What are the significant differences between the Models 6514 and 6517A Electrometers?

There are several differences between these two electrometers:

1. Voltage source. The Model 6517A Electrometer contains an independent, programmable, bipolar 1000V supply. The Model 6514 contains no voltage supply.
   
   
2. Method of resistance measurement. The Model 6514 Electrometer sources a constant current and measures the voltage drop across the sample. It then calculates resistance using Ohm's Law. The source current is dependent upon the resistance range and is listed in the "Ohms" section of the Model 6514 specifications. On the other hand, the Model 6517A Electrometer sources a constant voltage and measures the current drawn by the sample. It then calculates resistance using Ohm's Law. The user may program the source voltage or use the electrometer in Auto Ohms mode. In Auto Ohms mode the source voltage is automatically selected by the instrument and is dependent upon the resistance range in which the sample falls. The Auto mode source voltages are listed in the "Ohms" section of the Model 6517A specifications. For more information on either measurement technique, please refer to Keithley Low Level Measurements handbook or Application Note #312, High Resistance Measurements.
   
   
3. Alternating Polarity Method. This difference is related to the previous two. The presence of the voltage source in the Model 6517A Electrometer allows it to employ the alternating voltage technique where the instrument sources a positive voltage and makes a current measurement and then sources a negative voltage and makes a current measurement. A series of such measurements is performed and the results averaged in order to average out the background currents and some of the electrical noise in the circuit. The Model 6514 cannot perform this technique. For more information on alternating polarity and its advantages in measuring high resistance please see the Keithley White Paper, "Improving the Repeatability of Ultra-High Resistance and Resistivity Measurements".
   
   
4. Scanner Card Options. The Model 6517A has two optional plug-in scanner cards the Model 6521 and the Model 6522. Each of these multiplexer cards permits connection to up to ten devices. The Model 6514 Electrometer has no scanning options.
   
   
5. Default charge ranges. The Model 6514 electrometer has coulombs ranges from 20nC to 20uC. The Model 6517A has coulombs ranges from 2nC to 2uC. The user may use either instrument to read higher charge by using the external feedback function. Please refer to the Instruction Manuals for the electrometers for additional information.
   
   
6. Environmental monitoring. The Model 6517A electrometer is supplied with a type K thermocouple probe to measure temperature in the range of -25 to 150 deg C. Also available is an optional humidity probe by which the user can monitor the humidity of the environment. The temperature and humidity probe may be used with the Model 6517A only.