if it's over cooling, for example. Kev Quote Link to comment Share on other sites More sharing options...
MrKev Posted September 13, 2011 Share Posted September 13, 2011 something like this: http://www.google.co.uk/products/catalog?q=usb+temperature+logger&hl=en&prmd=imvns&bav=on.2,or.r_gc.r_pw.&biw=1024&bih=475&um=1&ie=UTF-8&tbm=shop&cid=13784429004931203445&sa=X&ei=MbpvTvyyKo75sgbO_bCRBw&ved=0CIUBEPMCMAQ Kev Quote Link to comment Share on other sites More sharing options...
secondjeremy Posted September 13, 2011 Share Posted September 13, 2011 Is the wattage the same on mains as it is on 12 volts? I ask as my My Electrolux camping fridge is something like 120 watts on mains but only 72 watts on 12 volts - (6amps) and really will only maintain itself on this voltage rather than cool down rapidly. This means that in warm weather I can expect it to run most of the time on 12 volts which is no problem when its in the car with the engine running but I have to be careful if leaving it in the car (especially if I don't have a Jump Start with me!) Quote Link to comment Share on other sites More sharing options...
Guest Posted September 13, 2011 Share Posted September 13, 2011 Is the wattage the same on mains as it is on 12 volts? I ask as my My Electrolux camping fridge is something like 120 watts on mains but only 72 watts on 12 volts - (6amps) and really will only maintain itself on this voltage rather than cool down rapidly. This means that in warm weather I can expect it to run most of the time on 12 volts which is no problem when its in the car with the engine running but I have to be careful if leaving it in the car (especially if I don't have a Jump Start with me!) The two fridges are compressor fridges so I think work the same on both types of input, they work well on cool down swapping beer in etc and I believe more efficient than the Peltier effect type of fridges, the 7amps is peak in theory freezing cycle hence my requirement to measure it and see what's going on. Jason. Quote Link to comment Share on other sites More sharing options...
secondjeremy Posted September 14, 2011 Share Posted September 14, 2011 Jason - As its a compressor I'd think that the consumption is the same on mains and 12 volts as I can't imagine the compressor motor would function on anything less than full power especially when starting. The fridge I refer to is an ammonia fridge which simply heats water/ammonia mix - with the ammonia taking in heat as it dissolves in water again. This allows the unit to function on gas as well as the power simply heats liquid. Quote Link to comment Share on other sites More sharing options...
simonr Posted September 16, 2011 Share Posted September 16, 2011 OK - the simplest data logger I can think up is an analogue meter with the tip of a felt pen glued to the needle. Then glue the meter to the hour hand of a clock so the pen draws a line on the face? In place of the meter you could use a small DC motor turning against a spring - or an elastic band. That would develop more torque and move a bigger pen. Bit of a blue-Peter solution - but it could work! Si Quote Link to comment Share on other sites More sharing options...
Guest Posted September 16, 2011 Share Posted September 16, 2011 Thanks Simon, I may give that a go If other things don't work. I have ordered one of the USB multi-meters I can run that with a log file to see what happens, I have also ordered one of the temperature data loggers so that I can run them together and compare power consumption, On a third note I brought some insulation yesterday got a big roll of the stuff you find inside cool bags from B&Q of all places for only £11 a zip and a bit of mesh for the fan and material for tensile strength and I will have a fridge bag so can test with and without. Jason. Quote Link to comment Share on other sites More sharing options...
CwazyWabbit Posted September 16, 2011 Share Posted September 16, 2011 OK - the simplest data logger I can think up is an analogue meter with the tip of a felt pen glued to the needle. Then glue the meter to the hour hand of a clock so the pen draws a line on the face? In place of the meter you could use a small DC motor turning against a spring - or an elastic band. That would develop more torque and move a bigger pen. Bit of a blue-Peter solution - but it could work! Si You'd have to remember to remove the minute hand I also imagine you'd need to support the outside of the hour hand so it didn't flex under the weight of the meter and jam everything up. Quote Link to comment Share on other sites More sharing options...
Cynic-al Posted September 16, 2011 Share Posted September 16, 2011 you could connect a 12v clock upto an old battery, put a known load on (say a bulb) and see how long it took to go flat enough to stop the clock, use that to calculate the capacity of the battery then repeat with your fridge running until it stopped the clock and compare the two to work out what its drawn over a given period. Or just use something like a picoscope, they go for nothing second hand and they work really well and you can dump the logged data straight into excel to see whats going on Quote Link to comment Share on other sites More sharing options...
redneck Posted September 25, 2011 Share Posted September 25, 2011 You could try one of these http://uk.rs-online.com/web/p/pc-based-data-acquisition/4901070/ Quote Link to comment Share on other sites More sharing options...
simonr Posted September 25, 2011 Share Posted September 25, 2011 Now, they look good! Si Quote Link to comment Share on other sites More sharing options...
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