The Pyro-C sensors have a "user threshold" feature allowing the user to adjust the measurement threshold in noisy environments. Increasing the threshold will prevent triggering on noisy signals and allow accurate measurment of energy and frequency, as long as the laser pulses are larger than the noise.

The trigger level can be adjusted up to 25% of full scale, however operation depends on the pulse width setting. For pulse width settings below ~0.25ms, the minimum energy that can be measured accurately is approximately 40% above the user threshold setting. Pulses below this energy level will trigger the sensor down to the user threshold level, but accuracy is compromised.

For pulse width settings above ~0.25ms, accuracy is good all the way down to the threshold. If the laser pulse width is less than 1/2 the setting, the minimum energy corresponds to the setting. However, with longer laser pulse widths, the minimum energy will be higher, rising to approximately twice the user threshold level when the laser pulse width is equal to the sensor pulse width setting.

It is recommended always to set the user threshold to the minimum possible setting to retain best energy accuracy in any given situation. See the user manual for further information on how to use the user threshold.

The catalog specification states the maximum power a sensor can be used with and without the heat sink. The purpose of the heat sink is to keep the sensor temperature below the maximum permitted at higher average powers. If you use the sensor for a short time only, on the order of 1-2 minutes at a time, you should be able to measure up to the higher power given in the spec even without the heat sink.

Unless otherwise indicated, Ophir sensors and meters should be recalibrated within 18 months after initial purchase, and then once a year after that.

Yes, with certain limitations. Here are the points to be aware of:
Vega, Nova II, StarLite, StarBright meters and Juno PC interface: Full support of all features
All other instruments (Nova/Orion and LaserStar meters, as well as USBI, Pulsar, and Quasar PC interfaces): Support the Pyro-C sensors, except for the following features: Only 2 of the 5 pulse width settings are available.
User selectable threshold is not available.
In addition to the above: When using a Pyro-C sensor with the Nova (or Orion) meter, the "Nova PE-C Adapter" (Ophir p/n 7Z08272) is required. 

All Ophir pyroelectric sensors can measure average power with Ophir Power and Energy Meters. The instrument measures the number of pulses each second and multiplies the energy reading by the pulse rate. If the pulse rate is constant, then the accuracy of power measurement will be the same as the energy accuracy since the pulse rate measurement is very accurate.

Let’s look at an example. In the spec of the PD10-C, it gives the lowest measurable energy as 1 nJ at wavelength 900 nm. Let’s say the wavelength you work at is 633 nm. Looking at the relative spectral sensitivity curve for this sensor (shown below), we see that the sensitivity at 633 nm is only about 2/3 of what it is at 900nm (as seen in the attached spectral curve). That means that the lowest measurable energy at 633 nm will be (1nJ / ⅔), or about 1.5 nJ.

The Power and Energy Meters simply decides it is time for a sample and takes the next pulse that comes after that time, e.g. if it samples at 400 Hz, then every 1/400th of a second it is ready to take the next pulse that comes along.