SPice10212: Calibration

You need to calibrate the board yourself. You may find it easiest to calibrate with a special purpose program loaded into the SPLat controller, at least initially. When you gain experience with the process you could build a calibration mode into your final SPLat program.

Caution: Chances are you will be working with lethal voltages. Only do this work if you fully understand how to work safely with dangerous voltages. The use of the term "isolation transformer" does not imply they make it safe.

The process is easiest if you have a SPLat board with an onboard LCD display. In that case the readings can be displayed on the LCD. Otherwise a calibration program can store readings in RAM for readout using SPLatLink.

Assuming you are calibrating your system to measure 3 phases of a 240V circuit, you will need the following for calibration:

• Your SPLat board set up with the SPice10212 and wired via the isolation transformers to the 240V source.
• A multimeter capable of accurately measuring the 240VAC and a safe way of connecting it to the 240V.
• A DC power supply for the SPLat
• Computer and programming cable
• A calibration program, translated and downloaded into the SPLat board. Some example programs are provided later under Programming.

Because transformers can have some unit to unit variation, it is best to use the actual transformers, not proxies. You absolutely must not use a different model of transformer for calibration to the one you are using "live".

Before you start the actual calibration you need to decide what full scale range you are aiming for. The SPice10212 fitted to a MMi99/200, an SL99 or an MS12 will produce an analog reading of 255 counts at full scale. If for example you selected 255VAC full scale that would give you exactly one count per volt. If now your nominal voltage being measured is 240VAC, a 255VAC full scale would allow 6.25% over-range (255/240=1.0625). In practice it would probably be better, when monitoring mains (line) voltages, to go for 10% or even 15% over-range, so with a 240V nominal system you would plan for 264V or 276V full scale.

Once you have decided on a full scale voltage determine the scale factor between voltage and analog input readings (count). If 255 counts represents say 264VAC, then for any given voltage

Counts = Voltage * 255/264

So the scale factor for converting voltage to counts is 255/264=0.966. Call this factor "K"

You must calibrate each of the 3 channels separately. Repeat this procedure for each channel.

1. Apply 240V to the transformer primary (the actual voltage will be whatever is present on the line).
2. Check the secondary voltage with a multimeter, measuring at the input pins of the SPice10212. It should be in the range 24 to 33V (assuming the primary is at its nominal voltage). The high upper limit is brought about because a transformer will always produce more than its nominal voltage when unloaded. It is designed to drop to the nominal voltage when fully loaded. In general, the smaller the transformer the larger the voltage difference will be. The SPice10212 is designed to accommodate transformer voltage ranges between 24V and 33V.
3. Observe the raw analog to digital reading produced by the calibration program for the channel you are working on. If you are using a controller with an LCD you should be using a program that displays it on the LCD. Otherwise you should observe it via the SPLatLink feature of SPLat/PC. This is documented in the SPLat/PC help file (not in the SPLat Knowledge Base). Just search for SPLatLink. Make sure you are working with a recent version of SPLat/PC.
4. Measure the input voltage to the transformer, e.g. the 240VAC line. Chances are it will not be exactly at the nominal value, because the mains fluctuates. Say for example you read 236V.
5. Calculate what the raw analog input count should be, given the current mains (line) voltage. You get this by multiplying the voltage by the scale factor K you calculated earlier. With the example numbers, where K=0.966 and the the voltage is 236V, the raw reading should be 236*0.966=228 counts.
6. Adjust the calibration potentiometer associated with the channel you are working on until the reading matches the required value. While doing this check that the input voltage is remaining steady.

It can be very helpful if you have a Variac to vary the input voltage. This helps you select your calibration point and helps you gain confidence in the system.

Make sure you don't calibrate at the full scale value, i.e. with 255 counts. If you are adjusting for 255 counts there's no way of knowing if the input is over 255 (e.g. should read 256).