Depth Measurement in general
As always, the equipment used depends on the application. The simplest depth gage is one you probably already have in your possession: the dial caliper. The depth rod which protrudes from the end of the calipers, as you open them up, is used to measure depths with a resolution of .001" and an accuracy of .001", at least for the first 4 inches of travel. Expect an accuracy of .0015" from 4 to 6 inches. Obviously you can measure depths equal to the extended length of your calipers.
The maximum width of the depth rod is .145" on Brown & Sharpe, Tesa and Etalon 6" calipers. This dimension may limit its use, but the rod could be filed or ground to a specific profile, as long as it does not affect the length of the rod.
If you need to measure the depth of small holes you may be interested in the Shop-Cal which has a thin cylindrical rod of .060" diameter for measuring depth.
For best stability when measuring, attach a depth base to the end of the calipers. These simply slide on and are clamped in place with a screw. Before tightening the screw, place the caliper on a flat surface to make sure the depth base is flush with the end of the caliper. Some calipers come with the depth base. In other cases you may need to buy it separately.
The base allows you to firmly position your calipers over the opening in order to get a perpendicular entry into the hole. This is absolutely necessary for an accurate reading.
Because the thickness of the caliper beam varies from manufacturer to manufacturer, depth bases may not always be interchangeable between makes. Depth bases designed for use with calipers are shown on page 173.
Advantages of using the dial caliper for depth measurement:
Dial Depth Gage
This is, in effect, a dial indicator with a depth base attached. The depth base spans the opening, keeping the indicator steady and giving you perpendicular entry into the hole, slot or groove.
A lifting lever at the top of the indicator allows you to easily lift and release the gaging plunger (the indicator spindle). Since the spindle pressure is constant and slight, the readings are not likely to be distorted because of over-tightening in the case of micrometers, or unconscious fudging in the case of calipers. The use of dial indicators allows you to choose from a variety of resolutions and accuracies up to .0001"
It's possible to buy depth bases with 2-1/2" or 4" spanning widths and attach these to any dial indicator you already own. Check to make sure they'll fit your indicator's stem diameter (.375" or 8 mm). You will have to lift the indicator spindle on your own, since the lifting mechanism is probably not present. Lifting mechanisms can also be bought separately for many Mitutoyo models.
Each dial indicator probably has a spindle range of one inch. You can reach into deeper holes by attaching extensions, usually up to 8 inches. All extensions are included with your dial depth gage, but these can also be bought separately. They are used for the same purpose with any dial indicator. Be sure the thread sizes match, either 4-48 or M2.5
Before use, calibrate the gage by placing it on a flat (granite) surface and adjusting the position of the depth base so that the indicator reads zero.
Advantages of using the dial depth gage:
Digital Depth Gage
The same features as the dial depth gage (above) with the added bonus of quick and accurate interpretation of the read-out. No skill is needed in interpreting dial hands.
Additionally, you can switch between inch and metric systems at the touch of a button. Gages with SPC output can be used where large quantities of measurements are involved and statistical analysis is required.
Additional advantages of using the digital depth gage: