The Agilent 34970A Data Acquisition/Switch Unit

The 34970A Data Acquisition Unit consists of the 34970A and an Agilent 34901A 20-channel multiplexer. This instrument is used to measure AC/DC voltages, resistance, frequency, period, DC/AC current, or temperature as measured by a thermocouple, RTD, or thermistor. The 34970A features a 6 1/2 digit multimeter which is capable of making very accurate and stable measurements. The 34970A will be used to make all measurements in Ch 444, so you will need to come up to speed on it's use quickly.

The Basics

Inputs to the instrument are connected to the 34901A 20-channel multiplexer. On the backplane of the 34907A chassis are three slots: two are unused (and covered), and the top slot contains the 34901A. Facing the back of the instrument, depress the latch on the left side of the top card and gently pull the 34901 multiplexer out. Notice the three numbers (100, 200, 300) on the slots on the back of the 34970A chassis: these numbers are the slot addresses. Since the 34901A is in slot 100, all measurements using this instrument will address channels that look like 101, 102, 103, etc. Carefully remove the cover from the 34901A multiplexer (move the gray clip in the direction indicated by the arrow) and slide off the top.

Look at the multiplexer board: starting at the narrow end of the board are 22 channels, each marked as CH01, CH02, etc. Each channel has low (L) and high (H) terminals to which connections can be made and read by the multimeter. For example, if we connect a thermocouple to the H and L terminals of CH01, we can read the voltage produced by the thermocouple by reading channel 101 (slot 100, channel 01). Similarly, if we connect the leads of a pressure transducer to channel 02, we can measure the voltage produced by reading channel 102.

Connect a type-T thermocouple wire to channel 01 of the multiplexer. See page 20 of the Agilent 34970A User's Guide (Ref 1) for more information. For a type-T thermocouple, the blue (copper) lead is + (H) and the red (constantan) lead is - (L). Make the connection, replace the cover, and slide the 34901A back into slot 100 on the 34970A.

Turn the 34970A on (button in front at lower left.) The panel reads ADDRESS 9 (this is the bus address for interfacing the 34970A to a computer with an HPIB interface.) Turn the knob on the lower right of the instrument until 101 appears in the channel box on the display. If channel 101 was not previously configured, the instrument will say MUX OPEN (this means the channel is open. If the display reads MUX CLOSED, press the OPEN button on the right side of the front panel).

Press the Mon (Monitor) button to display data read from channel 101 in real time.

To configure channel 101 to read a thermocouple:

  1. press the Measure button; CHANNEL OFF appears
  2. turn the knob (lower right) to the right; TEMPERATURE appears in the display (it is dimmed out.) To select TEMPERATURE, press Measure.
  3. THERMOCOUPLE appears in the display' press Measure to select
  4. J TYPE T/C appears; rotate the knob (lower right) to the right until T TYPE T/C appears (dimmed); press Measure to select
  5. UNITS C appears; press measure to select
  6. DISPLAY 0.1 C appears; press measure to select
  7. DONE appears

The temperature should be now be displayed. To complete the configuration for a temperature measurement, do the following:

  1. press Advanced
  2. INTEG 1 PLC appears; press Advanced
  3. T/C CHECK OFF appears; rotate the knob until T/C CHECK ON appears, and press Advanced
  4. INTERNAL REF appears; press Advanced
  5. CH DELAY AUTO appears; press Advanced
  6. DONE appears

You are now reading (provided the Mon function is on) the temperature as measured and calculated, relative to an internal reference, from the thermocouple wire on channel 101. If there is something wrong with the thermocouple wire, OVLD will dislpay on the screen (this is why we turned the T/C CHECK function ON.)

To Connect a resistance temperature detector (RTD) to the 34970A:

  1. Consult page 21 of the Agilent 34970A Data Acquisition / Switch Unit User's Guide
  2. Note that we are using 4-Wire RTDs. Both of the lab RTDs have adapters wired into place so the RTDs and the 34970A units can be moved. The RTD adapter is a 4-conductor with red, black, black, and yellow conductors.
  3. To connect the 4-conductor adapter to the 349701A multiplexer, note the following: Channel n is paired with channel (n+10) - i.e., if you want to connect the RTD to channel 03, connect the red (+) and black (-) leads to channel 03, and connect the yellow (+) and black (-) leads to channel 13.
  4. Connect the adapter to the desired channel in the 34901A, put the cover back on, and slide the 34901A back into place in the 34970A.
  5. Connect the adpater to the RTD.

To configure a channel to read an RTD:

  1. Turn the knob on the 34970A until the channel number where the RTD is connected appears.
  2. press the Measure button; CHANNEL OFF appears
  3. turn the knob (lower right) to the right; TEMPERATURE appears in the display (it is dimmed out.) To select TEMPERATURE, press Measure.
  4. THERMOCOUPLE appears in the display; turn the knob until RTD appears. Press Measure.
  5. Press measure to accept the default value for resistance.
  6. Press measure to accept Alpha=0.00385.
  7. Press measure to accept celcius units.
  8. Display 0.1 oC appears; rotate the knob until DISPLAY 0.01 oC appears; press measure to accept.
  9. DONE appears

If Mon is pressed, you should be reading the temperature as measured by the RTD (provided you're reading the correct channel.)

To set up a scan:

  1. go through the above procedure to configure the channel you wish to read (i.e., if you connect pressure transducer leads to channel 02, make the connection on the multiplexer, put the multiplexer back in the 34970A, turn it on, and use the knob to select channel 102. Here, you would press Measure and select DC VOLTS rather than temperature.)
  2. Turn the knob and find the channel you wish to read
  3. Press Interval; INTERVAL SCAN appears; press Interval
  4. H:M:S 00:00:00 appears; press Interval
  5. 00000 SCANS appears; using the left/right arrows above the knob, and the knob itself to scroll through digits, set for 25 SCANS and press Interval. Select the digit you wish to change by pressing the left/right buttons above the knob; digits are selected when they are dimmed. Turn the knob to set the digit to the desired value.
  6. Press Interval
  7. DONE appears

    To make a scan:

    Press Scan; you'll hear the channel open and close rapidly and the * annunciator will flash on the bottom of the display.

    Now press View; READINGS appears

    Press View again; notice that at the lower right of the display, LAST MIN MAX AVG appear

    The measurement being displayed is the LAST reading; press the right arrow button above the knob until AVG is displayed. This result is the average of the 25 scans you configured above.

    Follow a similar procedure to make connections to the multiplexer and make measurements. Do not hesitate to consult the Agilent Manual if you have questions!

    Here's a quick little exercise to complete in your lab notebook:

    1. Leave the type T thermocouple connected to channel 101. You've configured channel 101 to read a thermocouple with an internal reference.
    2. Leave the RTD connected to whatever channel you happened to connect it to.
    3. Place the thermocouple lead and RTD in a beaker containing some room temperature water.
    4. Another way to make a measurement with a type T thermocouple is to connect the constantan leads of two thermocouples, and then connect the copper leads to the voltmeter. We therefore have two thermocouple junctions, one of which is held at a constant, known temperature and the other of which is used to make the measurement. In this case, the voltage displayed corresponds to the temperature difference between the reference and measuring junctions. A polynomial (ref 2) is then available to convert the voltage (referenced to ice water) to a celcius temperature. A theromocouple circuit will be provided; connect the copper leads to a channel on the multiplexer.
    5. Place one of the thermocouple junctions in the same beaker of room temperature water as the thermocouple and RTD in step 2.
    6. Place the other junction in a dewar flask filled with an equilibrium ice-water mixture.
    7. Configure the channel with the second thermocouple circuit to make a DC voltage measurement (NOT a thermocouple measurement.) Configure to scan 25 times.
    8. Read the average temperature on channel 101 and record it.
    9. Read the and record the average temperature from the RTD.
    10. Read the average voltage on the thermocouple circuit (switch to whatever channel you connected to) and record it.
    11. Using the polynomial for a type T thermocouple given in ref 2, calculate the corresponding temperature, and compare it to that obtained from the temperature measurement using the internally referenced thermocouple and that obtained from the RTD.
    12. Disconnect the thermocouple and RTD leads from the multiplexer. Set the 34970A to channel 101, press Measure, and rotate the knob to select CHANNEL OFF. Set the other channel you connected to OFF similarly. Turn the 34970A off and leave the work area cleaner than you found it.

    References

    1. Agilent 34970A Data Acquisition/Switch Unit User's Guide, Agilent Technologies Inc., 2003.
    2. Hewlett-Packard Application Note No. 290, "Practical Temperature Measurements".
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