The direct relationship between the electrical values from a pH sensor and textbook pH and temperature measurements makes it easy to determine if a Signet 5700 pH Monitor or a Signet 8750 pH Transmitter is functioning properly.

Why this works:

• Because 0 mV = 7 pH in all pH systems; shorting the mV input produces 0 mV into the instrument
• Because the Signet 5700 and 8750 pH instruments are designed to accept the input from a 3000 Ohm thermistor, where 3000 Ohm = 25°C.

Procedure:

1. Remove the BROWN wire (mV Input) from the instrument terminal.
2. Connect a shorting strap between the mV terminal and the ISO Ground terminal (BLUE wire).
3. Remove the GREEN wire (TEMP+) from the instrument terminal.
4. Connect a 3000 Ohm resistor between the TEMP+ terminal and TEMP- terminal.
5. The display should read 7.0 pH and 25°C. There should be no evidence of instability.
(The readings may deviate slightly from ideal values if any calibration adjustments have been made in the instrument.)

Conclusion:
• If the readings are correct, you can assume that the instrument is functioning properly.
• The problem is related to the wiring, the preamplifier or the electrode.