Brett, If your apparatus is already preloaded and you want to use the preloaded value as the initial condition reference point, you will want to use either hardware or software offset nulling.
Offset nulling is a method used to offset a series of measurements with respect to an initial condition (I.C.). If your load cell is preloaded, you can reference all future load conditions with respect to the preloaded I.C. On a hardware level, offset nulling is possible if you're using a SCXI-1520 or PXI-4220. In this case, you will turn on offset nulling in LabVIEW or Measurement Explorer. On a software level, offset nulling is accomplished by taking a series of initial condition readings from the load cell. Then, you average these readings and subtract this value from all subsequent readings in order to offset the initial preloaded value. If the load cell is initially unloaded, you will not want to incorporate any form of offset nulling (assuming I.C. voltage reading is 0V). Now, I understand you're looking to have the arrow blink up and down with loading and unloading, respectively. In order to accomplish this effect, you will want to incorporate a series of Boolean tests. First, measure the initial load reading (with or without the offset nulling dependent on whether/not preloading is existent). Now, use the imbedded LabVIEW Boolean tests to measure whether each subsequent reading is greater than the previous measurement. Make a case structure with the switch condition 'true' and 'false' dependent on the Boolean test condition. In the 'true' condition structure, place the property node coding (answered in your last post) that causes the up arrow to appear. In the 'false' condition structure, place the property node coding for the down arrow to appear. If you need instruction on case structure programming, please refer to this <a href= "http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=BD18D9452FC21649E034080020E74861&p_node=DZ52000_US&p_source=External";> link </a>. In effect, your program should now display the up and down arrow with respect to increasing and decreasing loading of the load cell. If you store all of the signal values to an array as they are acquired, you can use the Array Max & Min function to determine the signal value respective to the maximum displacement. To access this function, [right-click] on your LabVIEW block diagram, click [All Functions]>>[Array]>>[Array Max & Min]. I hope this helps. Please let me know if you need further assistance. Best Regards, Joe Des Rosier National Instruments
