what type of batteries does this use and are they expensive?

It uses Lithium Battery 3V CR2032. They're not expensive. http://www.walmart.com/ip/INSTEN-5pcs-CR-2032-CR2032-3V-Lithium-Button-Cell-Coin-Battery/31515503

Two questions: 1. Will the scale stay 'on', giving you more time to add or subtract from the substance being weighed?

Yes. Seems like they all should do that since you are using it to precisely measure ingredients. This one does give you plenty of time to add or subtract.

is this weight calibrated?

This scale comes precalibrated with a readability of 1 gram and a weight capacity of 11 lbs. / 5 kg.

can you calibrate unit to a known weight if it gets off? ie the scale initially weighs an object to be 5g but over time the scale shows it as 7g

To calibrate, press the tare button or turn off and back on. Scales that weigh in small increments can easily be influenced by as little as a breeze, product spilled on the scale, or product left in a cup, etc. Placement of product at various positions can also affect the reading. Weight should not creep if item being weighed is left in a static situation. And lastly, a good set of calibration weights are always necessary to assure true readings.

Best Sellers Rank	#158,180 in Kitchen & Dining ( See Top 100 in Kitchen & Dining ) #500 in Kitchen Scales
Brand	Smart Weigh
Color	Black
Customer Reviews	4.7 out of 5 stars 2,914 Reviews
Display Type	LCD
Recommended Uses For Product	Home Business, Mailing, Postal, Shipping, Warehouse
Special Feature	Auto Shut Off, Backlit Display , Large Display, Portable, Tare Function
Weight Limit	11 Pounds

Smart Weigh Professional USPS Postal Scale with Tempered Glass Platform, Multiple Weighing Modes and Tare Function, Silver Shipping Scale, Platform Scale, 11 pounds/ 5 Kilograms

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