New from West Australia electronics dealer Altronics comes the Brymen BM202 digital multimeter. The meter is priced at A$155 - a very low price considering its functions and data-logging abilities. And why are we covering a multimeter in AutoSpeed? Many modifications of a modern car requires the use of a multimeter - from measuring injector duty cycle, to checking on resistances of sensors, and measuring temperatures. (For more on using a multimeter, go to "Using A Multimeter" ) With appropriate inputs, this meter will measure pretty well anything electronic that you want it to. It will even download the data to your PC...
Sized at about 80 x 160 x 40mm in its protective bright red rubber holster, the meter is a little more compact than many others that feature such a wide range of functions. However, at a glance, you could be forgiven for thinking that there isn't much to the meter - there are only seven positions on the rotary selector, with the other input controls adding only another four pushbuttons. However, the 4-digit LCD display gives an indication that perhaps there's more to this meter than first meets the eye - when the screen is examined closely, no less than 22 enunciators (ie small symbols indicating other factors) can be found. An example of the latter is that two small horizontal lines appear to the left of the display to show when Direct Current (DC) is selected, while a (~) appears when Alternating Current (AC) is selected.
The seven selections that can be made with the main knob are Volts, Ohms, Amps, Milliamps, Micro Amps and K-Type thermocouple. Within each of these, sub-selections are made by pressing the 'Select' pushbutton. For example, when the main knob is set to Ohms, pressing the Select button toggles through Diode Test, Audible Continuity (a buzzer sounds when a circuit is made), Capacitance, and Relative ? (allowing more accurate readings of very small capacitances). The small display enunciators indicate each of these selections. One criticism of taking this approach is that the symbols are in fact very small - you have to peer closely at the display to make sure that you've made the right selection. This applies even more so in poor light - there is no backlighting facility.
The meter is auto-ranging. This means that the meter will automatically select the right scale on which to show the parameter that you are measuring. For example, if you wish to measure battery voltage when the car is running (~13.8 volts), you do not need to select an appropriate voltage range (eg 2-20 volts). Instead, you just make sure that the meter is set to Volts (it defaults to DC), and apply the probes. However, if you are making a lot of measurements that are around the same value, you can set the range of the meter so that it doesn't need to spend even a moment working out which scale to use. Manually setting the range of the meter is done by pressing the Range button. This moves the decimal place, or brings up a different enunciator (eg symbols for ohms, kilo-ohms or meg-ohms).
The selection of Frequency (Hz) or Duty Cycle (%) is made by pressing the 'EF' button, once you're in the AC Volts mode. One press takes you through to Hz, and the next to %. When in either of these modes, the meter auto-ranges, ie you can't select the measuring range - it does it for you. A Hold button, which freezes the display, is also fitted.
The meter is supplied with conventional probes, with the leads about a metre long. All parameters but current and temperature are measured with the probe plugs in the same pair of sockets. The measurement of small currents and large currents require movement of the positive probe plug, and temperature measurements with a K-type thermocouple require the use of an adaptor (importantly, not supplied) to match the normal small yellow K-type plug to the banana sockets of the meter. Incidentally, currents of up to 20 amps DC can be measured for up to 30 seconds continuously.
One very good point is that the meter's 'auto power off' function can be disabled - and there's nothing worse than trying to monitor something, only to find that the meter has switched itself off! The meter is - unusually - powered by two AA batteries.
That's about it for functions, except for one quite odd one - Electric Field Detection. When in this mode, the meter will beep and a bar graph display will lengthen when the meter is brought near to live mains-power wiring. However, it also did this when brought up close to my cat's head....
One attraction of the meter is that it can be hooked up to a PC to allow data logging to occur. The data link comprises an optical interface - instead of actual voltages being transferred from the meter to the PC, two LEDs on the end of the interface lead talk to two LEDs buried in the back of the meter. This isolation gives greater safety to both the PC and the meter, should anything go wrong in either device when (for example) high voltages are being measured.
The interface kit (normally an additional A$25 but free at the moment*) comprises the special optical interface lead and two 3.5 inch discs of software. The lead uses a 9-pin female connector to plug in to a PC COM port, with COM 2 being the default port. However, changing the port that is used is very straightforward - there's an option in the meter software. Talking about the software, it is easy to load and get running. The instructions state that the minimum requirement of the PC is a CPU of at least 486DX2-66, Windows 95/98, and at least 16MB of RAM with 32MB preferred. If you're more into cars than computers (I know I certainly am!) in plain English this means that an old PC may not be able to hack it - especially an old laptop.
The PC display can be configured in a number of ways. Firstly, a large digital display that mimics the display on the meter can be brought up on the PC screen. This is good for legibility purposes rather than anything else. Secondly, an analog meter (which auto scales) can be used to show by means of a needle and meter scale the magnitude of the variable being measured. Another option (and many of these options can be displayed simultaneously) is the comparator display. This takes the form of an analog meter with various colour-coded segments on its scale - blue, green and red. The green is marked 'go' while the red and blue have 'NG' (ie no go) next to them. The needle enters the blue segment of the meter when the signal level is below a user-definable lo-limit, and enters the red segment when it is above a user-definable hi-limit. Importantly, the internal PC speaker can be made to sound different notes when the needle enters either NG segment. This is a very useful function if you are monitoring something and want an audible indication of when the signal goes too high or too low.
In addition, the PC can display a line graph of the data being collected. The sampling increments can be set from a minimum of 0.5 seconds to as high as 3600 seconds (ie an hour), and the time for which the sampling will occur can also be set. At the end of this period (and the logging can be stopped at any time) the data can be seen in graphical or numeric form, the latter having detailed timing information and being able to be exported to other programs (eg a spreadsheet) or printed.
One major advantage that the data logger has over using a PC plug-in module or board is that the 'front-end' of the logger is a multimeter. This means that it is resistant to overloads, cannot be easily damaged by high voltages, and can directly measure parameters like current, frequency, temperature and duty cycle. In contrast, most PC data logging boards require an input of 0-5 or 0-12 volts - anything else needs to be converted to this type of signal input.
As you've seen, the BM202 is a pretty sophisticated multimeter, capable of measuring pretty well anything that you might need to measure on a car (or even if building an electronics kit for a car!). But how well does it work in practice?
Let's take the last first - the data logging. Initially I tried the logging on a laptop PC - a 486DX66 with 8MB of RAM. This is less RAM than the system is stated as needing - and they're right. While most of the software worked fine, logging of a rapidly changing variable caused the PC to get further and further behind, until what the PC was doing wasn't very closely related to what the multimeter was doing! I then tried the logging on a desktop PC equipped with an AMD K-7 and 64MB of RAM. On the desk system, the PC interface software worked fine - but most people with laptop PCs who are into cars don't have really fast laptops with heaps of RAM!
Another disappointment of the logging is that the quickest sampling speed is only 2Hz - ie a sampling rate of once every 0.5 seconds. In many car applications it would be good to be at least five times fast, ie a sampling rate of 10Hz - ten times a second. However, with the exceptions of these points, the displays, logging system and data collection procedures worked fine.
The other functions of the meter were easy to use and worked well. The duty cycle measurement, for example, immediately gave injector duty cycle 'on' time when the meter was hooked up in parallel to an injector on a running engine. (Interestingly, reversing the leads gave the injector 'off' duty percentage.) In general use we found that the continuity piezo buzzer was a little faint when the meter was mounted in its rubber protective holster, and the four pushbuttons could do with more tactile feedback.
With a very wide range of functions, the ability to log measurements to a PC, and a low price, this A$155 meter that would make an excellent addition to the toolbox.
* As an introductory special, every Q1080 meter has the Q1082 RS232 interface included free.
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