Power Meters FAQ's

Laser Power/Energy Sensors

I need to place a power sensor (thermal) in a closed, sealed chamber and connect it to a meter outside that chamber. How do I do that?

02/14/21

If you can’t just arrange for a feedthrough of the cable through the chamber wall, then you’ll need to have panel-mounted connectors on the inside and outside of that wall; one cable segment connects from the sensor to the connector on the inside of the wall, and another cable segment continues from there to the meter. The trick is where/how to split the cable (you can see a short video about this question here. The starting point is that the D15 connector with the sensor’s EEPROM inside must be connected to the meter, and any split will be elsewhere along the cable. From there, for power sensors the simplest solution is to cut the sensor cable between the D15 smart plug and the sensor itself. There are only 2 internal wires and a screen/shield to connect, and the panel-mounted connectors will be simpler. (If you need to do this on multiple sensors, you’ll need to be careful to keep each sensor paired with its own D15 plug, as the EEPROM in the D15 plug is where the individual sensor’s calibration data is stored.)
 
More generally, such as for other sensor types, the usual way to approach this would be to use a “split cable” solution, where there would be a panel-mounted D15-to-D15 feedthrough connector on the vacuum chamber panel (the 2 D15 connectors would be “dumb”, i.e. without the EEPROM inside that holds the calibration data – their purpose would be just to feed-through), and from the outer connector there would be a second segment of cable terminating in a “Smart” D15 connector that connects to the meter. Consult with Ophir if this is of interest for you.

Any precision measurement is normally associated with a tolerance. You may be wondering how a user of laser measurement instruments, say a laser power meter, quantify the tolerance in the measurements obtained with the instrument.

02/14/21

Each Ophir sensor data sheet states its accuracy, typically something like ±3%. This refers to the absolute uncertainty of the measured value. For example, for a 2 Watt reading, the actual "true" value would be between 1.94 W to 2.06 W (with reference to NIST, to which all our calibration is traceable). To this must be added other uncertainties, if they exist. This assumes the reading is from about 5% of full scale up to full scale. It should be noted that our accuracy specification is, in general, based on a 2 sigma standard deviation.
 
This White Paper explains in detail exactly how we arrived at the accuracy numbers we specify, and what exactly they mean: Ophir Power/Energy Meter Calibration Procedure and Traceability/Error Analysis.