Frequently Asked Questions

We have been requested to provide a FAQ page for our customers as this technology, although very mature, is quite new to so many people. These are the most common questions we have been asked and our attempt to provide adequate answers.

1. How do the QCMs measure mass?

QCMs measure mass flux indirectly by measuring the effect a change in mass has on the activity of an oscillating crystal; as more mass is added to the crystal the frequency slows down. This sensitivity to change in mass is linear over a wide range.  Here is a mass conversion example.

2. How much mass range do the QCMs actually have?

A general rule for the maximum frequency shift is to divide the fundamental frequency by 100. For example: a 15 MHz QCM has a sensitivity of 5.09 x 10⁸ Hz/gm/cm². The maximum mass it would be able to measure would then be a 150 KHz shift, or 9.33 x 10^-5 grams. This is assuming that you could get that much mass to adhere well to the crystal. There is the distinct possibility that some of the mass would be in a gelatinous state, that it would not all move with the crystal because it was not a solid. In this case, the crystal movement would be dampened at a substantially lower frequency.

3. Are the QCMs sensitive to change in temperature?

Crystals have a well defined response to temperature; in fact, they are sometimes used for very accurate temperature measurement. This would greatly diminish the ability of the QCM to measure mass flux, independently of temperature if we were not able to do anything about it. We overcome this problem by matching two crystals and then measuring the frequency difference so that the thermal effects are canceled out.

4. Why do you offer optional dielectric overcoatings?

There are presently two schools of thought. Proponents say they have found that matching the surface of the crystal to the surface they are interested in makes a measurable difference in what adheres to the QCM. These folks are usually looking at very small amounts of mass flux over long periods of time. The other side of the argument is that only the first couple of layers actually come into contact with the surface of the crystal and after that the molecules are just piling up on one another. If the pressure around the QCM is 10^-5 Torr and the temperature is low enough, one layer of molecules per second will be collected on the surface of the crystal.

5. Are we able to provide ultra-high vacuum QCMs?

All of the QCMs we build are manufactured using ultra-high vacuum approved materials and procedures. Standard lab quality Mark 18 CQCMs are used in our Vacuum Outgassing Deposition Kinetics Apparatus (VODKA), which operates in the 2 x 10^-11 Torr range.

6. Can the QCMs be used in space?

Our QCMs have a long space flight history. One of the first QCMs we developed (in 1965) was for use in space. At the time of this writing we have quite a number of flight units, both in space and being built for upcoming flight projects. In fact, we have a QCM on the JPL Mars Rover.

7. Can the QCMs be used to measure particles as well as molecules?

Yes they can. The QCM on the JPL Mars Rover has a thin film of "sticky" polymer on the surface to hold small dust particles so that they may be measured. Some ultra-cleanrooms use QCMs to monitor particle contamination in real time. The "sticky" polymer has no effect on the sensitivity of the QCM.

8. Are QCMs ever used to measure dew-point in manufacturing facilities?

Yes. For instance, in the manufacture of hard disks it is important to monitor the dew-point and the QCMs work well to track it and send real-time data for alarms and record keeping.

9. What controllers are available for the QCMs?

There are two classes of controllers available for the QCMs, depending on the operating environment. We make several laboratory quality controllers, from the older Model 1800 and 1900 Controllers (found in facilities all around the world) to the latest Model 2000 (fast becoming the standard controller), and we make a whole line of customized flight controllers.

10.  What is the CRC I hear about while using the M2000 software?

The CRC is an error detection scheme.  It is used for both the commands sent from the host (PC) to the Model 2000, as well as the command responses and data lines sent from the Model 2000 back to the PC.  The CRC count displayed by the M2000 software running on the PC refers to the number of command responses and data lines received from the Model 2000 that failed the CRC test (transmission error).  The CRC used is the CCITT version (polynomial 1021 hexadecimal).

For a primer on CRC check out:  C Programmer's Guide to Serial Communications by Joe Campbell.

11.  How can I clean the crystals of accumulated mass deposits?

There will come a time when the sense crystal becomes heavily loaded with mass.  If a standard bake-out does not adequately clean contamination off the crystal then a more rigorous, last-ditch method must be employed.  Follow the recommended procedure that is included with your particular QCM Sensor.